an idea for your ridicule

"Chung Leong" writes (after correcting the sin of
top-posting):

Uzytkownik "Henry Spencer" napisal w wiadomosci
...
In article ,
aSkeptic wrote:
Would preheating the H2, to break it down to H, before it enters the
combustion chamber improve a chemical rocket's "gas milage"/exhast
velocity?

If you could do that... yes, very considerably. You could forget the
oxidizer, and just let the H recombine to H2 -- an *IMMENSELY* energetic
reaction, which would not only make most other chemical rockets obsolete,
but would eliminate all interest in solid-core nuclear-thermal rockets.
Nothing short of gas-core nuclear could compete.

Trouble is, all that energy has to *come* from somewhere. As you might
guess from the above, you need extremely high temperatures to break down
H2 to H. This isn't some little add-on to the propulsion system; it
*becomes* the propulsion system.

Practical interest in such approaches centers on finding a way to
stabilize H, so you can invest all that energy on the ground, and release
it in flight without having to carry the powerplant along. Unfortunately,
nobody has yet found any workable stabilizing technique.

Magnetic confinment? Then again, the energy density of a plasma is probably
pretty low.

You appear to have confused monatomic hydrogen, which still has an electron,
with a plasma, which is a dissociated mess of protons and electrons.
Notwithstanding the ridiculaously low density issue, unless you are
constantly heating the plasma, its electrons and protons will recombine ---
and if you had that kind of heat-source available, you could just heat the
propellant directly!

BTW, it _is_ possible to store "triplet state" monatomic hydrogen in a
"magnetic bottle," but again, the maximum practical densities are absurdly low,
because the collision rate goes up as the square of the density, and each
collision has some probability of flipping the monotomic hydrogens back into
the "singlet state," upon which they are no longer confined, and will
shortly thereafter hit the walls of the confinement chamber and find
something to react with --- such as another singlet monatomic hydrogen
that has recently hit the walls...


-- Gordon D. Pusch

perl -e '$_ = \n"; s/NO\.//; s/SPAM\.//; print;'

In article ,
aSkeptic wrote:
The silly idea I have is using something like NERVA to preheat one or
both components of a chemical bipropellant...

There have been proposals to do something somewhat related: injecting LOX
into the nozzle of a NERVA, as an "afterburner". It hurts Isp, of course,
but increases thrust, and if the LOX is "free" -- e.g., if it comes from
the Moon while the LH2 has to come from Earth -- apparently you can see a
net benefit.

Ok heres annother wild example. Yes this is probably one of the
dumbest/impractical ideas you'll find here.. The combustion
temperature of the SSME is around 4000 K (if memory serves). Could a
greater combustion temperature be achieved by heating both components
(O2/H2) to say 2000 K before they are burned? Or would the burn still
be close to 4000 K as they are now?

The latter. If memory serves, even at SSME pressures, flame temperature
is limited more by dissociation of the reaction products than by energy
available. To put extra energy into the exhaust of a good rocket engine,
you need to use non-thermal means. (Running an electromagnetic thruster
on the exhaust of a NERVA, with the reactor supplying the power, has been
suggested.)
--
MOST launched 30 June; science observations running | Henry Spencer
since Oct; first surprises seen; papers pending. |

In article ,
Zoltan Szakaly wrote:
You can have the H2 dissociate in the reactor core, absorbing energy
and then have them recombine in the nozzle. This phenomena was studied
in the 60s and may have been partially used in rover and kiwi. It
takes a high temperature in the reactor core.

The assessment I saw -- a Bussard paper which may not have been
definitive, I haven't studied nuclear-thermal rockets carefully -- was
that for orthodox solid-core systems, hydrogen dissociation isn't very
useful because of the finite recombination rate and the rapid expansion
of gas in the nozzle.

If you run your engine at quite low pressure, which permits plenty of
dissociation, then after only a bit of expansion, the recombination rate
is too low for much to happen within the nozzle. At higher pressures,
where recombination can occur much farther downstream, there's not much
dissociation. Intermediate values suffer both problems. Only if you can
run substantially hotter -- with a liquid-core reactor or perhaps an
unorthodox solid-core design -- can you achieve both substantial
dissociation and usefully rapid recombination.
--
MOST launched 30 June; science observations running | Henry Spencer
since Oct; first surprises seen; papers pending. |

In article ,
Chung Leong wrote:
Practical interest in such approaches centers on finding a way to
stabilize H, so you can invest all that energy on the ground, and release
it in flight without having to carry the powerplant along. Unfortunately,
nobody has yet found any workable stabilizing technique.

Magnetic confinment? Then again, the energy density of a plasma is probably
pretty low.

Yes, you really need some way to do it in a liquid, preferably a
reasonably pure one (the problem gets easier if the atomic hydrogen is
dissolved in a larger amount of ordinary LH2, but the performance is
greatly reduced). Schemes using strong magnetic fields to help have been
suggested, but nothing has worked out very well.
--
MOST launched 30 June; science observations running | Henry Spencer
since Oct; first surprises seen; papers pending. |

Scott The silly idea I have is using something like NERVA to preheat one or
Scott both components of a chemical bipropellant. Dissociation would be
Scott interesting, but what I'm getting at is.. uh..

