At Livermore in the 80's George Chapline came up with the fission fragment
drive - which exhausts the fission products directly instead of heating up
a working fluid.
High thrust (especially in afterburner mode), relativistic exhaust
velocities and much more efficient than NTR or nuclear electric.
Not quite an Ortega Torch, but close.
Chapline's original 1989 paper: https://www.osti.gov/servlets/purl/6868318
Grassmere (Clark, Sheldon) 2005 paper:
http://www.rbsp.info/rbs/PDF/aiaa05.pdf
NIAC Study: https://www.nasa.gov/pdf/718391main_Werka_2011_PhI_FFRE.pdf ;(with
discussion of the afterburner and the thrust/Isp trade)
Chapline is still alive and working at the Lab. I've been intending to go
see him, but haven't gotten around to it - need to make it a priority
Best,
Bill
Bill Bruner, Col., USAF (ret.)
New Frontier Aerospace
bill@xxxxxxxxxx
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On Mon, Aug 26, 2019 at 7:58 PM John Schilling <
john.schilling@xxxxxxxxxxxxxx> wrote:
Lower efficiency is to some extent inherent in the technology. Unless you
have a way to use electricity to accelerate neutral atoms or molecules, you
have to ionize everything first and that's a fixed energy cost
(theoretically ~10 eV/ion, in practice ~100 eV) before you get any
acceleration. The more you accelerate, the less energy you're wasting.
So, maybe 50% at 1500 seconds, 65% at 3000 seconds, and possibly up to 90%
when you are dominated by losses in the power-conversion and acceleration
mechanisms. And we're looking for less wasteful ways to do the ionization.
Xenon is definitely preferred as a propellant; Krypton will also work but
cost you 5-10% in efficiency because of the higher ionization energy per
unit mass (roughly the same per ion, but lighter ions. Light elements like
hydrogen are problematic because it takes a *lot* of hydrogen ions to make
a kilogram. Anything with oxygen is problematic because it is still too
light and because you basically can't ionize oxygen without turning it into
atomic oxygen in the process, and that stuff is about as corrosive as Alien
blood. Also, if your way of making ions involves a cathode, that's going
to electrostatically beckon "Here, oxygen. Here, oxygen oxygen oxygen!
Come get your free electrons!", and then dissolve into goo.
Heavy metal vapors will work quite well if you can find a metal with a
high vapor pressure at reasonable temperatures and can avoid having it
plate out over your feed system, insulators, etc. I've seen preliminary
work done with bismuth, zinc, and iodine. Mercury would be ideal if it
weren't for the toxicity problems. But if you're in an environment where
everybody has to wear a spacesuit anyway, and if I recall correctly the
LCROSS measurements suggested there was quite a bit of mercury in the cold
trap at the Lunar south pole.
I'm not sure we need to get Mars travel times down to weeks, but your math
looks right for what it would take to do that.
John Schilling
john.schilling@xxxxxxxxxxxxxx
(661) 718-0955
On 8/25/2019 10:48 AM, Henry Vanderbilt wrote:
65% electricity-to-thrust efficiency? Not bad at all. I assumed
something like 70% in coming up with my ~1 kW/kg power supply requirement
for a usefully faster-than-chemrocks space transport. (More on what went
into that in a moment.)
Is the lower efficiency at lower Isp inherent in the technology, or a
matter of the chosen design optimization point being ~3000 seconds Isp?
And, AIUI, Hall-effect thrusters tend to use inert gases for propellant.
What are the prospects, if any, for hitting similar efficiencies with
cheaper and/or more common propellants - say, for example, water? Or
liquid oxygen? With, perhaps, electric thruster technologies other than
Hall-effect?
I ask because for a number of reasons, ~2000 seconds Isp, AKA ~20 km/s
exhaust velocity, seems to me about right for a high-energy space transport
intended to supplant chemical rockets for inner solar system runs. And a
network of such transports would one hopes expand traffic to the point
where cheap common (and dense and easily storable) propellant would be
important.
Per various Jim Wertz papers (and grossly oversimplifying) Earth-Mars
transfers start getting down to weeks rather than months for one-way delta
V's on the order of 20 to 30 km/s. (Earth-nearby asteroid and Earth-Venus
transits require somewhat less.) A basic ship Mass Ratio of around three,
AKA two parts propellant to one part everything else, is reasonable (and
reasonably expandable) designwise, and yields mission delta V of ~1.1 times
exhaust velocity, or about 22 km/s baseline delta V. Double the propellant
via auxiliary tanks for MR=5 and the mission delta V increases to ~32
km/s. Meanwhile, exhaust velocity roughly matching the lower end of the
expected range of mission velocities seems to me the way to shade things to
minimize the acceleration-time issue, AKA make it easier to get past
"mousefart thrust" requiring months of acceleration to reach
transit-in-weeks velocities.
