# Thread: rocket fuel for interstellar travel

1. ## rocket fuel for interstellar travel

I once read about a rocket for interstellar travel that would fly in a stream of it's own fuel. The fuel would be accelerated to the target and the ship would fly down that path, gobbling up fuel as it went. How much fuel would you have to accelerate to do such a thing, assuming you were using hydrogen? It seems like it would be a ridiculous amount of fuel, but I can't even imagine how much that would be.

Would all the hydrogen on Earth be enough or are we talking about harvesting fuel from a gas giant?

2. It wouldn't be that much, because it would stop being useful as soon as your ship accelerated to a velocity relative to the fuel equal to its exhaust velocity. This would probably happen at a much lower velocity than if it'd been a conventional rocket with a decent mass ratio. (You only need a mass ratio of 7.4... for the craft to have a delta-v equal to double its exhaust velocity, so assuming that's how fast you could launch the fuel ahead, that's the mass ratio a conventional rocket would need to beat one ingesting an external propellant stream.)

3. I suspect what you're talking about is the preseeded trajectory for a Bussard ramjet. (Useful search terms.)
It has the advantage that you don't need to burn fuel to accelerate fuel to be used later in the voyage - the idea is that the fuel packets are launched using a rail gun. The velocity profile of the fuel packet trail is tailored to match the velocity of the spacecraft, so that drag is minimized.

Grant Hutchison

4. Member
Join Date
Jul 2010
Posts
70
The correct answer is NONE! After your craft has reached sol escape velocity it can turn off its engine and coast to the star! it will need fuel to get it to orbit ( and what ever) around the target star.

Space craft are not like cars --- there is no friction in space --- it does not work the same way as on earth!

Mark

5. ## rocket fuel for interstellar travel

I think Solfe was describing a technique in which a constant supply of fuel was available so that the ship could reach relativistic speeds, or at least some significant percentage of the speed of light. That was the intent of the ramjet method which would ingest interstellar hydrogen.

Sent from my iPhone using Tapatalk Pro
Last edited by schlaugh; 2019-Jan-07 at 01:22 AM.

6. But you would need the same amount of fuel to reverse the process at your destination. You will need the time, the fuel, the astrogation ,
and the conscious operation of that vehicle , unless you expect to trust your machine to do everything for you , at precisely the correct time
and place. Unless you want to slip past your destination at some terrific velocity .

7. Interesting thought to send a factory bot to a Kuiper Belt mini-object made from various ices, then build engines on one end designed to use the mini-object's mass as fuel.

8. Doesn't that assume that the Kuiper Belt object (from which fuel manufacture is suggested ) is moving at the same vector and velocity as the primary ship in question? It would appear that you would have to decelerate to that Kuiper belt object (certainly a substantial cost in
delta V , and than try to accelerate on vector , just to reach the velocity you had before . Where do you realize a significant gain or advantage? Maybe I missed something.

9. Originally Posted by danscope
Doesn't that assume that the Kuiper Belt object (from which fuel manufacture is suggested ) is moving at the same vector and velocity as the primary ship in question? It would appear that you would have to decelerate to that Kuiper belt object (certainly a substantial cost in
delta V , and than try to accelerate on vector , just to reach the velocity you had before . Where do you realize a significant gain or advantage? Maybe I missed something.
The Kuiper Belt mini-object is the ship itself. You add the parts.

10. Originally Posted by danscope
But you would need the same amount of fuel to reverse the process at your destination. You will need the time, the fuel, the astrogation ,
and the conscious operation of that vehicle , unless you expect to trust your machine to do everything for you , at precisely the correct time
and place. Unless you want to slip past your destination at some terrific velocity .
There are other proposed braking methods as well. A magsail "parachute", for instance.

11. It sounds like a rocket using a pre-seed flight path isn't riding in a stream of material, it's in a bolus* of fuel to be consumed. Does the image of a "stream" come from the idea that the system to create that mass of fuel probably would continue to feed it as the ship travels or perhaps is seeding the same area for another ship to make it's passage later?

*I like that word. Its organic sounding while describing a "simple" physical principal of delivering stuff to the right place at the right time, which is a necessarily complex problem.

12. It's a stream, rather than a bolus. If braking at destination is required, the first packages to be launched are those that will be consumed at the destination, which are moving slowly and have a long way to go. Then faster packages for the earlier braking trajectory. Then the acceleration trajectory is seeded, with slower packages for early acceleration, and then faster packages which will overtake the slower ones, and provide fuel later in the acceleration phase. Then the ramjet launches. The whole is carefully tailored to place a fuel package in the right place at the right time with the right velocity, as the ramjet passes through. Any delay in the ramjet launch, and the stream will compact at the far end and disperse at the near end, eventually becoming unusable.

Grant Hutchison

13. Established Member
Join Date
Jun 2002
Posts
1,943
Originally Posted by holmes4
-- there is no friction in space --
A rogue planet, wandering between stars would provide enough friction to vaporize any reasonable interstellar craft.
We are starting to get some stats on how come such things are, one time star occultations; but you are not going to fend such a thing off with your magnetic deflection screens. It remains to be seen how big a problem interstellar debris will prove to be. Radar may not be good enough to pick such things up before collision is unavoidable.

14. Originally Posted by grant hutchison
It's a stream, rather than a bolus. If braking at destination is required, the first packages to be launched are those that will be consumed at the destination, which are moving slowly and have a long way to go.
I imagine fuel 'packages' with a small amount of autonomous steering, that can adjust their position just before they are intercepted. The 'package' could release a cloud of fuel just before the decelerating ship passes by, in response to a proximity signal emitted by the ship. This seems to be a fairly sensible solution to the problem of deceleration in an interstellar craft, which is one of the trickiest bits of the whole operation.

