Bill,
Fusion is an engineering problem (no, not an easy one) with the basic
physics relatively well understood. I mentioned it as an example of
advanced nuclear propulsion because I suspect it's not all that much
farther out of current practical reach than useful affordable fission
propulsion. IOW, I don't think either is anything like close. (No
complaints if I turn out wrong, mind.)
As for zero point energy, I must confess to harboring a suspicion (not
mathematically defensible by me, I hasten to add) that all the gyrations
physics is currently going through to explain distant observations
inconsistent with the local experimentally-determined rules will end up
being the 21st century version of epicycles. It seems more likely to me
that the local rules simply don't precisely hold at great distances in
time & space - what we see is what we get. Now, maybe there's a job
opening for a new Kepler who can spot new underlying meta-rule(s), and
maybe the universe is just stranger than we can imagine. Above my pay
grade! But I avoid investing in business plans predicated on either
epicycles or zero-point energy in the meantime.
RE Fermi, indulge me as I wander even further, into outright theological
speculation. A one-galaxy-one-intelligence rule might well be in force
- perhaps to allow for observing ultra-longterm results from varied
starting conditions undisturbed by interactions with other experiments?
- and would explain observed absence of others in this neighborhood.
(Such a wealth of speculation possible in the absence of evidence!)
Henry
On 7/13/2021 6:05 PM, William Claybaugh wrote:
Henry V:
My SciFi fantasies of the future leap past things as silly as fusion: what is dark energy and how can I use it?
Which question I once asked Lisa Randall who replied that the only answer that fits the Standard Model is that so-called “zero point energy” is somehow leaking into observable reality. To which I commented that we might could want to look at the history of dark energy as the driver of excess expansion of the universe: if that history shows some sort of exponential growth that begins some considerable time after the Big Bang, then we might be looking at the history of technical civilizations—across the observable universe—that have figured out how to use dark energy.
OTOH, Fermi teaches us that the fact that they aren’t here suggests that they don’t exist in this galaxy, yet.
Bill
On Tue, Jul 13, 2021 at 5:10 PM Henry Vanderbilt <hvanderbilt@xxxxxxxxxxxxxx <mailto:hvanderbilt@xxxxxxxxxxxxxx>> wrote:
K. Yeah, I saw somewhere recently that DOD is looking at NTR
again too. Means costs might not be quite up to NASA tradition,
but still not historically a low-cost developer. And over the
life-cycle, they'd still be nukes, with all that implies for
extraordinary expenses.
Overall costs aside, "the same size" motor I take as the same
installed thrust? There's still the issue that what an NTR gains
on the Isp side, it loses a great deal of from the far lower
thrust-to-weight of the engine itself plus the far higher ratio of
propellant storage arrangements mass to propellant mass than with
LOX-whatever.
Now, if someone is looking past NTR to some sort of nuclear
radiative coupling of heat to reaction mass arrangement, that'd be
INTERESTING. Show me a directionally leaky fusion reactor with
provision to inject coolant to increase the mass flow, and I'll
perk right up and pay attention.
Henry
On 7/13/2021 3:22 PM, William Claybaugh wrote:
Henry V.:
NASA is not the only organization funding nuclear rocket
development…and I am being generous to NASA in characterizing
these paper studies as development.
Bill
On Tue, Jul 13, 2021 at 4:18 PM Henry Vanderbilt
<hvanderbilt@xxxxxxxxxxxxxx <mailto:hvanderbilt@xxxxxxxxxxxxxx>>
wrote:
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
