PDA

View Full Version : How Fast to Go From Saturn to Venus in 3 Hours?



Tuckerfan
2007-Sep-25, 10:56 AM
I was listening to an old radio serial (I think it was Space Patrol) and a couple of characters who were in orbit around Saturn are ordered to report to Venus, they say that it'll take them three hours. Now, obviously, they're crowding the speed of light really closely, but how close? And how many gees would they be pulling to be able to do this?

G O R T
2007-Sep-25, 11:30 AM
Light would travel between the planets in 1 hour 24 minutes plus or minus 6 minutes depending on orbital position. This puts the average velocity required at only half lightspeed. Piece of cake for a rocket with a sparkler hanging out the back. :)

Tuckerfan
2007-Sep-25, 11:37 AM
Heh. Wonder if NASA knows about that? Oh, have you heard about this? Remake of The Day the Earth Stood Still (http://imdb.com/title/tt0970416/). I can't really picture Reeves as Klaatu, though.

pghnative
2007-Sep-25, 01:14 PM
I can't really picture Reeves as Klaatu, though.
He'd be a great Gort, though...

astromark
2007-Sep-25, 07:14 PM
All very interesting a... but when writing fiction you can say what you like. It does not make it possible. 'Eta Carina. Warp 9, make it so...'
As a small exercise in mathematics you can calculate that velocity from Saturn to Venus in three hours... but its still fiction.:)

speedfreek
2007-Sep-25, 08:51 PM
Some very rough math here:

Venus is around 0.7 AU from the Sun, and Saturn ranges from around 9AU to around 10AU. Let's choose a distance of, say 9AU for this journey.

To make a journey of 9AU, in 3 days, requires a constant acceleration at around 8g until turnover at the halfway point and then a constant deceleration at around 8g until you arrive!

I won't show my working, as I cheated and used this calculator (http://home.att.net/~srschmitt/script_starship.html) to do it. :whistle:

Saluki
2007-Sep-25, 09:10 PM
How are you going to accelerate toward Venus from Saturn? If you acelerate from Saturn, you head outward. You need to decelerate to get to Venus from Saturn.

cjl
2007-Sep-25, 09:27 PM
You need to accelerate. Acceleration is a vector, not a scalar, In the case of heading towards venus, the acceleration vector would be towards the sun and slightly backwards in the orbit.

speedfreek
2007-Sep-25, 10:00 PM
Yes, deceleration is just acceleration in the opposite direction.

If you are orbiting a body and you accelerate whilst pointing in your direction of orbit, you move away from that body. If you turn and face backwards to the direction of orbit and accelerate, you move towards that body. So if you were orbiting Saturn you would start your burn a little before your orbit pointed you in the opposite direction to Saturn's movement in space, in order to "decelerate" towards the Sun.

But in my earlier example really did oversimplify things in as much as I didn't take into account the difference between Venus and Saturns orbital speeds. In fact, to do it "properly" requires something like a Hohmann transfer to be calculated. Well, the more I think about it if we want to do it in 3 days, then I'm really not sure what kind of calculations you use to do it properly, as you are using constant thrust! A transfer orbit isn't going to work - with constant acceleration you move in straight line (subject to the influence of other planets). So either my previous post was just a rough ballpark estimate, or not! ;)

Van Rijn
2007-Sep-25, 10:19 PM
But in my earlier example really did oversimplify things in as much as I didn't take into account the difference between Venus and Saturns orbital speeds. In fact, to do it "properly" requires something like a Hohmann transfer to be calculated. Well, the more I think about it if we want to do it in 3 days, then I'm really not sure what kind of calculations you use to do it properly, as you are using constant thrust! A transfer orbit isn't going to work - with constant acceleration you move in straight line (subject to the influence of other planets). So either my previous post was just a rough ballpark estimate, or not! ;)

Right, if you can manage those accelerations, the trajectory is very nearly a straight line. Sure, you need to aim for where Venus will be, and you need to match velocity at the end, but that's trivial if you can manage constant 8g acceleration.

Van Rijn
2007-Sep-25, 10:31 PM
I was listening to an old radio serial (I think it was Space Patrol) and a couple of characters who were in orbit around Saturn are ordered to report to Venus, they say that it'll take them three hours. Now, obviously, they're crowding the speed of light really closely, but how close? And how many gees would they be pulling to be able to do this?

Very roughly, using that calculator speedfreek found, and assuming 9 AU at constant acceleration with turnaround, with a three hour flight (as seen from Earth - it would be a little less on ship) I got about 5700 Gs, and a maximum velocity of about .71 C

speedfreek
2007-Sep-26, 12:45 AM
Oops! I misread the OP. Was supposed to be 3 hours, not 3 days. What a doofus I am :( Well at least all the principles still apply. (I must remember to check my posts more carefully)

5700g eh? All I can say is.. ouch.

grant hutchison
2007-Sep-26, 02:22 AM
It gets a bit relativistic in the middle.
If we assume an average distance between Saturn and Venus of 9.54AU (just Saturn's distance from the Sun) then our travellers need to accelerate towards Venus for 4.77 AU (covering that distance in an hour and a half) and then decelerate for the same distance, in the same time.
Assuming the three hours is time measured by the travellers' clocks, then that's an acceleration of 4710g, with a peak velocity of 0.68c, and a total elapsed time of 3.36 hours for those at rest in the vicinity of Venus and Saturn.
But if our rocketeers need to get there in three hours by the observers' clocks, then they need to rush a little more: 6200g, peak velocity of 0.74c, and an elapsed time by their own clocks of just 2.59 hours.

