Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: remoulton on 17/03/2009 00:18:21
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I hope this is the correct category in which to ask this question...
A number of people (Robert Bussard among others) have worked for decades on the concept of a fusion drive. Assuming this proves to be possible...
A vessel accelerates to cruising velocity v, which is an appreciable fraction of c. At some point, as it nears its destination, it must perform a turnover and decelerate, shedding most of this momentum. But, with a fusion drive, isn't the ship now moving into its own ionized and superheated exhaust? What kind of shielding would be necessary to protect the craft from this? And, could navigational instruments function in the presence of so much ionization or would the vessel be travelling essentially blind?
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My guess is that were we able to construct such a vehicle, we would have the technology to avoid the problems you mention. It might be as simple as making sure any ejecta moved fast enough so that it would not be overtaken; this should be simple enough; the ejecta must naturally move faster than the ship. Another way would be to direct the ejecta in a cone shape so that it spread outward. Another way would be to eject only light-speed stuff, like gamma rays.
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Why gamma rays? There is no need, you can use visible light, or infrared radiation. With a 3 Terawatt laser you get a push of ~ 10,000 N (~ 1000kgp): F = W/c where W is the beam's power.
Nothing however imposes to use a laser, a heated body's radiation would be the same.
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The OP mentioned a fusion reaction; I guessed there would be lots of gamma rays there. Yep; you're right; just plain old light would work.
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I'd also imagine that if you decelerate slowly enough, the engine-wash particles that did catch up with you would have relatively low velocities with respect to your rocket. Low velocity particles are generally easier to shield against.
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How would engine wash particles ever catch up; wouldn't it have to accelerate toward the decelerating ship? The wash should always be moving faster than the ship in the direction the ship was moving.
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The wash should always be moving faster than the ship in the direction the ship was moving.
Are you talking about the engine wash it released prior to decelerating or after decelerating. I agree with you in the latter case. In the accelerating phase, I think the only requirement is that the particles be moving in the opposite direction of the ship in the ship's reference frame. Once the ship slows down, they might catch up.
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I guess maybe it would be possible to collide with your accelerating ejecta while decelerating. Of course any respectable space vehicle capable of interstellar travel would accelerate half way there and decelerate the other half; all at a comfortable one g.
We could alter course just a smidgen at the half way point for a slight dog leg and avoid the possibility of collision.
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The turnover will be at the half-way point; if you were able to wait until you were near your destination before decelerating it would mean that either you could decelerate faster than you could accelerate, or that you didn't accelerate as quickly as you could and made the journey-time greater than necessary.
I can't see how any exhaust from the acceleration phase can catch up with the craft when it starts decelerating because in a reaction engine powered craft, it's speed cannot exceed that of it's exhaust. Even if the craft reaches, let's say, 0.9999% of the exhaust velocity, the exhaust will still be moving in the opposite direction at 0.0001% of it's velocity.
The issue of decelerating in to its own exhaust will only happen if it's decelerating in to something that can slow the exhaust, like an atmosphere, but then you wouldn't be using a Bussard ram-jet in an atmosphere. In the effective vacuum of space, the exhaust will always moving away from the craft as there's nothing to slow it. Rather, it will draw away from the craft as the craft slows.
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I can't see how any exhaust from the acceleration phase can catch up with the craft when it starts decelerating because in a reaction engine powered craft, it's speed cannot exceed that of it's exhaust.
What makes you say that? The exhaust is vented from the spaceship. In the spaceship's reference frame, this exhaust always has momentum "backwards" moving the ship "forwards." So essentially as long as you're burning fuel, you're accelerating, which shouldn't put a cap (aside from relativistic limits) on your maximal speed.
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Umm... I'm not sure why I thought that, but it must have been for a very good reason.
Now that I'm a bit more awake, I think I might have been thinking about something else related to Bussard ram-jets, and I agree that rockets can exceed their exhaust velocity. Rockets do become less efficient though, once they've exceeded a certain speed - perhaps that was what I was thinking about?
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Yes when you are decellerating you do run the risk of being hit by the reaction mass you used earlier, but I would have thought that because it is essentially a very hot gas it will have dispersed to fill a huge cross sectional area by the time it hits you so your relatively small spaceship will be hit by relatively few particles, and as someone mentioned earlier you would only have to make a tiny course correction to be missed by it entirely.
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Why have people suggested using photon drive in this topic about ion drive?
The momentum change for photons is, surely, less than the momentum change for ions with an equivalent energy. So why consider it?
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The advantage of photons is that whilst the momentum change you get per unit energy is very small, the momentum change you produce per unit mass is the largest that is theoretically possible. This becomes very important when you have to accelerate your fuel as well as the spaceship and you are going a long way. Essentially the faster you throw stuff out of the back of your ship the less energy efficient the ΔV but the more mass efficient it is.
The mass efficiency of a rocket is known as the specific impulse, oddly it is the momentum produced by a rocket per unit weight (on earth) of its fuel. A water rocket has a very low specific impulse as it only pushes the water out at say 5m/s so it would have a specific impluse of
5m/s / 9.81m/s2 = 0.5 s
but it is very energy efficient because it is pushing material out slowly.
a chemical rocket can have a specific impulse of up to about 400s
Ion drives that have been designed up to now can have an impulse of up to 30 000s but theoretically you could go a lot higher than this.
Some kinds of antimatter rockets could have an impulse of 10 000 000s launching material out of the back
If in theory you could anihilate antimatter and send all the resultant photons in the same direction you would have a specific impulse of about 30 000 000s which is the highest theoretically possible.
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When you accelerate the craft you will do that until you get the velocity you are looking for. Then I would presume those engines to be 'shut down' so that there would be no 'ejecta'. the 'ejecta' that then was pushed in the opposite direction to give you the velocity I would then expect to disappear in space. After coasting :) you want to stop your velocity, turning it you don't need to use 'fusion', and as the ship thereafter 'brakes' using the oposite reaction from your 'ejecta' with a higher velocity than your ship, that too should leave your ship free, it seems to me. It will always be in a 'ejecta free zone' I think (except from inside that 'engine':)
Awh, I think you are right, that ejecta thrown out earlier will travel with you. On the other hand? Isn't it traveling the opposite way at near/or lightspeed??
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I was assuming that we accelerate at one G for half the distance, and decelerate at one G the other half. For short distances, like within the solar system, I see no possibility of the ship ever being within the zone of its ejecta. I don't have a handy-dandy equation at hand to calculate the ejecta zone for great distances, but my intuition tells me that you should remain free of the ejecta zone there also.
But I am well aware that intuition is wrong a lot [:)]
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Daveshorts
I take your point about lost mass. However, if you are generating energy using a fuel, nuclear or otherwise, there is bound to be some waste product which can't be worth keeping. Why can't it be ejected as the propellant? It may be just a matter of the 'numbers' but the sums would need to be done to get it right.
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Doesn't the idea of a conical beam (second post in the thread) solve the problem of running into propellant?
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Doesn't the idea of a conical beam (second post in the thread) solve the problem of running into propellant?
It would greatly diminish it. The problem with a conical beam would be that you'd be cutting into you drive's efficiency, since you'd lose some thrust to the transverse momentum within the beam. A collimated beam is probably the best bet.
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Daveshorts
I take your point about lost mass. However, if you are generating energy using a fuel, nuclear or otherwise, there is bound to be some waste product which can't be worth keeping. Why can't it be ejected as the propellant? It may be just a matter of the 'numbers' but the sums would need to be done to get it right.
The photon engine is the best choice when you need to reach very high speeds and a lot of energy and time for the travel are available; of course you can also eject waste massive particles, but you can't do it (at high rates of mass ejected) for a long time, because you will soon expire all the available mass.
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That equation would depend upon how the amount of energy available related to the waste mass produced during the process. Are there actual figures about this or is your conclusion based on gut feeling, like mine? If you were using a fusion reactor, how much energy would you get for each He nucleus formed and could it all be used to eject another He ion at an appropriate speed? The He nucleus would be no more use on board the ship, in any case, so you may as well use it as propellant.
In deep space, there could be no other (solar) source of energy, so you'd need to carry it all with you. A lower mass of craft would be less to accelerate as your journey progressed.
I was just thinking that a fusion reactor could produce the propellant directly - if you could direct the pressure in some way. The He nuclei would be moving a fair bit.
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http://en.wikipedia.org/wiki/Nuclear_photonic_rocket
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Thanks, but I'm not sure that helps to resolve the question. It discusses the options and agrees with my idea that the thrust is greater for an ion drive.
There is still the issue of limited energy availability and efficiency would be important (the gamma ray drive would need to be massive, with so much lead screening!). For photon drive, you still need energy and that has to come from something with mass. If you don't eject this mass, then you are wasting energy on propelling it further. If you need to dump it anyway, you may as well dump it at high velocity and call it an ion drive - even if there are photons too!
I couldn't fine a definitive answer in there.
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Okay; so we need an ion drive capable of ejecting ions at near light speed. By the time we are capable of constructing this interstellar voyager we should also be capable collecting the stray ions in empty space so that we can use them for fuel and ejecta.
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A huge fishing net out at the front, perhaps? It would have to be very big and very strong to prevent debris punching holes through it. In deep space we would only get 1 proton per m cubed.
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A huge fishing net out at the front, perhaps? It would have to be very big and very strong to prevent debris punching holes through it. In deep space we would only get 1 proton per m cubed.
This is exactly how the Bussard ramjet is designed to work, except that the scoop is purely magnetic and may be several thousand km across.
http://en.wikipedia.org/wiki/Bussard_ramjet (http://en.wikipedia.org/wiki/Bussard_ramjet)
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It would appear that the limitation is available Energy at least as much as available Mass of propellant.
I tried to make sense of the idea in the link which involved pre-seeding the path of the craft with material to use as propellant. If you had to get the mass there, 'artificially' that would take as much energy as taking it with you - not to mention the wastage through spreading of the beam of material.
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Yeah, the idea of having to 'seed' the path of the craft with fuel in the manner proposed seems inefficient to me too.