Henry (S):
My concern with any ground testing is that in the early ‘90’s I priced it:
$100 Billion in infrastructure (according to DOE) and a maximum of 4000
lbsf. thrust using a special then existing building that has since been
torn down.
We can apply our favorite correction factor to a government estimate of
anything but inflation is close to double (1.81) since then which leaves me
hard pressed to not be thinking in the tens of billions.
All of which can be avoided by building a conservative NTR of around 800
seconds Isp and testing it at around 1000 km altitude. A couple of hours
thrusting should “qualify” the design (and dispose of the hardware, if
desired) and allow a low cost transition to operations.
Bill
On Fri, Aug 23, 2019 at 11:15 AM Henry Spencer <hspencer@xxxxxxxxxxxxx>
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