Some of the necessary maths is laid out at The Relativistic Rocket (http://www.math.ucr.edu/home/baez/physics/Relativity/SR/rocket.html).

Grant Hutchison

Warren Platts
2007-Sep-26, 02:37 AM
Oops! I misread the OP. Was supposed to be 3 hours, not 3 days. What a doofus I am :( Well at least all the principles still apply. (I must remember to check my posts more carefully)

5700g eh? All I can say is.. ouch.
The g forces could be minimized if the passengers were immersed in a neutrally bouyant tank of liquid. It would then be a matter of the total pressure a human body could withstand.

Van Rijn
2007-Sep-26, 02:47 AM
You'd need a bit more than neutral buoyancy at 5000 Gs or more. But that's okay, because you'd be hard pressed to manage that kind of acceleration over that time even with a matter/anti-matter rocket. And if you had one, it would make a nice death ray.

a1call
2007-Sep-26, 02:54 AM
I can't really do the math in an acceptable amount of time, but if you accelerate away from the direction of Saturn's orbit around the sun, effectively deorbiting yourself around the Sun and free falling towards the sun (feeling 0g) into an orbit around Venus, then you probably get there with minimal fuel expense and accelerating of the order of 600 km/s or so.

Of course not having any expertize in this area I expect to be corrected.

Deceleration around Venus, don't know how it works. Should involve some sort of gravitational pull.

grant hutchison
2007-Sep-26, 02:54 AM
You'd need a bit more than neutral buoyancy at 5000 Gs or more. But that's okay, because you'd be hard pressed to manage that kind of acceleration over that time even with a matter/anti-matter rocket. And if you had one, it would make a nice death ray."Hard pressed" indeed. :)
The problem you refer to, I think, is that not all body tissues have the same density. So although you may be floating in some "neutral buoyancy" fluid, your bones will still tend to sink, while your fat will tend to float. At high enough acceleration, your skeleton rips out of your body, while your adipose tissues rise to form a scum under the lid of the buoyancy tank.

Grant Hutchison

Warren Platts
2007-Sep-26, 03:18 AM
I see your point. A decent laboratory centrifuge will deliver 25,000 g's--enough to suck the DNA out of a fat cell. But I would think a buoyancy tank would increase the g's a human could withstand--at least that's a premise of some science fiction stories. I wonder if NASA has done any research in this area.

publius
2007-Sep-26, 03:34 AM
You all have to got stop thinking in terms of convential proper acceleration where you feel forces. The way to do is artificial gravity, and I mean that in a different sense than it is typically used. Space-time engineering would be a less confusing term. You "simply" warp space-time so you fall toward your destination, feeling no force. :)

Now, figure out how to do that without needing solar masses worth of mass-energy, and *negative* mass-energy at that, or other exotic tricks, and you'll be, well, what's his name from Star Trek lore, Zephrim Cochran. :lol:

-Richard

astromark
2007-Sep-26, 06:49 AM
How are you going to accelerate toward Venus from Saturn? If you acelerate from Saturn, you head outward. You need to decelerate to get to Venus from Saturn.


This is completely wrong. Only if you are orbiting the sun would an increase in velocity take you away. Going to Venus from Saturn is as simple as pointing in the general direction and going there.

m1omg
2007-Sep-26, 07:37 AM
If someone invents Halo drive or another reactionless drive like that in Orion's arm then you will have acceleration and fuel solved.
Unlikely but possible.
More;
http://www.orionsarm.com/ships/Drive_Technology.html#halo
http://www.orionsarm.com/ships/Reactionless_Drives.html

Van Rijn
2007-Sep-26, 08:28 AM
I see your point. A decent laboratory centrifuge will deliver 25,000 g's--enough to suck the DNA out of a fat cell. But I would think a buoyancy tank would increase the g's a human could withstand--at least that's a premise of some science fiction stories. I wonder if NASA has done any research in this area.

It probably would help some for moderate g forces. I think the first limitation would be blood pooling. Laying flat helps, but that has its limits. Assuming extremely strong magnets were available, diamagnetism (http://en.wikipedia.org/wiki/Diamagnetism#Diamagnetic_levitation) also might be able to help some, though I expect there would be differential effects with different tissue.

Tuckerfan
2007-Sep-26, 08:53 AM
He'd be a great Gort, though...

Exactly what I was thinking.

So, the answer is that the only way you could do it is if you didn't mind being turned into goo. I figured the g load would be high, but never did I imagine that it'd be in the thousands! Here's a thought that I don't think anyone's came up with: Humans gradually loose bone while in space, so perhaps the first humans to fly a ship approaching the speed of light will be boneless because they've spent so many years in space.

eburacum45
2007-Sep-26, 09:09 AM
According to Miguel Alcubierre and Chris Van Den Broeck, spacecraft inside a warp bubble would be immune to these acceleration forces, as Publius suggests.

