Trying to put hard figures on some bizarre assertions...
Let's imagine a missile propelled by a radioisotopic souce
. Not even a hypersonic scramjet, just an easy subsonic cruise missile with extended range.
I take 20t mass, flying low at 250m/s = Mach 0.73, with L/D=5 as cruise missiles resemble a V1 more than a glider. The small engine must push 40kN and 10MW, for which it needs >40MW heat
, more in a ramjet.
A big nuclear reactor running at full 4.2GW heat output is full of radioisotopes. I take the fission products 99
Mo as an example.https://en.wikipedia.org/wiki/Fission_product_yieldhttps://en.wikipedia.org/wiki/Isotopes_of_molybdenum
If it had equilibrium concetration in the reactor, is separated and used immediately, its 6.05% fission yield and mean 389keV beta energyhttps://www-nds.iaea.org/relnsd/vcharthtml/VChartHTML.htmlhttps://www.nndc.bnl.gov/nudat2/reCenter.jsp?z=42&n=57https://www.nndc.bnl.gov/nudat2/decaysearchdirect.jsp?nuc=99MO&unc=nds
compared with mean 200MeV for uranium fission let this isotope produce at most 500kW heat
. Its daughter 99m
Tc provides a little additional energy as gammas. Keeping half a dozen main fission products lets reach 3MW heat, not >40MW. Some leave more decay time than 99
Mo but are heavier for the same power. The user would need to stop 10 big nuclear reactors, process and bring the waste quickly to the weapon waiting for use.
Impossibility can't be proven in technology, and proofs belong to mathematics only, but under the very broad assumptions above, I comfortably claim that radioisotopic power doesn't propel a missile
. It's not only impractical: the figures don't fit.
But a nuclear reactor could propel that cruise missile
. Reactors can be powerful and light, as Nerva showedhttps://en.wikipedia.org/wiki/NERVA
18t for the whole engine, where the reactor produced 1GW. An engine heating air instead of hydrogen would push more than Nerva's 246kN. The mass of a nuclear reactor doesn't scale like its power, but the 20t missile looks feasible, or a bigger one, and it can fly for long and faster, like Mach 2 or 3.
Hypersonic speed is more doubtful. If ejecting hydrogen from a tank, 800s specific impulse and L/D=3 let fly for <<40min or <<5000km, rather 4min and 500km if bulky hydrogen makes 10% of the start mass. Long flight demands to heat air in a ramjet, despite compression from high speed heats it already. It's conceivable with low efficiency as in a scramjet if the flow remains supersonic hence lukewarm before the heater which is then quite difficult. Or the reactor must heat the air by the emitted neutrons rather than by contact with reactor parts that are cooled below the ramjet's chamber temperature. Though, neutrons carry a minor part of the fission energy, so cooling the reactor takes a bigger air flux than the ramjet has, but with less drag than the ramjet pushes. Badly difficult too.
If such weapons are more dangerous for the shooter than for the targets, it's good news for the targets.