There are two aspects of this process that strike me immediately: the fuel packages would need to be launched many decades or even centuries before the ship itself (the last ones that the ship intercepts will need to be moving quite slowly, so would be launched first at a relatively low speed). Secondly, the fuel packages would probably include some incombustible hardware that could cause damage if it collided with the decelerating ship. But overall, this seems like a reasonable solution. In particular, the fact that the fuel packages are moving more slowly than the craft means that less energy is required overall.

15. Whatever happened to the Bussard ramscoop? Feeding off interstellar hydrogen? I should look that up.

LATER: https://en.wikipedia.org/wiki/Bussard_ramjet

Oh, won't work without major changes.
Last edited by Roger E. Moore; 2019-Jan-29 at 09:23 PM. Reason: addition

16. Originally Posted by Squink
A rogue planet, wandering between stars would provide enough friction to vaporize any reasonable interstellar craft.
We are starting to get some stats on how come such things are, one time star occultations; but you are not going to fend such a thing off with your magnetic deflection screens. It remains to be seen how big a problem interstellar debris will prove to be. Radar may not be good enough to pick such things up before collision is unavoidable.
There may be rogue planets in interstellar space, but I think the odds of colliding with one would be very, very small.

Sent from my iPhone using Tapatalk

17. Originally Posted by Jens
There may be rogue planets in interstellar space, but I think the odds of colliding with one would be very, very small.
Interstellar debris of significant size could be discovered and tracked by detecting transits of stars. A starship equipped with a telescope or series of the same capable of monitoring unexpected transits could map out a way forward to avoid trouble.

18. kzb
Established Member
Join Date
Apr 2005
Posts
2,366
Originally Posted by Roger E. Moore
Interstellar debris of significant size could be discovered and tracked by detecting transits of stars. A starship equipped with a telescope or series of the same capable of monitoring unexpected transits could map out a way forward to avoid trouble.
I was thinking of posting this in the interesting arxiv papers thread, but it seems to be relevant here as well:

Two new free-floating or wide-orbit planets from microlensing

Planet formation theories predict the existence of free-floating planets that have been ejected from their parent systems........these detections are consistent with low-mass wide-orbit or unbound planets being more common than stars in the Milky Way.

https://arxiv.org/abs/1811.00441

I've also been looking at Mroz et al previous papers. According to these we should expect about 2.5 Mars-mass rogue planets per star. Plus a huge number of smaller bodies. It's these smaller bodies that are the biggest worry for interstellar navigation I would say. You are looking at about 20 Earth masses of material ejected from each stellar system during its formation.

19. Just hitting a fleck of dust at interstellar travel speeds would be disastrous.

20. Hence many star ships that have been envisioned include some form of ice shielding, either as cladding on the hull or as a small comet/asteroid launched a few hundred kilometers ahead of the ship to clear a path.

Neither of which helps if you run into a dark rogue planet. Maybe a constant radar (or ladar) return would be useful but it would have to be quite powerful in order to detect an object far enough away so that the ship has time to evade destruction.

Sent from my iPhone using Tapatalk Pro

21. Originally Posted by kzb
I've also been looking at Mroz et al previous papers. According to these we should expect about 2.5 Mars-mass rogue planets per star. Plus a huge number of smaller bodies. It's these smaller bodies that are the biggest worry for interstellar navigation I would say. You are looking at about 20 Earth masses of material ejected from each stellar system during its formation.
I don't think the smaller bodies are really much of a problem. 2.5 Mars masses per star is just insignificant. I think it's really the small dust particles that are the issue. Even if you passed through the asteroid belt you would have a pretty hard time hitting one of the asteroids.

22. kzb
Established Member
Join Date
Apr 2005
Posts
2,366
Originally Posted by Jens
I don't think the smaller bodies are really much of a problem. 2.5 Mars masses per star is just insignificant. I think it's really the small dust particles that are the issue. Even if you passed through the asteroid belt you would have a pretty hard time hitting one of the asteroids.
I'm sure you are right, but just to clarify, it was 20 (not 2.5) Earth masses of smaller bodies.

Reportedly they expect 2.5 Mars-mass free-floating planets per star.

In some ways this is good because these could be refuelling posts. But it does mean you have to slow down and stop, then accelerate again.....

23. In the solar neighbourhood, that comes out to about 0.08 earth masses per cubic lightyear, or an average of half a picogram per cubic kilometre.

Grant Hutchison

24. Order of Kilopi
Join Date
Feb 2005
Posts
11,130
Originally Posted by danscope
But you would need the same amount of fuel to reverse the process at your destination.
Braking is easier. It was said a Ramscoop would be a better brake than a ramjet--so there is your fuel for a return journey at least.

I always liked the idea of the NSWR. Instant starship, just add water.

Now to keep it from melting....

25. Established Member
Join Date
Jun 2002
Posts
1,943
Originally Posted by Roger E. Moore
Just hitting a fleck of dust at interstellar travel speeds would be disastrous.
I worry more about bricks. Craters on barren bodies shows us that the count of debris goes up as square of inverse size.
Not many planets, but lots of dark bricks. We'll need good scopes to find them, and good lasers to fend them off.

26. Originally Posted by Squink
I worry more about bricks. Craters on barren bodies shows us that the count of debris goes up as square of inverse size.
Not many planets, but lots of dark bricks. We'll need good scopes to find them, and good lasers to fend them off.
If you were flying behind a stream of fuel, wouldn't the fuel take care of the debris in your path? It seems to me that the impact formula works both ways. At the very least, debris would get warmer as they get peppered with fuel.

#### Posting Permissions

• You may not post new threads
• You may not post replies
• You may not post attachments
• You may not edit your posts
•