The very generic rough system breakdown I come up with: For
(semi-arbitrary) 10 MW ship electrical power available, at ~70%
electric-to-thrust conversion efficiency and 20 km/s exhaust velocity, you
get ~700 newtons of thrust. I assume the ability to accelerate at 1 km/s
per day at basic full-propellant-no-auxiliary-tanks mass ratio of 3 as
being well into the usefully "zoomy" region - at 86,400 seconds in a day,
this means you need to accelerate at .011574 m/s^2, which means you can
accelerate 86.4 kg at that rate per newton of thrust, which means your 10
MW power supply will propel a 60 (metric) tone ship - 40 tons of
propellant, 20 tons of everything else.
1 kW/kg from your power supply then makes it 50% of "everything else", IE
of the entire ship dry mass, which is definitely squeezing the rest of
"everything else." But then you can back off a fair amount from that
initial full-tanks acceleration of 1 km/s per day and still reach 10-15
km/s delta V in days not weeks. (I need to run those numbers again at some
point, but not this morning - I have a real-world ditch to dig.) But ~1
kW/kg does give a reasonable ballpark idea of what's needed in the way of
watts per unit mass from a power supply for a usefully faster space
transport.
Henry
On 8/24/2019 5:47 PM, John Schilling wrote:
Roughly 3000 seconds Isp at 65% wall-plug efficiency. If you want higher
Isp that's easy and the efficiency will go up a bit; if you prefer thrust
to power you can go down to ~1500 seconds and still keep the efficiency
above 50%.
We'd like to do better on the efficiency front, to be sure, but that's
probably going to be a long slog and a few percent at a time.
John Schilling
On 8/24/2019 9:47 AM, Henry Vanderbilt wrote:
What Isp does this setup operate at? With what efficiency turning watts
into thrust?
RE nuke-electric versus solar-electric, yes, solar-electric current SOTA
is much closer to useful fast-transit power/mass efficiency levels than
nukes. With the added assumption of remote lasers to bring solar panels up
to near their typical practical upper limit of roughly 2x Earth-solar input
levels whether they're here or out near Mars, from what you say, roughly
one order of magnitude away in the system you describe. I tend to assume
some additional economies from larger scale installations plus a certain
amount of subsytem-marketing-brags cherrypicking and assumed engineering
tradeoffs, in saying that current optical-electric is about a half OofM
short of what's needed.
Conventional fission nukes I suspect are a dead end (for transport apps at
least) that will never get near the ~1 kW/kg needed for general-purpose
fast inner-system transports. The energy just shows up in way too
inconvenient a form, requiring lots of mass for shielding/conversion to
thermal energy, then conversion from thermal to electrical energy. Some
unconventional nuke technology that does NOT produce lots of messy neutrons
might get there - positron-electron? He3 fusion? The ability to convert
the energy directly to high Isp thrust without an intermediate electrical
step would also help. But, as best I know out here in the unclassified
world, those are a LONG way off.
Solar/beam-electric strikes me as the most promising lowest-risk approach
to while-we're-still-alive fast inner system transportation. I mention
nukes for fairness. And you never know, someone may next year come out
with the He3 1 MW Home AC Power Fusor, purchase conversion kit for thrust
applications separately, warranty void if not installed by a
factory-trained technician. It'd be nice! But I don't count on it.
And yes, in this business, always check the math. It eliminates huge
amounts of nonsense, and occasionally leads you someplace unexpectedly
useful.
Henry
On 8/23/2019 11:37 AM, John Schilling wrote:
The NASA "HERMes
<https://urldefense.proofpoint.com/v2/url?u=https-3A__en.wikipedia.org_wiki_Advanced-5FElectric-5FPropulsion-5FSystem&d=DwMDaQ&c=clK7kQUTWtAVEOVIgvi0NU5BOUHhpN0H8p7CSfnc_gI&r=rPTfWqtJdrL0Ber-yr0E_hSjRXuvJH6ZmQx03u8-2as&m=lgmr_Ur1YnNMRUlLyXe3ZjZyBN8dvIL9I1VaXb7uGUo&s=IZRXE6wZhHUzf8OmvyiYCr1MBZnHLI8F9tfhz7oz3Gw&e=>"
Hall thruster is targeting 100 kg mass for a complete 12.5 kW system
including power processing unit but power supply. Actual input power is
13.3 kW, which using the quoted performance of the Northrop-Grumman "
Ultraflex
<https://urldefense.proofpoint.com/v2/url?u=https-3A__www.northropgrumman.com_Capabilities_SolarArrays_Documents_UltraFlex-5FFactsheet.pdf&d=DwMDaQ&c=clK7kQUTWtAVEOVIgvi0NU5BOUHhpN0H8p7CSfnc_gI&r=rPTfWqtJdrL0Ber-yr0E_hSjRXuvJH6ZmQx03u8-2as&m=lgmr_Ur1YnNMRUlLyXe3ZjZyBN8dvIL9I1VaXb7uGUo&s=qJWjaZ9lOLcDhYOJzQV5E5-fB2giSoxV1sWLzcRhazs&e=>"
solar array would be an additional ~90 kg BOL. The HERMes numbers are
conservative, IMO, and Ultraflex has flight heritage. Accounting for solar
array degradation and system integration probably puts you at ~250 kg for a
state-of-the-art 12.5 kW solar-electric propulsion system, or 20 kg/KW.
