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cjackson
2014-Jun-11, 11:38 AM
Exactly what kind of technological leap and how much energy would be needed to accelerate/decelerate at 1g on a trip to another star system? I understand that, barring wormholes, constant 1g acceleration is the only way to achieve relativistic effects and get a spacecraft across significant distances in a human lifetime as measured aboard the spacecraft.

Jeff Root
2014-Jun-11, 12:13 PM
There should be online calculators that will allow you to input
one or more of the following, and then calculate the others:

- Distance you want to go

- Acceleration

- Maximum speed

- Time for the voyage by your clock

- Specific impulse of your engines

- Thrust of your engines

- Payload mass

- Propellant mass

- Structural mass

For specific impulse, you would probably choose one of
a few different propulsion options.

Many combinations of inputs aren't possible. You could
not specify a large distance, large payload and short trip
time but a small propellant mass, for example.

I'm hoping that someone will know of and link to such a
calculator. Or make one.

With such a calculator you will be able to see how much
propellant is required to move a given payload a given
distance in a given time.

Somehow you should also have a choice of whether to
accelerate continuously (for shortest time) or just at the
start (and optionally end, if you don't want to just whizz
past the destination) of your trip, to minimize propellant
mass and save wear on the engines.

The amount of propellant required, the duration of engine
operation, and the duration of the trip as a whole should
give pretty good indications of what technological leaps
are needed.

-- Jeff, in Minneapolis

antoniseb
2014-Jun-11, 01:29 PM
Exactly what kind of technological leap and how much energy would be needed to accelerate/decelerate at 1g on a trip to another star system?

Just using Newtonian math to keep it simple, let's imagine that you want to get something about twice the mass of the ISS (on the small side for interstellar flight, but lets start there), so 1 million kilograms to accelerate at 1G (10 meters/second(squared)) for 30 years (a billion seconds).

F=ma so the Force required is 106 * 10 Newtons.
Distance in the first second is 5 meters
Energy per second = FD = 50,000,000 Joules per second.
Energy stored for the whole trip: 5x1016 Joules.
This is roughly the amount of energy stored in a kilogram of antimatter.

Currently we can store about one atom of antimatter, so we need to improve that by a factor of 1027.
We also need to create a process for efficiently converting antimatter energy into acceleration, preferably using as little reaction mass as possible. Failing that we might need to increase the mass of the ship by six to ten orders of magnitude, so as to have a less dense method of storing energy and converting it to accelerating your vessel.

So, I can't say exactly what kind of technical leap we'd need, because we are so far from it that no clarity is really possible.

JustAFriend
2014-Jun-11, 01:54 PM
You'd definitely need a new form of propulsion; no chemical rocket has burned for more than about 10 minutes.

Jeff Root
2014-Jun-11, 04:47 PM
Just using Newtonian math to keep it simple, let's
imagine that you want to get something about twice
the mass of the ISS (on the small side for interstellar
flight, but lets start there), so 1 million kilograms to
accelerate at 1G (10 meters/second(squared)) for
30 years (a billion seconds).
That's an awfully long time. Just one year at 1 g will
get you close to the speed of light. When relativistic
time dilation is taken into account, 30 years should
permit travel to distances of hundreds of light-years.



F=ma so the Force required is 106 * 10 Newtons.
Distance in the first second is 5 meters
Energy per second = FD = 50,000,000 Joules per second.
Energy stored for the whole trip: 5x1016 Joules.
This is roughly the amount of energy stored in a kilogram
of antimatter.
Wow! That's several orders of magnitude less than I
would have guessed! Starts out sounding pretty good!



Currently we can store about one atom of antimatter, so
we need to improve that by a factor of 1027.
I have no idea how it might be done, but I'm a little bit
optimistic that a way can be found to do that.



We also need to create a process for efficiently converting
antimatter energy into acceleration, preferably using as
little reaction mass as possible.
The problem is that mixing antimatter and ordinary matter
produces gamma rays, which go right through everything.
You want a gamma-ray mirror. Neutronium, maybe? I'm
not optimistic about that at all. Something to absorb the
gamma rays and turn them into heat? Lots of mass for all
known materials. Lots of shielding for the payload. Huge
problem to solve.

-- Jeff, in Minneapolis

Ara Pacis
2014-Jun-11, 05:37 PM
You might try pre-positioning fuel on route to reduce ship mass.

Hornblower
2014-Jun-11, 05:55 PM
You might try pre-positioning fuel on route to reduce ship mass.

How do you propose delivering the fuel to those pre-positioning stations?

NEOWatcher
2014-Jun-11, 06:06 PM
How do you propose delivering the fuel to those pre-positioning stations?
Not only that, but how do you rendezvous with them to refuel without slowing down?

Ara Pacis
2014-Jun-11, 06:54 PM
How do you propose delivering the fuel to those pre-positioning stations?
Not only that, but how do you rendezvous with them to refuel without slowing down?

Seriously guys? How do you think?

Jeff Root
2014-Jun-11, 07:20 PM
Have them placed there, moving in the right directions, at
the right speeds, by friendly alien time-travellers. Obviously.

If God wanted man to travel to the stars, He would have
surrounded us with friendly space aliens. With fuel tanks.

-- Jeff, in Minneapolis

NEOWatcher
2014-Jun-11, 07:26 PM
Seriously guys? How do you think?
I was being serious. I don't see the advantage.

Getting the fuel there, and then up to speed to match your craft, or slowing down the craft to pick them up is going to use nearly the same fuel since the weight requirements of such a craft is going to be almost all fuel anyway.

The only way you can be ahead is if those fuel depots collected fuel from local sources after they got in position.

