[AR] Re: Circa 1968 video about NERVA nuclear rocket program
- From: Henry Spencer <hspencer@xxxxxxxxxxxxx>
- To: Arocket List <arocket@xxxxxxxxxxxxx>
- Date: Sun, 15 Apr 2018 15:28:03 -0400 (EDT)
On Sun, 15 Apr 2018, John Dom wrote:
...Many science fiction writers (like Clarke in 2001 and before him)
were advised to present their fantasy spaceship reactor mounted on a
long stick pushing forward behind the crew module on top. The stick
spacing would then sufficiently protect without requiring radiation
mass penalty. Any calc on this issue?
As John S. has already posted, it turns out that you need a *lot* of
distance to cut the radiation dose from a gigawatt-range reactor down to
something tolerable. Practical designs put the engine as far away from
the crew as possible *and* include serious shielding mass on the crew
side of the engine (a "shadow shield").
The power required to create thrust T by expelling an exhaust jet at
velocity Ve is 0.5*Ve*T/e, where e is efficiency. In fact, conveniently,
rocket efficiency is often somewhere in the neighborhood of 50%, so Ve*T
is a plausible first guess at the power of a rocket engine. So at an Isp
of 900s, i.e. Ve circa 9000m/s, you need 9kW for every newton of thrust,
and if your goal is 75klb of thrust -- the design spec of the later NERVA
engines -- you're going to need about 3GW of power. (NERVA's Phoebus
engine prototypes were the most powerful nuclear reactors ever built.)
3GW of reactor puts out a lot of neutrons and gamma rays.
The same subject. In the late 50-ies or so, there was a plane based on
the B36 bomber configured to fly with a fission (or was it two?) reactor
aboard for propulsion. It did fly a few times...
That was the NB-36H. Single reactor, and actually not for propulsion --
the regular B-36 engines did all the propelling. This was purely an
experiment, exploring radiation effects on equipment and the operational
issues of nuclear-powered aircraft. Even with a four-ton slab of lead as
a shadow shield, it needed heavy shielding around the cockpit too, because
radiation scattered off the wings and the air bypassed the shadow shield.
(Nuclear-powered spaceships should preferably put the whole vehicle, not
just the crew quarters, behind the shadow shield.)
It flew quite a few times, although not all the flights had the reactor
operating. The general conclusion was that the equipment problems looked
solvable, the required shielding mass was awfully high, and the
operational problems were severe. It contributed considerably to reducing
the enthusiasm for nuclear aircraft.
So I wonder what the Russians worked out for their recently advertised
fission motor cruise missile bomber with enormous range. Of course such
contraptions are not designed to fly with a crew inside.
Unmanned vehicles still have to worry about their electronics, but the
lack of a crew does help.
See also Project Pluto, aka SLAM, the late-50s US effort aimed at an
intercontinental cruise missile that would fly at Mach 3 at treetop
height. Prototypes of its nuclear ramjet were tested successfully in the
early 60s, but the project was canceled before it came anywhere near a
flight test.
Henry
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