Serious propulsion

Probably such things have been hashed out before, but allow me to
solicit the opinion of the readership on this question that has
come up in some off-line discussions:

Supposing you wanted a reactive propulsion system -- say a
nuclear rocket, electrical thruster, mass driver or other
throw-mass-out-the-back widget that had continuous thrust =
1 kN, ISP = 10,000 sec (= 100 km/sec Ve) and a total run
time = 1 megasecond or greater, preferably getting toward
10 Ms.

If it needs a reactor or other power supply, plumbing,
radiators to get rid of excess heat, etc., ignore that and
concentrate on the thrust-producing part. It's ok to have a
system composed of shorter lifetime parts if that helps
(e.g., banks of 100 N thrusters with individual lifetimes
of 100,000 seconds that get shed as they burn out).

Does current technology or anything that can be reasonably
foreseen in the next 20 years support anything like that? If
so, what might it be?

10 Ms = 116 days: That is definately achievable for ion engines,
though the power source (dutifully ignored) will be impressive.
Remember, you have to take all the energy with you - interstellar space
is remarkably energy-free.

However, your big problem is propellant. For 1 kN @ 10,000 sec Isp,
you are using about 10 g/s. That means you have 100,000 kg of
propellant in orbit (expensive, probably impossible with the best
propellant, Xenon), and you are only pushing it at 1kN - so you aren't
accelerating very fast.

Where do you want to go?

-David

However, your big problem is propellant. For 1 kN @ 10,000 sec Isp,
you are using about 10 g/s. That means you have 100,000 kg of
propellant in orbit (expensive,

100 tonnes in LEO is indeed expensive, but doable with existing
technology, even if we don't revive Saturn 5 or Energiya.


probably impossible with the best propellant, Xenon),

This assumes, I think, an electrical thruster.

Maybe that's the right solution; maybe a mass driver slinging
sand is. That's part of the question.

and you are only pushing it at 1kN - so you aren't
accelerating very fast.

That's ok. Actually, that's where the run-time requirement
came from, though, admittedly, it kind of begs the question/
assumes something about the solution. Something that can
provide one to a few cm/s^2 would be fine.

The Orbiter part of the Space Shuttle is around 100 tonnes.

David Summers wrote:

Where do you want to go?

Inside the solar system outwards of Earth, say low Mars orbit at
a minimum, but also major asteroids, Europa orbit, Titan orbit,
etc. as you think you can do it.

Massive-as-possible unmanned cargo payloads on Hohmannish
trajectories and fast-as-possible passenger vehicles are
the idea.

Well, you have lots of options in that area - solar sails, ion drives,
even direct LOX/LH propulsion will work. Mass slingers have some very
hard problems to overcome - and in the end would almost certainly at
best have the same thrust and Isp as ion drives. The areas you
mention still receive a lot of energy from the sun, so ion drives (or
solar sails if you can afford the risk) are probably the best bet. The
main problem with ion drives for this is that the best propellant,
Xenon, is rather rare - there may not be enough available in the
forseeable future for this mission.

-David

In article .com,
Allen Thomson wrote:
Supposing you wanted a reactive propulsion system -- say a
nuclear rocket, electrical thruster, mass driver or other
throw-mass-out-the-back widget that had continuous thrust =
1 kN, ISP = 10,000 sec (= 100 km/sec Ve) and a total run
time = 1 megasecond or greater...

Mmm... Even at 100% efficiency, this requires 50MW of continuous power,
which will not be easily had. (Jet power is 0.5*thrust*exhaustvelocity.)

That kind of exhaust velocity is almost certainly impractical with a mass
driver or anything similar.

Most electric thrusters are out too. Ion thrusters can get up into that
range easily enough, although they are more usually optimized for lower
Isp; you will need a whole bunch of them to get 1kN. The power source and
thruster hardware will be quite heavy.

There is no way that solid-core nuclear can do 100km/s. Even gas-core
probably tops out around 50km/s. 100km/s or more should be feasible with
systems that don't try to separate fission fuel and propellant -- NSWR or
imploded-pellet fission, for example -- but operating costs will be high
and the exhaust generally rather dirty.

Fusion or antimatter rockets can deliver the performance, but fusion is
beyond what we can build now, and antimatter is beyond what we can fuel
now (the hardware technology is rather easier than fusion [!!], but
large-scale antimatter production would require massive infrastructure).

Does current technology or anything that can be reasonably
foreseen in the next 20 years support anything like that? If
so, what might it be?