Ara Pacis
2014-Jun-11, 09:05 PM
I was being serious. I don't see the advantage.

Getting the fuel there, and then up to speed to match your craft, or slowing down the craft to pick them up is going to use nearly the same fuel since the weight requirements of such a craft is going to be almost all fuel anyway.Sure, but that depends on what propulsion method you're using.


The only way you can be ahead is if those fuel depots collected fuel from local sources after they got in position.By "ahead", do you mean efficiency or position?

Jeff Root
2014-Jun-11, 09:37 PM
He means efficiency. The reason you gave for "pre-positioning"
fuel along the route was to reduce ship mass. It might reduce
the mass of that ship, but at the cost of requiring additional
ships with a far larger total mass. Getting them to rendezvous
without slowing the ship whose mass you are trying to reduce
would be at the expense of sending the fuel out ahead of time,
slowing it down, and speeding it back up again for rendezvous.
Wasteful, pointless, and kinda goofy.

-- Jeff, in Minneapolis

Amber Robot
2014-Jun-11, 09:50 PM
Is anyone including the resistance from plowing through the interstellar medium?

grapes
2014-Jun-11, 10:46 PM
That's an awfully long time. Just one year at 1 g will
get you close to the speed of light. When relativistic
time dilation is taken into account, 30 years should
permit travel to distances of hundreds of light-years.

1g for 1 year is 3% more than the speed of light ( (acceleration of gravity)*(1 year) at wolframalpha.com ), and since the ship can not exceed light speed, the occupants may feel 1g, but their acceleration relative to the universe would be less, right?

Jeff Root
2014-Jun-11, 10:50 PM
The radiation would be terrible at really high speeds.
Aside from that, and maybe running into a few rocks,
the interstellar medium shouldn't be significant.

No forward-looking windows. Just cameras that can be
poked out the side when you want to see what's ahead.

-- Jeff, in Minneapolis

Jeff Root
2014-Jun-11, 11:05 PM
grapes,

I don't think I've ever calculated a relativistic acceleration
in anyone's frame but that of the one accelerating. It must
depend on how you define "acceleration". A change in speed,
or force per unit mass. Although continuing to accelerate
doesn't increase speed much relative to outside observers,
in addition to providing artificial gravity for passengers, it
gets them to their destination a lot faster by their clocks.
So I think it would be worth it if you can afford it, and can
deal with relativistic collisions.

-- Jeff, in Minneapolis

Jeff Root
2014-Jun-11, 11:13 PM
Some fraction of all starships moving at near-light speeds
must be totalled when they run into an object comparable
in size to the front bumper. I hope it's a small fraction.

-- Jeff, in Minneapolis

NEOWatcher
2014-Jun-12, 11:51 AM
1g for 1 year is 3% more than the speed of light ( (acceleration of gravity)*(1 year) at wolframalpha.com ), and since the ship can not exceed light speed, the occupants may feel 1g, but their acceleration relative to the universe would be less, right?

I don't think I've ever calculated a relativistic acceleration
in anyone's frame but that of the one accelerating.

Yes; it gets complicated considering the frames of reference. A linear calculation based on 9.8m/s2 just doesn't do it.
As the traveler is accelerating, their 1g is not our measurement of what we would see as their 1g. Our view of their 1g becomes less and less over time because of length contraction.

I have no clue how to compute this, but I know its not t=c/g.

caveman1917
2014-Jun-12, 12:23 PM
I have no clue how to compute this, but I know its not t=c/g.

Use relativistic kinetic energy E_K = (\gamma - 1)m (c=1), so given a certain amount of energy expended to accelerate the craft its velocity will be

\frac{E}{m} + 1 = \gamma = \frac{1}{\sqrt{1-v^2}}

\frac{m}{E + m} = \sqrt{1-v^2}

\left( \frac{m}{E+m} \right)^2 = 1 - v^2

v = \sqrt{1 - \left( \frac{m}{E+m} \right)^2}

caveman1917
2014-Jun-12, 12:33 PM
But if you're using constant acceleration you can simplify a little, such as given here (http://math.ucr.edu/home/baez/physics/Relativity/SR/rocket.html). That link also includes some example calculations for accelerating constantly at 1g.

NEOWatcher
2014-Jun-12, 12:39 PM
But if you're using constant acceleration you can simplify a little, such as given here (http://math.ucr.edu/home/baez/physics/Relativity/SR/rocket.html). That link also includes some example calculations for accelerating constantly at 1g.
Thanks for the link. It helps to see it when someone else does the math and gives several values along the way (particularly that first table)

IsaacKuo
2014-Jun-12, 03:09 PM
Exactly what kind of technological leap and how much energy would be needed to accelerate/decelerate at 1g on a trip to another star system? I understand that, barring wormholes, constant 1g acceleration is the only way to achieve relativistic effects and get a spacecraft across significant distances in a human lifetime as measured aboard the spacecraft.
First off, a human lifetime interstellar journey is possible with "only" a fraction of c. The Alpha Centauri system is arguably the most interesting star system to visit in our neck of the woods, and it is conveniently also the closest. At "only" 0.09c, it would take less than 50 years to reach. That's less than a human lifetime.

My usual assumptions for a manned interstellar mission include a return journey, so I look for a baseline mission of .30c going there (about 1.5 decades), 1 decade of study in situ, and then .25c returning (about 2 decades). That's a total of 4.5 decades, a good scientific career for someone in their early 30s to retire in their late 70s.

To reach .30c, with an acceleration of 10m/s/s, would only take around 3.5 months.

