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View Full Version : What is max V for fusion powered rocket?

2002-Jan-29, 07:22 AM
Does anyone know?

Will an ideal fusion rocket release anything else than He?

Is the possibility of being incinrated in an
accident in a fusion inter continental shuttle ( due to mechanical failure )less than that of crashing in a 767?

Will we pass by Earth's surface at escape V?
Fiordland NZ;

lpetrich
2002-Jan-29, 09:57 AM
Actually, the theoretical maximum velocity is c, but practical propellant/structure ratios will limit this to some multiple of the exhaust velocity.

v = ve*log(m0/m)

where ve is the effective exhaust velocity, m0 is the initial mass, m is the final mass, and the logarithm is the "natural" one. Logarithms convert ratios to differences, meaning that that large values will have slowly-increasing logs. Thus, log(10) ~2.3, log(100) ~ 4.6, and log(1000) ~ 6.9.

That's why rockets are often very much bigger than their payloads.

A rocket engine's exhaust velocity is often given as the "specific impulse", which is (exhaust velocity)/(acceleration of gravity). This is due to an English-system convention for force units in which (pound of force) = (pound of mass)*(acceleration of gravity); this convention is uncommon among metric-system users. The acceleration of gravity used is the average on the Earth's surface, or 9.80 m/s^2.

The next question is to estimate the exhaust velocity.

lpetrich
2002-Jan-29, 10:14 AM
As to fusion-powered intercontinental shuttles, they do not exist, at least not yet. And how dangerous their fusion reactors are will depend on their design.

However, it is reasonable to expect a fusion reactor to have a good degree of intrinsic safety, since it would require some energy input to keep the fusion reactions going (magnetic-confined plasma or laser-imploded pellets). It's just like a car engine; interrupt its electricity supply and its spark plugs will not spark -- and the engine will stop.

Fission nuclear reactors are another story entirely; such a reactor will continue to generate heat even if its nuclear fission is interrupted, as a result of high radioactivity of some of the byproducts of that fission. So a fission reactor will have to be designed carefully so as to keep its core from accidentally overheating and melting.

lpetrich
2002-Jan-29, 10:25 AM
As explained earlier, the practical maximum velocity of a rocket is some small number times its exhaust velocity, which can easily be estimated as sqrt(energy released per unit mass).

A H2-O2 rocket can get up to 4.4 km/s, meaning that such a rocket cannot get a velocity much more than that -- a 1000/1 full/empty mass ratio implies a resulting velocity of 30 km/s.

By contrast, a nuclear-fusion rocket can get an exhaust velocity of something like 15,000 km/s (0.05 c), and one with a 1000/1 mass ratio will reach a velocity of 100,000 km/s (0.3 c).

My earlier-quoted formula is accurate in the Newtonian approximation; here is the Einsteinian version:

v = tanh(ve*log(m0/m))

where tanh is the hyperbolic tangent, and where c = 1 units were used (a common practice in relativistic-physics work).

It is easy to see that nuclear fusion will give a much bigger kick. But a serious question is how feasible it is.

lpetrich
2002-Jan-29, 10:28 AM
Finally, the most suitable fusion reactions are:

H2 + H3 -> He4 + n

H2 + He3 -> He4 + p

However, H3 (tritium) is radioactive, with a half-life of about 7 years, making it unsuitable for long voyages.

If one has H2 (deuterium) alone, one can do these less-energetic-reactions:

H2 + H2 -> H3 + p

H2 + H2 -> He3 + n

So in summary, fusion will produce any of hydrogen, helium, and neutrons.

Simon
2002-Jan-29, 10:13 PM
Erm, if you don't mind me asking...
Is a He3+He3 reaction possible? How would it compare to the others you listed? I recall reading a paper or presentation about various types of fusion that said He3+He3 gave off the most easily-capturable/least radioactive energy, but I don't recall exactly... Lemme see if I can dig it up...

ljbrs
2002-Jan-30, 02:10 AM
Actually, the theoretical maximum velocity is c, ...

At the moment, nothing with rest mass can travel at light speed (c). A problem with anything with mass approaching c would be that its mass would approach infinity and the mass of any fuel used would also approach infinity. It could not reach light speed (c) for this reason. Of course, if the laws of physics are changed, then perhaps... I am not very good at holding my breath...

ljbrs /phpBB/images/smiles/icon_smile.gif /phpBB/images/smiles/icon_biggrin.gif /phpBB/images/smiles/icon_wink.gif /phpBB/images/smiles/icon_wink.gif

2002-Jan-30, 10:45 AM
Further: Unpropelled suface to orbit launch.

Firstly, for an object to travel at escape velocity at the Earth's surface and not 'break up'.

Could be achieved by focusing large laser source at the 'leading edge' of the vehicle,
in effect reducing 'wind resistace' by vaporising a volume of air directly in the vehicles trajectory.