What you're getting at is that the input temperature boost would lead to
dissociation in the combustion chamber, which might severely cap the exhaust
velocity increase from the extra energy input.

But if you could get significant recombination to happen in the nozzle,
you could reconvert some of that chemical energy into velocity.

So, uh, how fast is recombination? The usual assumption is that it stops
at the nozzle throat, so I suppose that means you'd need a very long nozzle
and some way to reduce drag.

Is dissociation pressure dependent? Could you run the combustion chamber
at high enough pressures to keep stuff stuck together?

LOX afterburner, schweet! Not exactly what I was envisioning, but
very close. Just like an afterburner on a jet, the point is not to
increase efficiency but to temporarily increase thrust.

Maybe there is some utility to this concept.

Imagine a NTR like NERVA with 3 modes of operation. The first mode,
like the first stage of the Sat-V gets the rocket off the ground. Hot
hydrogen gas expelled from the solid core reactor gets mixed (at some
point, maybe the throat?) to greatly increase the mass flow rate so
that the rocket has the power to get off the ground. When the O2 has
run out and the rocket has some considerable delta-V, ditch the O2
tanks and keep the H2 going until LEO is achieved. When in orbit, use
the same NERVA-like engine to power an electric generator for a
VASIMIR type engine for the rest of the trip. Mars or Moon that is..

SSTM :-) Just like in the movie "Destination Moon"..

(Henry Spencer) wrote in message
aSkeptic wrote:
The silly idea I have is using something like NERVA to preheat one or
both components of a chemical bipropellant...

There have been proposals to do something somewhat related: injecting LOX
into the nozzle of a NERVA, as an "afterburner". It hurts Isp, of course,
but increases thrust, and if the LOX is "free" -- e.g., if it comes from
the Moon while the LH2 has to come from Earth -- apparently you can see a
net benefit.

Ok heres annother wild example. Yes this is probably one of the
dumbest/impractical ideas you'll find here.. The combustion
temperature of the SSME is around 4000 K (if memory serves). Could a
greater combustion temperature be achieved by heating both components
(O2/H2) to say 2000 K before they are burned? Or would the burn still
be close to 4000 K as they are now?

The latter. If memory serves, even at SSME pressures, flame temperature
is limited more by dissociation of the reaction products than by energy
available. To put extra energy into the exhaust of a good rocket engine,
you need to use non-thermal means. (Running an electromagnetic thruster
on the exhaust of a NERVA, with the reactor supplying the power, has been
suggested.)

Henry Spencer wrote:

Only if you can
run substantially hotter -- with a liquid-core reactor or perhaps an
unorthodox solid-core design -- can you achieve both substantial
dissociation and usefully rapid recombination.

For example, if you put an arc heater downstream of the reactor,
but before the nozzle.

Paul

"Henry Spencer" wrote in message
...
In article ,
Only if you can
run substantially hotter -- with a liquid-core reactor or perhaps an
unorthodox solid-core design -- can you achieve both substantial
dissociation and usefully rapid recombination.

In the Dumbo reactor paper cited here several times over the past decade,
Appendix D ("The Super-Dumbo") includes some analysis of the dissociation
potential of the Dumbo design:

"The Dumbo models given in Chap. 9 heat the propellant to a temperature in
the range of 2500-3050 degrees K at operating temperatures in the range of
15-100 bar."

Would that fall in the range you discussed?

Of course, Dumbo was pretty much a design study; only one unit was built,
and that one was tested with a suboptimal nozzle, so this remains largely in
the realm of guesswork.

Jim McCauley

Henry Spencer wrote:

The latter. If memory serves, even at SSME pressures, flame temperature
is limited more by dissociation of the reaction products than by energy
available. To put extra energy into the exhaust of a good rocket engine,
you need to use non-thermal means. (Running an electromagnetic thruster
on the exhaust of a NERVA, with the reactor supplying the power, has been
suggested.)

Would "microwaving" the output of a LOX/LH2 engine work? After all, the exhaust
is water, and you already have fast-spining turbopumps powered by gas turbines
to which you only need to attach more stuff. Or is this a crackpot idea?

--
Sander

+++ Out of cheese error +++

Sander Vesik wrote:

Henry Spencer wrote:

The latter. If memory serves, even at SSME pressures, flame temperature
is limited more by dissociation of the reaction products than by energy
available. To put extra energy into the exhaust of a good rocket engine,
you need to use non-thermal means. (Running an electromagnetic thruster
on the exhaust of a NERVA, with the reactor supplying the power, has been
suggested.)

Would "microwaving" the output of a LOX/LH2 engine work? After all, the exhaust
is water, and you already have fast-spining turbopumps powered by gas turbines
to which you only need to attach more stuff. Or is this a crackpot idea?

Why try to an additional load on turbines that are working like hell
just to deliver propellant?

And remember, the faster the exhaust jet, the better...which means it
won't be in front of any microwave source for any meaningful length of
time...but you now have the additional weight of all that marginally
assisting gear. I don't pretend to be able to do the engineering
analysis, but I'd bet you get a net performance loss....

Now, some of the ideas for O2 injection downstream of the combustion
chamber in an afterburner-like manner, that would be worth something on
some missions.

--

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