So basically, constant acceleration is not necessary. Even so, 1gee acceleration for 3.5 months is an extreme technical challenge. No existing propulsion systems can come within orders of magnitude of doing it. The only method demonstrated in the lab which could do it is microwave sail propulsion (demonstrated multi-gee acceleration of decimeter scale sail by James Benford).

One method which I used to like is "bombtrack" propulsion, which uses nuclear bombs to push against the magnetic field of a large superconducting magnetic loop. This is similar to the Mag-Orion concept, except that these bombs are not carried by the starship. Instead, they are pre-positioned drones that form a "track". The bombs detonate as the starship passes by--the starship consists of the magnetic loop along with payload modules strung on the loop like beads on a necklace.

There are numerous problems with the bombtrack proposal, which I won't detail. The key concept, though, that is relevant to questions in this thread is that you do not have to pick up "fuel" along the way. Rather than rendezvous with fuel supplies, you merely interact with them in passing.

In fact, the methods of interstellar propulsion I favor now use the high relative speed between the "supplies" and the starship as the entire source of energy. The sheer violence of relativistic kinetic impacts is so great that they exceed the energy density of nuclear bombs (with no annoying neutron radiation or minimum critical mass requirements!). Rather than try to pick up an incoming relativistic "fuel" packet, the starship merely puffs a bit of on board propellant, which instantly explodes upon impact with the "fuel" packet. This explosion of charged particles is deflected by the mag-loop's magnetic field, producing thrust.

This relativistic kinetic impact powered rocket/ramjet is, I think, the most practical method of achieving the sorts of sustained accelerations you're asking about.

caveman1917
2014-Jun-12, 03:36 PM
There are numerous problems with the bombtrack proposal, which I won't detail. The key concept, though, that is relevant to questions in this thread is that you do not have to pick up "fuel" along the way. Rather than rendezvous with fuel supplies, you merely interact with them in passing.

Does that make much of a difference? If the bomb is just sitting there and you pass by at a decent fraction of c then the energy imparted on the ship by the bomb will be significantly lowered. This also requires energy for the bombs to slow down again when they reach their destination, so it looks like a double loss to have them stationary waiting on a track.

NEOWatcher
2014-Jun-12, 03:44 PM
That's a total of 4.5 decades, a good scientific career for someone in their early 30s to retire in their late 70s.
I would worry about someones health in a mission that long. It would take a boatload of "in case" medical supplies, or a boatload of spare researchers as a risk mitigation plan.


There are numerous problems with the bombtrack proposal, which I won't detail.
The biggest I see is the nearly instantaneous accelerations. That's high g impacts and a lot of them.


Does that make much of a difference? If the bomb is just sitting there and you pass by at a decent fraction of c then the energy imparted on the ship by the bomb will be significantly lowered.
It's better than nothing. ;)


This also requires energy for the bombs to slow down again when they reach their destination, so it looks like a double loss to have them stationary waiting on a track.
Or, if they were sent out long before the craft, then we have the bombs at a closer speed and no need to slow down.

But; how long would it take to put this "infrastructure" in place. Without a relativistic way to place the bombs, you're wait for setup seems to be as long as a non-relativistic trip would be in the first place.
It just gains you less "man-time" in space.

caveman1917
2014-Jun-12, 03:49 PM
Or, if they were sent out long before the craft, then we have the bombs at a closer speed and no need to slow down.

Yes, that was my point, if you're going to do it with bombs prelaunched then it seems to be a lot better to keep them at top speed. The more i think about it, the more the idea of having the bombs slow down again and being stationary for when the ship passes seems like a huge efficiency loss.

IsaacKuo
2014-Jun-12, 05:54 PM
Does that make much of a difference? If the bomb is just sitting there and you pass by at a decent fraction of c then the energy imparted on the ship by the bomb will be significantly lowered.
It is indeed lowered--there is no free lunch--but that's simply part of the design. The key requirement for "bombtrack" propulsion is that some useful fraction of the bomb's charged particle products are moving fast enough to usefully push against the starship's magnetic field.

This also requires energy for the bombs to slow down again when they reach their destination, so it looks like a double loss to have them stationary waiting on a track.
No, there is no requirement to slow down the bombs at all. The track can be moving at whatever speed was suitable for deployment. For example, you might use high performance solar sails for track deployment, resulting in bomb units heading away from the Sun at 100km/s. This speed is far too slow for interstellar travel, but it's fast enough to deploy the bomb track in a reasonable length of time. The bomb units never brake at all...they just keep on coasting away from the Sun at 100km/s until detonated.

IsaacKuo
2014-Jun-12, 06:05 PM
But; how long would it take to put this "infrastructure" in place. Without a relativistic way to place the bombs, you're wait for setup seems to be as long as a non-relativistic trip would be in the first place.
It just gains you less "man-time" in space.
For the initial acceleration track, relativistic deployment is not required. An acceleration run of 3.5 months up to .3c requires an acceleration track only a few light weeks long.

For the deceleration track, things get more complicated to explain.

Maybe the simplest way to explain it would be to have the starship itself hold the deceleration bombtrack as cargo. Shortly after completing the acceleration run, the starship starts spitting out bomb drones behind it. This is not simply a matter of tossing them overboard; there is no drag in space. Rather, the bombs themselves have thrusters and/or a mass driver is used to impart a relative velocity. The goal is to deploy a bombtrack behind the starship, so that it reaches the desired length when the starship arrives at Alpha Centauri.

Then, when Alpha Centauri is reached, the starship uses this bombtrack to decelerate. In the frame of reference of Alpha Centauri, the starship and bombtrack both starts off moving at .3c; the starship slows down as the bombtrack screams onward at .3c. In the frame of reference of the bombtrack, the starship starts off moving at 0c and it accelerates up to .3c.

This balloons the mass requirements of the initial starship geometrically. But the mass growth is still only quadratic, which can be a win against the rocket equation. The rocket equation's mass growth is exponential.

NEOWatcher
2014-Jun-12, 06:18 PM
For the initial acceleration track, relativistic deployment is not required. An acceleration run of 3.5 months up to .3c requires an acceleration track only a few light weeks long.
Which means that the farthest one will take over 150 years to be in place for your ship (assuming 20 light days long) at 100km/s.


Maybe the simplest way to explain it would be to have the starship itself hold the deceleration bombtrack as cargo. Shortly after completing the acceleration run, the starship starts spitting out bomb drones behind it.
That's pretty much the same as carrying all the fuel you need to decelerate.

Ara Pacis
2014-Jun-12, 06:33 PM
He means efficiency. The reason you gave for "pre-positioning"
fuel along the route was to reduce ship mass. It might reduce
the mass of that ship, but at the cost of requiring additional
ships with a far larger total mass. Getting them to rendezvous
without slowing the ship whose mass you are trying to reduce
would be at the expense of sending the fuel out ahead of time,
slowing it down, and speeding it back up again for rendezvous.
Wasteful, pointless, and kinda goofy.

-- Jeff, in Minneapolis

Yes, it is less efficient, but that can't be helped, not if you want to complete a realistic mission in a suitable amount of time. The key is to establish an infrastructure that does a lot of the heavy lifting. And then, use non-chemical propulsion systems where you can, in order to reduce mass. See Isaac Kuo's posts in this thread for something close to what I'm thinking.

The bomblets (and other pods, possibly reactor fuel or food containers) in advance of the vehicle are accelerated by an extending laser relay station track. These laser relay stations push the bomblets and other pods to higher speeds along the track. The laser station also push each other, and then are pushed by some of the impactors and bomblets. The vehicle itself has a propulsion laser to help increase the speed of the bomblets to match velocity and position for capturing. However, yes, at some point the vehicle may over take them. The trajectories are design such that the reduction of speed from maneuvering/capturing those bomblets is towards the part of the mission where the ship is intended to decelerate. After using them for momentum transfer, the vehicle uses them for pulse propulsion to decelerate into the target star system. Then, the vehicle may loop around the target star and then interact with the follow-on impactors to further reduce its speed. The pulse system might use a pusher plate or a magnetic system, whichever is most efficient.

The initial launch might be a combination of solar sail dive, laser sail or magnetic sail, whatever works best for the laser station and bomblets and pods. The vehicle is launched later, years, perhaps decades later. Depending on mass and efficiency, it might make a solar dive and perform an Oberth maneuver as close to the sun as possible. It may use drop-off boosters/tanks using a short duration high thrust fuel (NSWR or similar) then it might drop those and switch to an electric drive. It might use solar/laser sails or mag sails too at some point, depending on efficiency. At some point it may also use pulse propulsion from inert object impacts caused by objects that take solar dives and receive propulsion from lasers and/or from objects that are fires from electromagnetic catapults. These pulse propulsion events may be periodic depending on infrastructure, at which point it switches back to electric drive (unless it didn't need to turn it off). Eventually, the vehicle's speed will be close enough to that of the impactors that it's no longer advantageous, at which point it may switch to the bomblets that were with it at launch. The vehicle uses these and then coasts until it gets to the point where it starts picking up more and prepares for deceleration (which might take years or decades).

I have to give Isaac Kuo props for a lot of this, from previous threads.

IsaacKuo
2014-Jun-12, 06:36 PM
Which means that the farthest one will take over 150 years to be in place for your ship (assuming 20 light days long) at 100km/s.
Yes, well it depends on what mission parameters you want. I'll admit I didn't bother carefully calculating that example out, because I haven't favored bombtrack propulsion in well over a decade.

That's pretty much the same as carrying all the fuel you need to decelerate.
Not as a rocket, though. As a rocket, the mass requirements grow exponentially. This is because you need to decelerate your fuel as well as the spacecraft--not just the spacecraft. With this bombtrack system, the mass requirements only grow quadratically. You have to accelerate the deceleration fuel up to the cruise speed, but you do not need to decelerate any of it.

Like I said, it is not easy to explain. In the past, when I have tried, my experience is that at least 50% of people simply can't wrap their head around the idea of a deceleration track at all.

NEOWatcher
2014-Jun-12, 06:44 PM
Not as a rocket, though. As a rocket, the mass requirements grow exponentially. This is because you need to decelerate your fuel as well as the spacecraft--not just the spacecraft. With this bombtrack system, the mass requirements only grow quadratically. You have to accelerate the deceleration fuel up to the cruise speed, but you do not need to decelerate any of it.
I think I see what you are doing.
As long as you dump all this "fuel" overboard before decelerationg, it certainly would reduce the requirements.
Now the issue is that as that the first one that went overboard keeps travelling at a constant rate as you decelerate. So; as you slow down, your bombs keep getting farther and farther away.
By the time you get down to (lets say 100km/s) your final bomb is a few light weeks away because it left before deceleration. We just put another 150 years on the trip.

caveman1917
2014-Jun-12, 06:47 PM
Maybe the simplest way to explain it would be to have the starship itself hold the deceleration bombtrack as cargo. Shortly after completing the acceleration run, the starship starts spitting out bomb drones behind it.

You do mean in front of it, right? In the sense of towards the destination.


This is not simply a matter of tossing them overboard; there is no drag in space. Rather, the bombs themselves have thrusters and/or a mass driver is used to impart a relative velocity.