Is this possible?

Rail Gun: 10 -> 20 km vertical in best natural environment. Pref' close to large hydro plant for cheap juice.

1. Vehicle has it's own coil for V+ , barrel
of godzilla has coil for V-.

2. Achive escape V then vaporise the air for smooth ascent to geo syncronous orbit;

For One ton of vehicle what is approx E needed to reach escape v over 10 Km? Assume 9.8 ms2

GrapesOfWrath
2002-Jan-30, 11:38 AM
On 2002-01-30 05:45, RPN wrote:
Could be achieved by focusing large laser source at the 'leading edge' of the vehicle,
in effect reducing 'wind resistace' by vaporising a volume of air directly in the vehicles trajectory.

Is this possible?
I'm not so sure about the practicality of vaporizing air.

Azpod
2002-Jan-30, 06:43 PM
Actually, NASA is looking into rail guns as a theoretical way to launching satellites. Wind resistance is actually not much of a factor b/c the satellites reach 100km in a very short amount of time. However, the objects would need heat shields and a heafty structural system to withstand the acceleration.

The amount of energy E needed to accelerate any object of mass M from rest to a non-relativistic velocity V is E = MV^2 / 2 If your goal is to launch a 100kg object to Earth escape velocity (11.2 km/s) then the energy required (ignoring wind resistance, which requires a level of math I haven't seen in years to calculate) is E = M(100kg) * v^2(11,200m/s * 11,200m/s) / 2 = 6.27 * 10^9 Joules, or about the amount of energy released from 1.5 tons of TNT.

That's still quite a bit, but is much less than is required for a rocket.

Of course, that is assuming a rail gun that is able to transfer 100% of the energy to the satellite. A realistic system would much, much less efficient.

Donnie B.
2002-Jan-30, 10:27 PM
Hmmm... pretty far from astronomy, but...

Assume you had some device that could (from some distance away) superheat a volume of air. Would this work as propulsion?

The superheated air would expand, leaving a region of lower air pressure. If you did this in front of your vehicle, the high-pressure air behind it might push you into the hot, low-pressure region.

But...
That hot air is expanding, and so the side of the "bubble" closest to you is moving toward you, which would tend to counteract the thrust of the cool air behind you. And even if you did move into the bubble, you'd have a hard time repeating the process... the air around you is now almost as hot as the next bubble you form in front of you.

Inefficient, at best. Of course, it doesn't work at all if there's no air. Not useful for a spacecraft.

If you're going to go to all that trouble, why not do the same thing more efficiently by putting wings on your craft and using an airfoil to produce lift? Or superheat the air in a combustion chamber and let it zoom out the back... Presto! Jet plane!

Ben Benoy
2002-Jan-31, 02:05 AM
On 2002-01-30 06:38, GrapesOfWrath wrote:

I'm not so sure about the practicality of vaporizing air.

Yeah, I mean, aren't there treaties limiting weapons of mass destruction?

(I'll put it in for you: Groan...)

Ben

Simon
2002-Jan-31, 11:29 AM
Donnie, tests have been done using lasers to superheat air in a combustion chamber to create thrust for a rocket. I don't think that vaporizing air to make a drop in air pressure above the rocket would be very effective, 'cause no matter how good a job you do you won't get more than 1 atmosphere of pressure working as lift...

Mnemonia
2002-Feb-01, 02:27 PM
On 2002-01-29 17:13, Simon wrote:
Erm, if you don't mind me asking...
Is a He3+He3 reaction possible? How would it compare to the others you listed? I recall reading a paper or presentation about various types of fusion that said He3+He3 gave off the most easily-capturable/least radioactive energy, but I don't recall exactly... Lemme see if I can dig it up...

Maybe it does, but fusing two helium nuclei is FAR more diffuclt than fusing two hydrogen or a single helium and single hydrogen together. So until we master H+H or H+He reactions, I think He+He is out of the question.

jkmccrann
2005-Dec-01, 09:40 AM
Yeah, I mean, aren't there treaties limiting weapons of mass destruction?

(I'll put it in for you: Groan...)

Ben

Indeed there are, but those treaties are flexible, and the mass of air is fairly negligble, unless you live in Mexico City of course.

gwiz
2005-Dec-01, 10:30 AM
The British Interplanetary Society's 1978 Daedalus study of a fusion rocket ended up with a two stage vehicle that reached about 0.12c.

TinFoilHat
2005-Dec-01, 02:48 PM
Erm, if you don't mind me asking...
Is a He3+He3 reaction possible? How would it compare to the others you listed? I recall reading a paper or presentation about various types of fusion that said He3+He3 gave off the most easily-capturable/least radioactive energy, but I don't recall exactly... Lemme see if I can dig it up...

This page gives out the energy levels of various practical fusion reactions: