"Question re: drive efficiency" Topic

I've been pondering different types of spaceship drives commonly seen in science fiction lately. I often hear people mention when talking about the reasons why FTL travel is impossible that an object's mass increases as it approaches the speed of light and the energy required to accelerate the object therefore also increases such that the speed of light can never be attained by the object.

Interestingly, I've never heard anyone suggest using this phenomenon to increase the efficiency of a drive.

I've read that particles can be accelerated to near the speed of light in particle accelerators. Would it not therefore be theoretically possible to accelerate some particles of otherwise miniscule mass up to some appreciable fraction of the speed of light such that the particle's mass becomes significant enough to be useful as reaction mass in a space drive thereby greatly increasing the remass fraction of your ship's design?

I know this would require some advances in particle accelerator design to make much smaller accelerators to be practical, but that doesn't seem implausible.

I also imagine that it would require a lot of energy to operate the particle accelerator, but it seems to me that remass is a more limited commodity in spaceship design than is energy production.

What do the physicists (both aspiring and actual) amongst us have to say about this?

Ultimately, you are discussing two different things. First, you are postulating FTL travel. We will return to this. But the idea of using a an accellerator to create reaction mass seems at first a good idea. However, the breakthroughs in accellerator design and the physics behind it will most likely yield something more efficient, probably involving gravity manipulation.

However, why is it necessary to accellerate to the speed of light to exceed it? Since C is the speed limit for anything with mass in four dimensional space time, it is necessary to operate outside of "normal space" or warp it around in order to bypass this limit. Neither of which necessarily requires accellerating at all.

Why worry about the mass---another theory is that as you approach the speed of light there will be a time dilation effect, such that the ship and occupant will experience almost no change in time. I know, not practical as far as getting places quicker.

For me the theory of folding "normal space" or making a worm hole seems to be the most practical.

'Ion drives' as currently used in prototype form are particle accelerators (ions being simply charged particles). The point of any rocket motor is to make what comes out of the business end go as fast as possible to maximize its efficiency. But there's no special way of getting more energy out of the system (i.e. the kinetic energy of the reaction mass) than you put in (i.e. through energy released through chemical or nuclear reactions in your 'fuel').

And as always – when you are asking both about the real and the fictional options – check out this link:

link

Yea, the basic idea is how ion drives work already. It IS a good idea if you can make the energy costs work.

-Will

The true mass, or rest mass is an intrinsic property of a body and never changes. The apparent increase of an object's mass approaching the speed of light is relative phenomena and does not represent addition of any actual mass that could be used as reaction mass. It's all relative!

To clarify, I'm not asking about accelerating anything to FTL speeds. Rather, I'm wondering if it is possible to inrease your effective reaction mass by accelerating it to near light speeds.

I never got much farther than Newtonian physics in school. I could do the math for the more advanced physics, but could never wrap my head around the more advanced concepts like time dilation, length contraction or superposition. It's had to advance your knowledge when you just can't fathom how things that seem fixed throughout the entirety of you experience are actually relative properties.

Don't get me wrong, I don't doubt the veracity of the science or that those things can happen -- I just find the results so unbelievably fantastic that I'll never understand how those things happen.

No.

Even if your model worked, the mass of the particle accelerator and the energy requirements are so large that it doesn't work out. Think about how huge the CERN particle accelerator is and what tiny masses they accelerate. As for power, the more power you need, the more fuel you need (or batteries, capacitors, or whatever).

TANSTAAFL

For me the theory of folding "normal space" or making a worm hole seems to be the most practical.

As they say, "Relativity, causality, faster than light travel: pick two."

Sure, I understand that such an idea would require an enormous amount of power. However it seems like you can liberate enormous amounts of power from small amounts of material using something like a fission reactor. So, the amount of reaction mass you can carry to produce delta-v seems like a far more limiting constraint than does power production.

To me, it seems like existing processes for generating power in space are more efficient than for generating thrust in space.

Of course, I'm probably mistaken or just don't have a proper sense of the enormity of the scales involved.

I think the point is that mass isn't increasing in the frame of reference of the matter itself, just as time isn't slowing down.

Doug

I've seen the Atomic Rocket site. It's a fascinating site that makes my head hurt if I spend too much time there. ;)

So, what you seem to be telling me, Doug, is that once again the universe inconveniently doesn't work the way I think it should.

As an object nears C, the mass increases. This is different from quantity, which sounds like what you are trying to alude to. If an atom of matter, and an atom of anti-matter give you a certain amount of energy upon contact, the combination will not change because of speed, if both are traveling in the same direction. Now, if they are moving in opposed directions, than there is a level of kinetic energy that would be involved, but the interaction of matter/anti-matter would result in the same amount of energy.