Why use thrusters on the bombs? Starting one month in advance a bomb moving away at tossing speed (say 1m/s) reaches over 2500 km away from the ship, more than you'll require (or even be able to use).

caveman1917
2014-Jun-12, 06:55 PM
No, there is no requirement to slow down the bombs at all.

Ok, i misunderstood that part then.

Jeff Root
2014-Jun-12, 07:16 PM
I think Isaac did mean behind the ship. But as you indicate,
he was mistaken about deploying the bombs after completing
the acceleration. They would be dropped off behind the ship
while the ship was accelerating. When the ship decelerates,
the bombs would start catching up with it, in reverse order
of which they were dropped. The bombs would have to pass
the ship and explode the instant they were past, or you would
have a mess. Explode too soon and the ship is destroyed.
Explode too late and the ship doesn't decelerate enough for
the next bomb to reach it. Explode a little bit off to one side
and the ship is pushed even farther off track.

-- Jeff, in Minneapolis

Jeff Root
2014-Jun-12, 07:26 PM
It looks to me like the most efficient way to use such bombs --
and maybe the only way to use them that could increase rather
than decrease the capability of the system overall -- would be
to launch them *after* the ship, giving them higher acceleration
and higher speeds. Even then, it might be useful only if the
limiting factor is the acceleration of the ship, with higher limits
on acceleration of the bombs. If the ship could be accelerated
as much as those bombs, you would just do that instead.

-- Jeff, in Minneapolis

IsaacKuo
2014-Jun-12, 07:31 PM
You do mean in front of it, right? In the sense of towards the destination.
No, I mean behind it. In the sense of away from the destination--toward Earth. NEOWatcher seems to have the seem misunderstanding, because he sees the problem with putting the track in front of the starship...if they're in front of the starship, then they can't be used to decelerate with. They'll just keep cruising away ahead of the starship as it tries to decelerate.

The track needs to be behind the starship.

Try to imagine being on an long schoolbus. You want to run from the front of the bus to the rear of the bus, so that by the time you reach the (open) rear door your velocity is the same as the ground. The bus is moving at, say, 30mph.

In the reference frame of the bus, you start off at 0mph and you accelerate toward the rear of the bus up to 30mph.

In the reference frame of the ground, you start off at 30mph and you decelerate down to 0mph.

The bus is analagous to the bombtrack. This scheme only works if you start at the front of the bus, and work your way to the rear of the bus.

Why use thrusters on the bombs? Starting one month in advance a bomb moving away at tossing speed (say 1m/s) reaches over 2500 km away from the ship, more than you'll require (or even be able to use).
The primary reason each bomb has thrusters is to maintain formation. Alignment of the track is very important! Since each bomb already needs to have an integral propulsion system, it may be used for bringing them into the necessary formation in the first place.

NEOWatcher
2014-Jun-12, 07:45 PM
No, I mean behind it. In the sense of away from the destination--toward Earth. NEOWatcher seems to have the seem misunderstanding, because he sees the problem with putting the track in front of the starship...if they're in front of the starship, then they can't be used to decelerate with. They'll just keep cruising away ahead of the starship as it tries to decelerate.

Ok; that makes sense, but I doubt it really helps. (each successive bomb is farther and farther behind but will catch up as you slow down)

Whether you dump them off the back or the front you still have the same problem unless you vary the speed of each of the bomb's dump.
If you do that, then you are accelerating your ship by doing the dump.
Otherwise each bomb will have to carry enough fuel to decelerate so it's not too far from the ship. The last one needing to decelerate almost to zero.
If not you (assuming your dump at 100km/s relative to the ship), you're back to the 150 year deployment to slow down.

Ara Pacis
2014-Jun-12, 07:48 PM
It looks to me like the most efficient way to use such bombs --
and maybe the only way to use them that could increase rather
than decrease the capability of the system overall -- would be
to launch them *after* the ship, giving them higher acceleration
and higher speeds. Even then, it might be useful only if the
limiting factor is the acceleration of the ship, with higher limits
on acceleration of the bombs. If the ship could be accelerated
as much as those bombs, you would just do that instead.

-- Jeff, in Minneapolis

Are you referring to Isaac's system or mine?

First, different masses accelerate at different rates depending how much much force you can bring to bear on it. Just because you can accelerate a small mass across a set distance inside the solar system does not necessarily mean you can manage to provide enough force to bring a massier ship to the same speed within the same distance using the same system when you consider efficiency and economics. So, you could launch the bombs later and expect them to catch up to the vehicle and be the better for it.

Second, You can send them ahead of the ship at a speed and trajectory calculated to be close to the speed of the vehicle as the vehicle accelerates or coasts past. Moreover, you can schedule the interception to coincide with that part of the trajectory where you want the vehicle to start slowing down from momentum transfer.

See my post on the previous page for more information, in case you missed it.

Ara Pacis
2014-Jun-12, 07:51 PM
The primary reason each bomb has thrusters is to maintain formation. Alignment of the track is very important! Since each bomb already needs to have an integral propulsion system, it may be used for bringing them into the necessary formation in the first place.

Or use a solar sail on the bomb and a laser from the vehicle to let it tack into the proper alignment. You might do this by having the bomb adjust it's sail actively, or maybe put it into a slow spin and let it power down and then let the vehicle use it's laser in pulses as the angled sail spins to provide directional thrust.

Jeff Root
2014-Jun-12, 07:51 PM
The primary reason each bomb has thrusters is to
maintain formation.
Okay.



Alignment of the track is very important!
Fer sure!



