Bill - Life-cycle cost presumably, not some selective subset thereof?
I'm guessing you have some possibility in mind that might make that
cost-equivalency less ludicrously unlikely than currently. Whatcha
thinking here?
Henry
On 7/13/2021 3:06 PM, William Claybaugh wrote:
Henry V.:
What if the nuclear rocket costs no more than a chemical motor of the same size? It is always are assumptions that get us….
Bill
On Tue, Jul 13, 2021 at 2:11 PM Henry Vanderbilt <hvanderbilt@xxxxxxxxxxxxxx <mailto:hvanderbilt@xxxxxxxxxxxxxx>> wrote:
What he said. Conventional nuclear-thermal is a dead end. You
can match the transit-time performance at lower overall cost by
simply adding more conventional rocket propellant.
And note that there may well be little or no NTR advantage even if
you ignore the (LARGE) additional development, hardware, and ops
costs that show up the instant you say "nuclear". Not all
propellant costs are equal. Conventional NTR requires pure LH2
propellant or there's no Isp advantage at all. Putting, say,
twice the total propellant mass of 6:1 LOX/LH2 into LEO when &
where needed may not cost much more than the NTR's required mass
of LH2 - might possibly cost less - because LOX is hugely easier
to transport (far higher-density thus more compact) and store (far
"warmer" thus far less relative mass of insulation and/or
refrigeration needed.)
As my estimable colleague alludes, solid-core NTR due to materials
temperature limits simply can't reach high enough Isp to justify
the poor thrust-to-weight and TERRIBLE costs. Better to spend
available nuke R&D funding on more advanced technologies that
don't require solid-material heat exchangers, thus have some
chance of delivering enough real performance advantage over
chemical rockets to be worth the very high nuclear overheads.
Henry's evil twin
On 7/13/2021 12:26 PM, Henry Spencer wrote:
On Tue, 13 Jul 2021, roxanna Mason wrote:
Perhaps the realities of long duration human spaceflight have
finally sunk in,i.e. zero g, radiation, one way
travel/isolation,etc.
Don't get too excited. These are small, short design studies,
which won't actually build or test *anything*, with no apparent
plans for followup. The purpose is "to establish the basis for
future technology design and development efforts".
It doesn't qualify as serious until the plans include testing
hardware.
All this said, nuclear is still not a cake walk with perhaps at
best only half the travel time...
And that, we could do without nuclear -- just add more fuel.
Fuel is cheap, even in LEO.
IMLEO (Initial Mass in LEO) is a dangerously misleading surrogate
for cost, because it assumes that a kilogram of LOX in orbit
costs about as much as a kilogram of nuclear engine in orbit.
You could launch many kilotons of chemical fuel into LEO for what
the first nuclear engine will cost. Especially if you set up a
fuel depot and announce that for the next fifteen years, you'll
buy X tons of fuel per year, priced on a sliding scale (scaling
down as volume builds), from the first people who deliver it to
the depot -- no exclusive contracts, no development subsidies,
payment on delivery only.
It's far from clear that plain old NERVA-style solid-core nuclear
can actually improve things enough to be worth the up-front
investment; at most it's a pathfinder for something better. Any
serious plan for nuclear propulsion should include starting real
work on a more aggressive Phase Two at the same time (not just
penciling it in for the vague and misty future).
Holding of breath is not recommended. :-(
Henry