Current technology can stack up an array of ion thrusters, but powering
them is problematic. That is much too big for current-technology solar;
the problem is not the solar cells but the structural dynamics of enormous
lightweight solar arrays. 100MW space reactors are not off-the-shelf
items either. Solving either in the next 20 years is conceivable, but not
a small project, and probably will not happen without specific need.

100km/s dirty-fission or antimatter systems are probably buildable without
major new technology, but again, it would be a big project and is unlikely
to happen within 20 years unless someone makes a major effort to do it.
--
"Think outside the box -- the box isn't our friend." | Henry Spencer
-- George Herbert |

On Thu, 3 Feb 2005 16:03:29 GMT, (Henry Spencer)
wrote:

There is no way that solid-core nuclear can do 100km/s. Even gas-core
probably tops out around 50km/s. 100km/s or more should be feasible with
systems that don't try to separate fission fuel and propellant -- NSWR or
imploded-pellet fission, for example -- but operating costs will be high
and the exhaust generally rather dirty.

This is rather old, and I doubt much research has gone into it...
but if it could be developed, it would be an *immense* improvement in
rocket technology...

From an article by Jerry Pournelle...

"Take Boron-11 (11B5). Bombard with protons. The result is a
complex reaction that ends with helium and no nuclear particles. It
could be a direct spacedrive. For those interested, the basic
equation is

11B5 + p = 3(4He2) + 16MeV

and 16 million electron volts gives pretty energetic helium. The
exhaust velocity is better than 10,000 kilometers/second, giving a
theoretical specific impulse of something over a million."

Len Lekx wrote:

= This is rather old, and I doubt much research has
= gone into it... but if it could be developed, it
= would be an *immense* improvement in rocket
= technology...

= From an article by Jerry Pournelle...

= "Take Boron-11 (11B5). Bombard with protons. The
= result is a complex reaction that ends with helium
= and no nuclear particles. It could be a direct
= spacedrive. For those interested, the basic
= equation is

= 11B5 + p = 3(4He2) + 16MeV

= and 16 million electron volts gives pretty
= energetic helium. The exhaust velocity is better
= than 10,000 kilometers/second, giving a
= theoretical specific impulse of something over a
= million."

Just guessing, but I suspect the "complex reaction"
is hiding a requirement that this process take place
at pressures typical only of stellar interiors.

One wonders a bit, if the reaction is so productive,
that it is not used in weapons or tokomak projects
compared to deuterium or tritium, since 11B5 seems to
be the _majority_ isotope, and thus abundant and
presumably cheap:

Boron 5B Ar(B) = 10.811(7)
Isotope Atomic mass Mole fraction in NIST SRM 951 boric acid [2,32]
10B5 10.012 9371(3) u 0.198 27(13)
11B5 11.009 3055(4) u 0.801 73(13)

http://www.iupac.org/publications/pa.../7410x1987.pdf

That would make it, one would guess, a good power
source for "big push" ion propulsion systems.

FWIW

xanthian.

wrote:
Len Lekx wrote:

= "Take Boron-11 (11B5). Bombard with protons. The
= result is a complex reaction that ends with helium
= and no nuclear particles. It could be a direct
= spacedrive. For those interested, the basic
= equation is

= 11B5 + p = 3(4He2) + 16MeV

= and 16 million electron volts gives pretty
= energetic helium. The exhaust velocity is better
= than 10,000 kilometers/second, giving a
= theoretical specific impulse of something over a
= million."

Just guessing, but I suspect the "complex reaction"
is hiding a requirement that this process take place
at pressures typical only of stellar interiors.

One wonders a bit, if the reaction is so productive,
that it is not used in weapons or tokomak projects
compared to deuterium or tritium, since 11B5 seems to
be the _majority_ isotope, and thus abundant and
presumably cheap:


It doesn't work in magnetic fusion reactors because
the bremsstrahlung losses exceed the fusion output
(this is true of anything beyond DT, D3He, or perhaps
DD).

In a bomb, the compressed fuel is enclosed in a capsule
that retards the escape of radiation. It comes into
thermodynamic equilibrium with the photons, which
assume a blackbody spectrum at the temperature of the
fuel. The energy per volume of this radiation goes
as T^4, so the hotter the fuel, the more energy bleeds off into
this radiation bath, and the more the fuel has to be
compressed to compensate. High Z materials are also
harder to compress.

Anyway, deuterium is also pretty cheap, as nuclear weapons
systems go.

BTW, 11B+p is not aneutronic, since there are neutron-producing
side reactions like 4He + 11B -- 14N + n, as well as
p + 11B -- 12C + gamma that produces a very hard photon.

Paul