Since each bomb already needs to have an integral
propulsion system, it may be used for bringing them
into the necessary formation in the first place.
Wait-- what?? What are you distinguishing between?
Why does each bomb already need to have an integral
propulsion system? To maintain formation? So how
is bringing the bombs into formation in the first place
different from maintaining the formation? Ideally, they
could just be dropped off as the ship accelerates, with
no need of any propulsion on them.

-- Jeff, in Minneapolis

IsaacKuo
2014-Jun-12, 07:53 PM
I think Isaac did mean behind the ship. But as you indicate,
he was mistaken about deploying the bombs after completing
the acceleration. They would be dropped off behind the ship
while the ship was accelerating.
Depending on the exact details of how things are done, this can be more efficient. However, it is more complex to explain and for others to understand.

When the ship decelerates,
the bombs would start catching up with it, in reverse order
of which they were dropped. The bombs would have to pass
the ship and explode the instant they were past, or you would
have a mess. Explode too soon and the ship is destroyed.
Explode too late and the ship doesn't decelerate enough for
the next bomb to reach it. Explode a little bit off to one side
and the ship is pushed even farther off track.
Actually, the situation is the same during the acceleration run as for the deceleration run. The only reason people have more difficulty understanding the deceleration run is because of the counter-intuitive nature of relative motion in space.

Exploding at the wrong time isn't something that really risks destroying the ship, because nothing in the ship ever gets excessively close to the trackline. The starship is a large magnetic loop with payload modules strung on it like necklace beads. If a bomb explodes too soon, it will end up accelerating the starship in the wrong direction, but each individual bomb does not accelerate the starship by a large amount.

However, the timing requirements for self-detonating nuclear bombs are extremely tight, so I'm actually not confident that nuclear bombs will be able to do the job at all. Unless the timing of the detonations is nearly perfect, the result will be that they explode too far away from the starship to have any significant effect at all. This is one of the reasons I prefer kinetic impact based propulsion instead. The collision between a puff of gas from the starship and an incoming impactor will automatically take place at the required position.

As for keeping the starship on track, to a certain degree the system is self centering due to the shape of the magnetic field. Higher order stability, though, would have to be actively maintained using weight shifting and/or electromagnets and/or stationkeeping thrusters.

Jeff Root
2014-Jun-12, 08:13 PM
Are you referring to Isaac's system or mine?
Isaac's. Your post described a jumble of separate
stream of conciousness thoughts, not a system.

My first post referred to Isaac's deceleration scheme,
dropping bombs from the ship, while my second post
referred to his acceleration scheme, launching bombs
from the Solar System. I don't think either scheme
would provide an overall advantage, even ignoring the
massive possible failure modes. The energy lost to
speed differences between the ship and the bombs
appears to negate the advantages of not having to
accelerate them along with the rest of the ship.

-- Jeff, in Minneapolis

Ara Pacis
2014-Jun-12, 08:28 PM
Isaac's. Your post described a jumble of separate
stream of conciousness thoughts, not a system.

Are you saying that we shouldn't use staging in spaceflight? The Apollo astronauts might have something to say about that.

You know that characterization is insulting, don't you? I'll ask you not to do it again.


My first post referred to Isaac's deceleration scheme,
dropping bombs from the ship, while my second post
referred to his acceleration scheme, launching bombs
from the Solar System. I don't think either scheme
would provide an overall advantage, even ignoring the
massive possible failure modes.

My proposal doesn't have those failure modes.


The energy lost to
speed differences between the ship and the bombs
appears to negate the advantages of not having to
accelerate them along with the rest of the ship.

Again, depends on the system, and mine is organized differently and so does not have those issues.

caveman1917
2014-Jun-12, 08:55 PM
Actually, the situation is the same during the acceleration run as for the deceleration run. The only reason people have more difficulty understanding the deceleration run is because of the counter-intuitive nature of relative motion in space.

That's not it, it's because it doesn't make sense to throw the bombs behind the ship. Take the frame of the ship en-route to alpha centauri with the bombs loaded. The ship is thus stationary with the earth moving away behind it and alpha centauri moving towards it in front of it, and it wants to decrease the speed at which they are doing that. It has a set of bombs each consisting of the bomb mass and some fuel mass for a thruster. Now consider two scenario's, in scenario 1 it throws the bombs behind it and in scenario 2 it throws them in front of it, with equal speeds. In scenario 1 the ship will, due to conservation of momentum, increase its velocity whereas in scenario 2, by the same token, it will decrease its velocity. So at this stage scenario 2 is strictly ahead. The bombs then use their thrusters to become stationary with the ship, in both scenario's the same acceleration profile has to be applied (other than its direction), so at this stage where the bombs are now stationary either behind or in front scenario 2 is still strictly ahead. Then the bombs accelerate such as to end up in front of the ship one by one, again the exact same acceleration profile has to be used but the result is that, with equal speed, in scenario 1 the bomb is moving away from the ship as it explodes, whereas in scenario 2 the bomb is moving towards the ship, so here scenario 2 is also strictly better in order to decrease the ship's speed.