All we need to do is make a warp bubble and get a spaceship inside; only two impossible things to do before breakfast.

Neverfly
2007-Sep-26, 11:05 AM
According to Miguel Alcubierre and Chris Van Den Broeck, spacecraft inside a warp bubble would be immune to these acceleration forces, as Publius suggests.

All we need to do is make a warp bubble and get a spaceship inside; only two impossible things to do before breakfast.

Ok, I made some bubbles. Now who can warp them?:neutral:

mugaliens
2007-Sep-26, 04:14 PM
Some very rough math here:

Venus is around 0.7 AU from the Sun, and Saturn ranges from around 9AU to around 10AU. Let's choose a distance of, say 9AU for this journey.

To make a journey of 9AU, in 3 days, requires a constant acceleration at around 8g until turnover at the halfway point and then a constant deceleration at around 8g until you arrive!

I won't show my working, as I cheated and used this calculator (http://home.att.net/~srschmitt/script_starship.html) to do it. :whistle:

8 Gs for three days is very tolerable to humans if they're lying in a saline solution and pressure-breathing.

The internal forces on the body would be roughly equivalent to saturation diving at around 6 feet depth.

The air/lung interface would survive the worst of it, but would be little different than breath scuba for three days.

speedfreek
2007-Sep-26, 05:44 PM
Yup.

It's just a pity the journey time was supposed to be 3 hours, not 3 days like in my mistaken example. 3 days seems almost possible, in comparison!

grant hutchison
2007-Sep-26, 08:15 PM
Another problem with neutral bouyancy, and one which arises much sooner than the whole skeleton-falling-out thing, is a difficulty with air breathing.
Because of the difference in densities, there is a much lower pressure gradient in the air that fills your lungs than there is in the blood that perfuses your lungs: the pressure difference across capillary walls is therefore higher in the dependent parts of your lungs than it is in the upper parts.
This mismatch gets worse as acceleration increases, and eventually you'll just stop perfusing the upper parts of your lungs, while developing pulmonary oedema in the lower parts. The "solution" to the problem is to produce a steeper pressure gradient in the air spaces of the lungs, by filling them with a liquid: the pressure gradient inside and outside the lung tissues is then much more closely matched at all accelerations.

(I put "solution" in scare quotes, because liquid breathing is far from being the doddle that it's portrayed to be in science fiction.)

Grant Hutchison

Warren Platts
2007-Sep-27, 02:55 AM
8 Gs for three days is very tolerable to humans if they're lying in a saline solution and pressure-breathing.

The internal forces on the body would be roughly equivalent to saturation diving at around 6 feet depth.

The air/lung interface would survive the worst of it, but would be little different than breath scuba for three days.
8 g's only equivalent to 6 feet underwater? How do you calculate that? Not saying you're not right, but 6 feet isn't very much. What is the g equivalent of say 300 or 1500 feet underwater?

Warren Platts
2007-Sep-27, 03:01 AM
Another problem with neutral bouyancy, and one which arises much sooner than the whole skeleton-falling-out thing, is a difficulty with air breathing.
Because of the difference in densities, there is a much lower pressure gradient in the air that fills your lungs than there is in the blood that perfuses your lungs: the pressure difference across capillary walls is therefore higher in the dependent parts of your lungs than it is in the upper parts.
This mismatch gets worse as acceleration increases, and eventually you'll just stop perfusing the upper parts of your lungs, while developing pulmonary oedema in the lower parts. The "solution" to the problem is to produce a steeper pressure gradient in the air spaces of the lungs, by filling them with a liquid: the pressure gradient inside and outside the lung tissues is then much more closely matched at all accelerations.

(I put "solution" in scare quotes, because liquid breathing is far from being the doddle that it's portrayed to be in science fiction.)

Grant Hutchison
Maybe you could have an intravenous heart-lung machine, induce unconciousness, and so breathing wouldn't be necessary. What about an intelligent halibut that lacks an air bladder?

grant hutchison
2007-Sep-27, 12:14 PM
Maybe you could have an intravenous heart-lung machine, induce unconciousness, and so breathing wouldn't be necessary.OK. You go first. :)

Grant Hutchison

grant hutchison
2007-Sep-27, 12:46 PM
8 g's only equivalent to 6 feet underwater? How do you calculate that?Six feet of water at 1g exerts the same pressure as nine inches of water at 8g. I presume mugaliens is making the conversion based on someone barely immersed in a saline bath.

Grant Hutchison

m1omg
2007-Sep-30, 01:00 PM
Exactly what I was thinking.

So, the answer is that the only way you could do it is if you didn't mind being turned into goo. I figured the g load would be high, but never did I imagine that it'd be in the thousands! Here's a thought that I don't think anyone's came up with: Humans gradually loose bone while in space, so perhaps the first humans to fly a ship approaching the speed of light will be boneless because they've spent so many years in space.

That is because of weightlessness.
If you will accelerate at 1 g or use rotational simulated gravity you will be fine.
And humans cannot live without bones.