Stogie, I'm not following you.

In Newtonian physics every action results in an equal and opposite reaction. So, if my rocket throws small pebbles out the back there is a small reaction thrusting the rocket in the opposite direction.

If I throw big boulders out the back then there is a bigger reaction in the opposite direction resulting in more thrust.

I was pondering accelerating my small pebbles to high enough velocities to make them behave more like boulders due to the mass increase an object undergoes as it approaches the speed of light.

Apparently, I don't understand that phenomenon quite right and it doesn't work that way.

Daricles, get a copy of Ken Burnside's "The Hot Equations" link The section on rocket thrust and delta-V will be enlightening.

It also explains some of why your reactor idea is going to have problems.

You have part of it right. The higher the exhaust velocity, the more efficient the drive. The problem is that sometimes you need raw thrust force (say, climbing out of the gravity well, or emergency jinks to avoid incoming fire), which means lower exhaust velocities.

The thing about relativistic effects is that C is a very long lever. ~300 thousand kilometers per second, for easy math (real number is 286 and change). You need to get up to somewhere around 0.6 C before mass doubles, though mass quickly increases past there.

But because of E=Mc^2, you're going to have to dump obscene amounts of energy into the particle accelerator to turn your pebbles into boulders. Getting just 1kg up to 0.6c will take (KE=0.5Mv^2=) 16,200,000,000 megawatts. I think that's more energy than the entire Earth generates in a year.

No, I don't know how long an accelerator that would take. But it would take a really damn long one, on the order of 1000km. Can't use a ring, because the forces won't balance.

I think the issue is what you refer to as reaction mass, Daricles. The engineer in me (Penn State Grad>> Go Buckeyes) reads reaction mass as matter which is either converted to pure energy (matter/anti-matter), burned (C-stoff/T-stoff), or ionized into plasma.

Each of these reactions produces a certain amount of thrust. In the case of the two former methods, an explosion is created and it expands in a spherical shape assuming nothing impedes it. The expansion would diminish some of the force you get from accelerating the RM. Furthermore, accelerating matter towards each other would not be easy. Think of the LHC, and all of the delicate equipment required to fire one atom at a target atom.

In the case of an ion drive, it would be somewhat similar in method, depending on the version.

All that said, and after reading some of the posts, are you asking about accelerating the post-reaction matter and energy towards C?

In rockets the reaction mass and fuel are not necessarily the same thing. Reaction mass is the propellant that provides thrust, which may or may not be the byproduct of a combustion reaction depending upon the manner in which the propellant is accelerated.

In an air-breathing jet engine the main propellant (reaction mass) is the air ingested into the engine. Cold air enters the engine and is compressed before entering a combustion chamber where the air is mixed with fuel and burned so that the hot air and combustion by products rapidly expand and accelerate through a nozzle creating thrust.

There are many different types of rockets, only some of which rely on a chemical reaction to accelerate the reaction mass to generate thrust. For example, you could employ a mass driver as an engine. This would use an electomagnetic rail gun type of driver that accelerates reaction mass consisting of metal slugs. In this case, your fuel might be uranium rods powering a nuclear reactor to generate the electricity to operate the electromagnetic mass driver and your reaction mass would be the metallic slugs you throw to generate thrust.

In short, reaction mass is a general term that refers to whatever you toss out the back of your rocket to generate thrust independent of the manner of accelerating the reaction mass.

I was referring to accelerating the propellant to near the speed of light.

Yup, nuclear rockets (both fission and fusion varieties) have separate 'fuel' and 'reaction mass'. Chemical rockets like the Space Shuttle or a Saturn 5 have unified fuel and remass (LOX+Kerosene, LOX+LH2, and a bit of hydrazine in the reaction control systems).

Remass is what gets thrown overboard to maneuver (could be almost anything, though hydrogen, water, and carbon powder are favorites), fuel is your He3 or U235 or whatever to power the reactor.

Actually, and you may not consider them 'rockets', but Project Orion pulse units are nuclear without a separate reaction mass.

Doug

Hmm…see I have always seen RM as the matter which undergoes a change. Propellant as a fuel.

Interesting that you mention air. The SR-71 and a number of other jets actually need to reduce air intake to keep the engines burning.

My comments still hold some water regarding RM. If the RM, air for example, has to react with a fuel, you will still lose some speed due to the nature of the reaction. Another item to consider is how the RM and fuel would interact. Would it be traveling so fast that the normal reaction would not occur? Possible.