IsaacKuo
2014-Jun-12, 09:31 PM
That's not it, it's because it doesn't make sense to throw the bombs behind the ship. Take the frame of the ship en-route to alpha centauri with the bombs loaded. The ship is thus stationary with the earth moving away behind it and alpha centauri moving towards it in front of it, and it wants to decrease the speed at which they are doing that. It has a set of bombs each consisting of the bomb mass and some fuel mass for a thruster. Now consider two scenario's, in scenario 1 it throws the bombs behind it and in scenario 2 it throws them in front of it, with equal speeds. In scenario 1 the ship will, due to conservation of momentum, increase its velocity whereas in scenario 2, by the same token, it will decrease its velocity. So at this stage scenario 2 is strictly ahead. The bombs then use their thrusters to become stationary with the ship, in both scenario's the same acceleration profile has to be applied (other than its direction), so at this stage where the bombs are now stationary either behind or in front scenario 2 is still strictly ahead. Then the bombs accelerate such as to end up in front of the ship one by one, again the exact same acceleration profile has to be used but the result is that, with equal speed, in scenario 1 the bomb is moving away from the ship as it explodes, whereas in scenario 2 the bomb is moving towards the ship, so here scenario 2 is also strictly better in order to decrease the ship's speed.

First off, you're assuming that the ship magically changes velocity when the bombs use their on board thrusters to either go rearward (scenario 1) or forward (scenario 2). But actually, the ship stays at the same velocity either way.

Second, the amount of velocity we're talking about is small compared to the velocity of the ship anyway.

Third, there is no reason for the bombs to match velocities with the starship.

Fourth, scenario 2 simply won't work at all. The bombs are simply not in the correct location to help the starship decelerate! They are cruising off through the Alpha Centauri system while the starship is trying to decelerate.

You seem to assume that the bombs have some propulsion system of their own which lets them accelerate toward the ship, but if such a high performance rocket system existed, then the starship could have used that rocket system directly.

IsaacKuo
2014-Jun-12, 09:37 PM
My first post referred to Isaac's deceleration scheme,
dropping bombs from the ship, while my second post
referred to his acceleration scheme, launching bombs
from the Solar System. I don't think either scheme
would provide an overall advantage, even ignoring the
massive possible failure modes. The energy lost to
speed differences between the ship and the bombs
appears to negate the advantages of not having to
accelerate them along with the rest of the ship.

On what do you base this? I see no calculations to that effect anywhere in this discussion. Many years ago, I posted my calculations, even dubbing the relevant formula the "bombtrack equation". Since then, I have turned my attention to superior alternatives, but these superior alternatives still share some features with the bombtrack concept.

Jeff Root
2014-Jun-12, 09:42 PM
caveman1917,

I believe that Isaac has had that correct from the start -- the
bombs for decelerating would be dropped off behind the ship.
As I said, and as Isaac agreed, dropping them while accelerating
would be more efficient than dropping them after acceleration
was finished. One big problem with the method is that the bombs
are moving in the direction opposite the direction the force needs
to be applied, wasting a lot of energy.

But putting the bombs in front of the ship would mean they would
have to be propelled forward and then slowed down tremendously.
They would be on the right side of the ship, moving in the right
direction relative to the ship, but at enormous expense. Dropping
them behind the ship during acceleration doesn't require the bombs
to have any propulsion at all. They will naturally catch up with the
ship as the ship decelerates.

-- Jeff, in Minneapolis

caveman1917
2014-Jun-12, 09:53 PM
First off, you're assuming that the ship magically changes velocity when the bombs use their on board thrusters to either go rearward (scenario 1) or forward (scenario 2).

No i am not, where are you getting that from? The magic by which the ship changes velocity as it is throwing out the bombs is called conservation of momentum. Other than that the ship doesn't change velocity (other than the bomb explosions of course). ETA: it doesn't even really matter if it is throwing out the bombs or they are getting velocity away from the ship by using their own thrusters (the reaction mass from the thrusters will still interact with the ship so as to produce the same result).


Second, the amount of velocity we're talking about is small compared to the velocity of the ship anyway.

When the ship wants to decelerate then decelerating it, even by a small amount, is still better than accelerating it by that amount.


Third, there is no reason for the bombs to match velocities with the starship.

The only way for the bombs not to match velocity with the ship at some point while changing their velocity from away from the ship to toward the ship would be if they can change velocity discontinuously. How would you propose doing that?


Fourth, scenario 2 simply won't work at all. The bombs are simply not in the correct location to help the starship decelerate!

The bombs, when they detonate, are in the exact same location relative to the ship in both scenario's. The difference is that in scenario 2 they are moving towards the ship and in scenario 1 they are moving away from it, meaning scenario 2 is strictly more efficient here too.


You seem to assume that the bombs have some propulsion system of their own which lets them accelerate toward the ship, but if such a high performance rocket system existed, then the starship could have used that rocket system directly.

I thought that was your assumption:

the bombs themselves have thrusters and/or a mass driver is used to impart a relative velocity.

But even so, if they don't have a propulsion system on their own then it makes even less sense to throw them out the back. The result will be that you have a bunch of bombs behind you slowly moving away from you. Then what? The ship would need to decelerate in order to be able to use its means of deceleration.

Jeff Root
2014-Jun-12, 10:05 PM
The ship would need to decelerate in order to be able to
use its means of deceleration.
Exactly. The ship is decelerating, so the bombs *have to be*
behind the ship in order to meet up with it again. Or else do
a bunch of pointless and wasteful thrusting fore and aft.

-- Jeff, in Minneapolis

caveman1917
2014-Jun-12, 10:13 PM
Exactly. The ship is decelerating, so the bombs *have to be*
behind the ship in order to meet up with it again. Or else do
a bunch of pointless and wasteful thrusting fore and aft.

-- Jeff, in Minneapolis

You're missing the point. It needs to decelerate to use the bombs, it needs to use the bombs to decelerate. Don't you see the problem here?

Jeff Root
2014-Jun-12, 11:00 PM
Obviously. I said as much in post #35:

> Explode too late and the ship doesn't decelerate enough for
> the next bomb to reach it.

