# which engine type is more efficient for space travel

Ok, after determining in another thread that I can indeed get here to Alpha-Centuri in apx 3 yrs by accelerating at g for 1/2 the trip and decelerating at g for the second half. For those who missed the earlier thread, we determined that for the traveler the distance between the spaceship and A.C. would shrink as it approached the S.O.L. allowing the 4.5 light-year trip to be accomplished in about 3 yrs.

Here is part 2. I will need a very efficient engine for this, one that uses as little fuel as possible to gain propulsion. I am torn over two types – 1st is the matter / antimatter engine while the second is a high-speed particle accelerator.

As I understand it the matter / antimatter engine works by total conversion to energy in the form of mainly gamma rays.

The particle accelerator would accelerate normal matter to nearly c and shoot it out the back.

The advantage I see to the second type of engine is that the mass would actually increase as the particle approached the S. O. L. so when it was shot out the back it would have more mass to push the spaceship forward. In the M / A-M engine wouldn’t only the rest mass be converted into energy and therefore only push a little?

Not really… Most physicists don’t talk about mass dilation, it’s just a trick to make the relativistic formula for momentum look like the classical one. The more sensible thing to do is to just leave the same mass, and change your definition of momentum.

What you do have when you’re travelling close to C is energy, significantly greater than the “rest energy” (E=mc[sup]2[/sup]) of the particle. However, this energy doesn’t just magically appear because you’re travelling fast; you have to get it from somewhere. Your most efficient propulsion (in the long run) is to throw the fastest particles you can out the back end of your rocket, and the fastest particle is the photon (well, OK, there’s a few that are just as fast, but the photon is the easiest to produce), so you can’t beat the antimatter drive.

[QUOTE]
*Originally posted by Chronos *
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I’m not sure if I can follow the physics involved here, but would a bunch of halogen lights work? They would produce photons, and I imagine that the acceleration would be, well sluggish. Would it work(in theory)?

Yes, in theory. In practice, you want a lot more photons. An antimatter drive would work admirably for that.

If you want a really good book on the subject, check out:

The Physics Of Star Trek … by Lawrence M. Krauss (forward by Stephen Hawking)

It is published by Harper Collins … I am sure you can find it at Amazon if your local book store does not have it.

It has a great section on warp functionality from a classic position. It covers various manners of ship propulsion in space.

The author is a well respected, authored, and awarded physics professor … but the book is well written as well!!! (as anyone who has ready physics books - even lay physics books can tell you , this is nothing short of a miracle!!)

Read, “Flying to Valhalla” by Charles Pellegrino. This one of the few realistic accounts of interstellar travel that I have ever read.

Trust me, read this and you will look upon interstellar travel in an entirely new way.

::in Scotty voice::
Are Ye daft man! The Dilithium crystals will ney take the strain!

Although this might not be considered a rocket strictly speaking, there is something much more efficient. Namely if you could somehow apply that energy to pushing yourself against some (preferably very large) external mass.
Picture a flashlight floating in space. You turn it on and the beam shines out until the batteries run down. The flashlight will have accellerated a very tiny bit, and by expending energy will be VERY slightly less massive. Now imagine if the output of the batteries was applied to an electric motor that pushed against something- a linear accelerator perhaps. By the time the batteries run down, it would gain quite a bit of velocity.

So far more efficient than a photon drive would be a total conversion engine that could somehow push (maybe using gravitational fields?) against the background gravitational field of the galaxy.

How about a FTL drive that’s powered by William Shatner’s ego? Alpha Centauri and back in twenty two days! WOO-HOOO!

What do you mean by “most efficient”? If you have a payload of a certain mass and you want to minimize the launch mass (payload plus fuel plus engine)? If so, the most efficient way is to leave the engine and fuel behind. You can do this by attaching a huge sail on your ship and pushing it with a laser on or near earth. Robert Forward used this in his novel “Rocheworld.”

An interstellar ramjet engine is almost as good. You use magnetic fields to collect interstellar particles, and use that for propulsion. You still have to carry the engine, but you don’t have to carry your fuel. This is an enormous saving - try calculating the energy needed to accelerate a ship at 1G for 3 years, even for an antimatter drive. Don’t forget that you have to accelerate the fuel as well.

Regarding antimatter vs. particle accelerator engine, Chronos is correct (as always). Yes, particles do get more massive as you accelerate it, but you use up energy(=mass) to accelerate it, so there is no gain.

That is the next step - it looks like the mass of the fuel will be several times the mass of the ship.

Thats one I forgot about, free hydrogen in space can be used for a fusion drive system - or for that matter converted to antimatter on board possibly

k2dave:

Converting hydrogen to antimatter? I didn’t know you could do that. And even if you could, wouldn’t you use more energy in the conversion than you could get out of it as fuel afterward?

And, just to make sure I’m clear on the OP, you meant you’d get to Alpha C in three years subjective (shipboard) time, right? Because if Alpha C is 4.5 light years away, you can’t possibly get there in less than 4.5+ years as observed from Earth.

Here’s a link from the Marshall Space Flight Center about antimatter and fusion engines. The stuff sounds like science fiction, but it’s being worked on now, along with other exotic propulsion systems.

Actually, I think that Bussard ramscoops (which would collect interstellar hydrogen) would brake you more than the propulsion you’d get from fusing the hydrogen obtained. If you could somehow free up all of the rest energy, as by converting half of it to antimatter and annihilating, you could make it work, but there’s no known way to do so.

scr4 is correct about light pressure systems where you leave a laser behind to push you-- The biggest problem with rockets is you have to carry the fuel to push your ship, and then more fuel to push your fuel, and so on. When I said an antimatter drive would be most effecient, I was restricting myself to a drive where you carry all of your reaction mass onboard. Pushing off from a big rock gives you a better initial boost, but you can’t sustain it, so eventually the lowly light drive will surpass it.

Hmm… In all the years reading science fiction I never thought about this. But still, wouldn’t you accelerate till your ‘exhaust speed’ matched the speed of the ship? (In the ship’s frame of reference that is) At that point, the momentum you lost by capturing a unit of mass is compensated by shooting the same amount of mass out the back. Though you’re probably right, it’d be difficult to shoot it out at 0.5c or anything close to it.

http://sec353.jpl.nasa.gov/apc/ I had another page that showed a ‘side-by-side’ comparison of about 12 different propulsion types but I lost the bookmark!

> How about a FTL drive that’s powered by William Shatner’s ego?

Or maybe by the rapid fall of Priceline’s stock price?

Exactly, I don’t really care how long an observer has to wait, but if your still around, I’ll send you a postcard.

I have looked into solar sails and lasers and they just don’t have enough omph to keep the ship accelerating at 1g. maybe it would be better as a supplemental fuel along w/ the ramjet. Actually the ramjet should be very effective in the deceleration part of the trip.

Thanx for the links

This would not work, as any force exerted by the decline is offset by the exploding hearts of day-traders.