[AR] Re: Nuclear Hydrogen Thruster
- From: Henry Spencer <hspencer@xxxxxxxxxxxxx>
- To: Arocket List <arocket@xxxxxxxxxxxxx>
- Date: Sat, 9 Jun 2018 18:10:16 -0400 (EDT)
On Sat, 9 Jun 2018, Craig Fink wrote:
New Horizon Probe propulsion design was driven by History...
Agreed in general, but it was also driven by a tight cost cap and the need
to build something well-understood that would work -- and work reliably
for many years -- for a unique mission opportunity that wouldn't recur.
And bear in mind that it *didn't* require particularly high performance.
This combination of requirements and non-requirements definitely favored
straightforward old technology available off the shelf.
Some folks I know *did* try to sell NASA on using piston pumps rather than
pressure feed for the propulsion system, to reduce tank mass. Those were
nearly off-the-shelf hardware, from work done at LLNL a few years earlier.
They got nowhere -- not needed, not wanted, forget it.
Your right, what if NASA had put a resistojet onboard, what whould it
look like? First, the power source for the arc jet...
Note: resistojets and arcjets are different things.
Propellant, Hydrogen by far the best propellant for the job. Hydrogen
is "Shelf Stable" at 3 K temperatures of deep space...
No, actually, 3_K is *not* the temperature of deep space, unless by "deep"
you mean intergalactic. That's the temperature of the cosmic microwave
background, but within the galaxy, and most especially within the inner
solar system, there are other sources of sky background. I forget what
the total adds up to, but I have a vague recollection that it ends up
being 2-3_K higher -- insignificant for most purposes, but not entirely so
for hydrogen storage.
Also, even if 3_K storage were possible, note that hydrogen at 3_K is a
solid. The triple-point temperature of hydrogen is just under 14_K, so
liquid hydrogen cannot exist at temperatures lower than that, regardless
of pressure. Storage as a solid is not impossible but adds serious
complications.
Hydrazine is not and requires temperature control of the entire system
to keep it from freezing.
Spacecraft electronics want to operate at roughly room temperature in any
case, so temperature control is mandatory anyhow. Keeping hydrazine from
freezing sets a temperature floor that's a little higher than you'd like,
but it doesn't make a huge difference.
RTG-powered spacecraft have all the heat they need, and often more. :-)
*Most* of the energy produced by the radioisotope stays as heat and has to
be radiated away; RTGs are quite inefficient. Some designs have just let
it all radiate, but others have exploited some of it to help with
spacecraft temperature control.
Passive "Sun Shield" is all that is required to stop thermal radiation
heating.
Would that it were that simple. Check out the JWST sunshield for what's
needed to keep things cooled down even to a temperature quite a bit higher
than what you're envisioning -- it's an elaborate and tricky design, was
very expensive to develop, and is by no means certain to work.
For one thing, the sunshield itself gets warm and starts to radiate.
This is why the JWST sunshield is multiple layers, fanned out somewhat so
the intermediate ones have some chance to radiate to space. (Radiators at
low temperature radiate very little heat per unit area, so ample area is
needed.)
For another, there's a warm electronics box at most a few meters away, and
both radiative and conductive paths from it contribute significant heat.
(Doubly so for an RTG-powered spacecraft, where there's also a blazing-hot
RTG, radiating furiously, not far away.)
For a third, the JWST design works only because the spacecraft is divided
into a sunward side and a starward side, which rather restricts spacecraft
pointing. (And woe betide you if you ever lose attitude control, even
temporarily.) Especially during encounter, New Horizons had to twist and
turn constantly to point cameras etc. (which is mostly why it was
completely out of touch with Earth throughout near encounter, because it
couldn't point its camera at anything interesting and its main antenna at
Earth simultaneously).
Space is the perfect Dewar so the pressure in the Hydrogen tank can
be kept very low (vapor pressure of Hydrogen at the boiling point of
Helium), resulting in very light weight storage tank...
Remember that you have to launch this thing somehow. It has to handle the
prelaunch and launch environments too, not just the cruise environment.
(Life would really be much simpler if we could design spacecraft only for
the space environment, believe me.)
New Horizion's mission profile requires 2 types of burns, one for mid
course corrections, one for planetary slingshots.
Actually, most of the New Horizons burns are for attitude control. It
also does very occasional small midcourse corrections.
Planetary Slingshot type burn being the short large thrust mode deep
within gravity wells.
Uh, New Horizons never did any such thing. The only gravity well it was
ever deep within was Earth's. It did one passive gravity assist, at
Jupiter, in which it stayed quite far away from the planet to avoid
Jupiter's extremely nasty Van Allen belts; there were no thruster firings
involved.
Low ISP (300 to 600) high thrust, blow-down mode so to speak
using an electric powered centrifugal compressor to bring the Hydrogen
propellant up to pressure (variable 100 to 10,000 psi).
For a small propulsion system, especially if quite high pressure was
wanted for some reason, piston pumps would almost certainly be preferable.
Battery, yes this engine is going to have a "Shelf Stable" battery to
make the trickle charger useful for the pulse arc-jet and power the
pump. Shelf Stable at 3 K temperatures, superconductor storage works
great at Liquid Hydrogen temperatures...
Make up your mind -- is it 3_K, or liquid-hydrogen temperatures (14_K and
up, see above)? Solid hydrogen isn't a good coolant, since it doesn't
flow much. And most superconductors don't work so great at LH2
temperatures -- they really need 4K and below. (The high-temp ceramic
superconductors, which don't need to be anywhere near so cold, are very
difficult to use for power applications and especially magnets, and it's
not because people haven't been working hard on it for decades.)
Battery capacity, may as well make this extremely light weight low
pressure hydrogen tank do double duty by choosing the right material to
build it... Fe, hydrogen annealed Iron tank...
If memory serves, pure iron's crystal structure is body-centered cubic,
meaning that it is almost certainly brittle at cryo temperatures, i.e.
useless as a cryo structural material.
Pulse Arc-Jet pressure tank, sized for one pulse of the Mid-course
correction. Liquid Hydrogen is pumped into the extremely small 10,000 (or
more) psi engine tank. This tank is heated by the nuclear battery, over
time, bringing it up to pressure ready for the Arc-Jet pulse where all the
energy stored in the superconducting loop is routed thru the Arc-Jet sending
the 10,000 psi extremely hot Hydrogen on its way. Not sure exactly what the
ISP of this engine would be, but I suspect it might be rather high.
Don't forget to add up all the dry mass of this system -- big main tank,
small pressure tank, compressor, high-pressure valves, superconducting
storage ring, high-current switching system, arcjet engine, power
conditioning for arcjet engine (often outweighs the thruster itself by
quite a bit), etc. -- and compare it to the mass of extra propellant
needed to just use hydrazine instead. You might be surprised at the
result. As noted in the previous discussion of cold-gas systems, boosting
your Isp to great heights can easily make the total wet mass higher rather
than lower; for low-impulse propulsion systems, *dry mass really matters*.
(Actually dry mass really matters for high-impulse systems too, but that's
a different rant...)
Henry
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