Although missing one bomb probably would not be serious --
They'd be spaced in such a way that at least the next few
would catch up to the ship pretty soon without any further
deceleration.

But -- How does a car engine get started? It depends on
turning over in order to turn over! The ship can start the
deceleration by tossing one or two bombs out the front.
Then the bombs behind will begin to catch up.

-- Jeff, in Minneapolis

caveman1917
2014-Jun-14, 10:03 AM
Obviously. I said as much in post #35:

> Explode too late and the ship doesn't decelerate enough for
> the next bomb to reach it.

Although missing one bomb probably would not be serious --
They'd be spaced in such a way that at least the next few
would catch up to the ship pretty soon without any further
deceleration.

But -- How does a car engine get started? It depends on
turning over in order to turn over! The ship can start the
deceleration by tossing one or two bombs out the front.
Then the bombs behind will begin to catch up.

-- Jeff, in Minneapolis

Yes you're right of course, the ship is constantly changing frames during its interactions with the bombs and hence the acceleration profiles for the two scenario's aren't the same. I completely missed that earlier.

publiusr
2014-Jun-14, 05:05 PM
You'd definitely need a new form of propulsion; no chemical rocket has burned for more than about 10 minutes.

A NSWR is the only thing I know of that could pull that off--if you can find a way to manage the heat.

cjackson
2014-Jun-20, 04:43 AM
Is it safe to say, that regardless of technological advances constant 1g acceleration will be infeasible?

Ara Pacis
2014-Jun-20, 04:47 AM
Is it safe to say, that regardless of technological advances constant 1g acceleration will be infeasible?

You could just rotate the craft to simulate 1 g constantly.

Noclevername
2014-Jun-20, 06:00 AM
Is it safe to say, that regardless of technological advances constant 1g acceleration will be infeasible?

A modified ramjet is plausible; the original Bussard Ramjet relied on using magnetically collected interstellar medium for fusion fuel, which does not seem to work out as it would create more drag than thrust, and the ISM is of the wrong composition to maintain fusion.

But using an onboard energy source to "throughput" electrostatically concentrated ISM as reaction mass (without decelerating it) might enable constant acceleration; whether it could reach 1G is for mathier people than me to decide.

WayneFrancis
2014-Jun-20, 06:21 AM
1g for 1 year is 3% more than the speed of light ( (acceleration of gravity)*(1 year) at wolframalpha.com ), and since the ship can not exceed light speed, the occupants may feel 1g, but their acceleration relative to the universe would be less, right?

Correct. It would get the ship to relativistic speeds but the acceleration, to outside observers, would appear to slow down.

Wooops I answered a post that was years old :/

Jeff Root
2014-Jun-20, 06:46 AM
You're okay, Wayne. The post is just over two weeks old.

-- Jeff, in Minneapolis

kzb
2014-Jun-26, 11:45 AM
According to this article, a ship travelling at more than 58% c will repel particles in its path. This is apparently from 2006, does anyone know if this was confirmed or denied later?

http://phys.org/news10789.html

mkline55
2014-Jun-26, 02:12 PM
According to this article, a ship travelling at more than 58% c will repel particles in its path. This is apparently from 2006, does anyone know if this was confirmed or denied later?

http://phys.org/news10789.html
The story is that he plans to present an exact solution to the Einstein Field Equations which shows an antigravity effect. I believe you will find Dr. Ferber's article here (http://arxiv.org/pdf/gr-qc/0604076.pdf). I don't know the history of the paper, but it appears the author claims that once you accelerate a much heavier mass to over 57% of c, then it might be used to accelerate a smaller mass. I am skeptical of the practicality and specifics.

kzb
2014-Jun-26, 05:30 PM
^ He seems to be saying that anything in the way of a relativistic spaceship would automatically be kicked out of the way? If true that would be handy.

WayneFrancis
2014-Jun-27, 12:46 AM
According to this article, a ship travelling at more than 58% c will repel particles in its path. This is apparently from 2006, does anyone know if this was confirmed or denied later?

http://phys.org/news10789.html

The article is very vague and a few things seem to pop out at me.
First is the 1 ton payload only needs 30,000,000,000 tons worth of tnt to accelerate it to .9c
That might seem a lot but that is only 30 gigatonne. That too might seem a lot and it is. It is ~600x more then the larges nuclear bomb we've detonated.
But think of it this way. That is 907kg of material. That means we should be able to, fairly easily, get a 1kg mass to .9c as it would take only ~30megatonne or ~125PJ
The LHC uses 362MJ or 3.62x108J/ beam. We'd need 33,000x that amount of energy or to get a 1kg mass to .9c.
Ok ... the feat is sounding a bit more difficult then I first thought but still not that much energy in the grand scheme of things.

Next is the article mentions "In the'antigravity beam' of a speeding star". What? We've gone from a 30 gigatonne explosion to a star travelling at .9c that the craft would "hitching a ride on"?
It sounds like they are saying the craft could rid in the wake effectively being dragged along with the star. But if you've got a star travelling at .9c why do you care what is in your path? Really
the star is in front of you and would absorb all the impacts anyway. What happens when you get to your destination? Don't aim that "star" at your target or your target will be gone. So you
aim a little off? But then You still have to jump off. Which means you need another 30 gigatonnes of energy.

Sounds like they've taken some normal liberties that pop-sci articles do and extrapolated way to far and I haven't even talked about the "antigravity" beam. Which makes it sound like a warp
situation where you shrink space in front of you. You ship continually free falls into that. The warped bubble just deflects stuff around your ship because that becomes the shortest path from the front to back.

I'd like to hear others explanation and I guess I will :)