If you need full power at Mars orbit, that would go to ~30 kg/kW.
There's room for improvement, of course, and plausible paths to same. It
isn't clear that nuclear-electric power supplies will outperform
solar-electric until you get somewhere past Mars; nuclear power plants are
heavy beasts, and people handwaving that the Magic N-Word means as much
energy as we need in as small a package as we need to fulfill the dream du
jour are probably selling snake oil. Or maybe aren't selling anything and
just haven't done the math. Insist on checking the math.
John Schilling
john.schilling@xxxxxxxxxxxxxx
(661) 718-0955
On 8/23/2019 10:26 AM, Henry Vanderbilt wrote:
A tangentially related thought: In looking over the years at the need for
~2000s Isp, ~ 1km/s/day delta V inner-system space transport to supplant
chemical rockets, I came up with a rule-of-thumb: That to do this with
nuclear-electric ships, the nuclear electrical generation part of such a
system needs sustained (weeks) power output on the rough order of one
kilowatt per kilogram of generating plant. Else we're back in the
mousefart thrust trap.
My impression is that the current nuclear-electric SOTA might approach 100
kg per kw sustained output. Anyone have better numbers than that, or
thoughts on alternative nuclear approaches?
Henry (V)
On 8/23/2019 10:14 AM, Henry Spencer wrote:
On Fri, 23 Aug 2019, William Claybaugh wrote:
The de facto Western ban on testing in the atmosphere means testing will
have to be off-planet.
And for something like NSWR, I'm not sure I can reasonably object to that.
For a solid-core design that can be expected to pretty much contain all
fission products, I could see testing on the ground using exhaust
scrubbers, *perhaps* moving to open-air testing after containment was well
verified. But for advanced systems like NSWR, where there's inherently a
lot of radioactive garbage in the exhaust and so even one unscrubbed run
would be bad, I'd worry about scrubber effectiveness and scrubber failure
modes.
Moreover, just a little bit off-planet isn't enough -- you want to test
(and operate) outside Earth's magnetosphere.
(Gosh, could this be something that the Lunar Gateway is actually good
for? :-) )
This favors "known" designs that can be relied upon to produce a usable
vehicle on the first try.
They may be favored, but if they can't meet the mission specs -- and the
"known" designs realistically have at best modest advantages over chemical
rockets -- then they're not even in the running. The objective is to be
useful, not just usable. A modest gain over chemical isn't worth the very
high development costs and nasty political hassles.
If reuse is a part of the plan, then some sort of refueling facility is
implied, all in a nuclear safe orbit (1000 km, generally). Getting a new
payload on a hot vehicle is left as a problem for the reader.
What probably makes the most sense is a concept that was seen in the late
60s: reusable nuclear tugs. They're not part of your mission-specific
vehicle; they dock to your ready-to-go vehicle, boost it from LEO to (say)
Mars trajectory, and then turn around and come back (immediately, using
brute-force high-delta-V retrofire, not an economy orbit) to return to
their base. So your payload never spends much time in their vicinity.
If you're going to have to fiddle around for a while during/after docking
but before departure, you can fit substantial shielding around the tug
docking interface, and have it moved aside five minutes before departure.
(Launching it will cost something, but it doesn't *go* anywhere, so that
cost only has to be paid once and then it's available for all future
departures. Think of it as the equivalent of airport de-icing trucks.)
I also observe that the whole notion of "nuclear safe" orbits implicitly
assumes that a failed nuclear-propelled vehicle would be abandoned and left
to come down at random. This makes no sense -- quite apart from the side
effects of doing so, it's too expensive to just abandon.
(Well, except perhaps in a spectacular one-shot program with no
continuation or follow-on, which is expected to crash and burn once it
meets its primary objective, and therefore isn't interested in building
support infrastructure -- e.g. tow trucks for disabled vehicles -- for
future use... and that doesn't make much sense either. Been there, done
that, know better now.)
Henry
