From: Doug Jones (random@qnet.com)
Date: Fri May 26 2000 - 23:23:23 MDT
Brian Atkins wrote:
>
> Well I am not a rocket scientist :0 But I can scan in a chart from their
> business plan (or perhaps it is on their old site at nanopropulsion.com)
> showing how their system is waaay better than the shuttle or other methods.
> The idea is that the nanoaluminum allows for a much denser mix with the
> other materials in the fuel, and therefore much faster reaction rates
> and therefore much much higher thrust than what you get from conventional
> mixing techniques.
I read through their site, and yes, it does appear that they can make an
end-burning solid motor work well. Their "specific thrust" figure of
merit merely shows that the end burning is possible- which I grant could
improve solid motor loading density by perhaps 35 to 40 percent, the
amount of empty volume normally cast into solid motors.
However, the simple fact remains that the sea level Isp of
Al/perchlorate/HTPB motors is at best about 260 seconds, while even low
tech pressure fed LOX/kerosene is about 280. The vacuum performance is
even more strongly in favor of the liquids.
> Also they go on about how this would also allow for
> nozzleless rockets that could even consume their casing on the way up.
> There would be no "hole" in the center of the fuel, it would just burn
> at the end. What do you think?
Here they are flat-out wrong. Without a de laval nozzle to efficiently
expand the hot gases to supersonic velocity and collimate the exhaust
into a unidirectional flow as opposed to a hemisphere, their Isp will
drop in half. The only way to get around this would be to use a
micro-orion method as proposed for laser launchers- detonating a thin
surface on a large area produces very high pressures so that you *can*
get good expansion off a flat plate. (Basically the expansion from
~50,000 psi to 1 psi takes only a few inches, so that the gases expand
in a 1-dimensional manner except near the edge.) Alas, the nozzleless
method only works with detonations, so either a gigawatt laser or a
block of TNT are needed. Deflagrating propellants need not apply...
> They seemed to think their biggest problem is that the missle tracking
> systems are unable to handle the higher speeds.
No, an end-burning solid rocket motor would only have slightly increased
Isp due to more efficient combustion and a small increase in enthalpy,
and a longer burn time due to the larger propellant charge. Software
changes would take that incremental change in stride. Basically, it's a
neat hack, but with limited potential.
The one big bad ugly thing is that any sort of disbond or crack in the
grain would be even more dangerous than for a center-burning motor.
Titan SRMs have a bad habit of blowing up when the buring area increases
by a few percent from a crack- imagine how critical propellant integrity
would be at 100x the burning rate. A small void that would make a
slower burn rate motor merely hiccup would double the burn rate of the
end-burner... instant pretty fireworks.
-- Doug Jones Rocket Plumber, XCOR Aerospace http://www.xcor-aerospace.com
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