From: Brian Atkins (brian@posthuman.com)
Date: Sat May 27 2000 - 11:12:14 MDT
First off can you explain to me what the difference is between Isp and
Fsp, thanks.
Doug Jones wrote:
>
> 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.
Ok I am going to verbally describe what I see in their biz plan, you
tell me if this makes sense. I am considering the fact that they handed
these out indiscriminantly at the conference as making it ok for me
to go on about these details :-)
First in the marketing section they claim to have validated their
orders of magnitude improvement in thrust through something called
The Aerospace Corporation. Ever heard of them?
In the technology section they show a graph of how in different kinds
of explosions (conventional explosions vs. thermite) the reactant
diffusion distance is inversely proportional to reaction velocity.
Farther away reactants have to move to get to each other, the slower
the reaction velocity.
They show a picture of a typical chunk of fuel/oxidizer from regular
fuel- about 200 microns wide (they relate it to a table salt grain),
and then make the claim that whereas in regular fuel this would be
a single pair of materials, in their fuel this would contain tens of
billions of fuel/oxidizer pairs.
Next page is a table showing burn rates and chamber pressures of three
technologies: stinger (.33 inch/sec and 1300 psi), boeing delta II
(.23 inch/sec and 800 psi), their end burning rocket (6.8 inch/sec and
25 psi). Below that is a graph showing Fsp of various rocket types along
with their burn rates. What that seems to show is most conventional stuff
such as Shuttle SRB, Titan IV, sidewinder are all grouped near each other
at around .4 burn rate and .005 specific thrust (psi/psi). Meanwhile
they show their caseless motor up close to 8 inch/sec burn rate and a
specific thrust of 1. Assuming this graph is correct, what are the
consequences? The graph also shows a different version of their system
which is apparently not caseless... this would be used in sidewinders
and such I guess. This one has multiple points along the graph line,
from much slower than the others to about the same point as the caseless.
So I guess they are saying they can control the reaction rates pretty
well.
On the next page they show a cutway pic of a regular solid rocket motor
vs an end burning. Both show nozzles. They point out that their motors
with the highest specific thrust are nozzleless and sometimes caseless,
and they make the claim that a caseless rocket would be capable of SSTO.
But no real details on how that would work. :-)
Most of this seems to be on the web here if you didn't see it:
http://www.nanopropulsion.com/propulsion_technology/index.html
In fact after re-reading that page just now it has all this info I
am talking about, but presented in a better format.
I guess what I would like to know is what impact if any do these Fsp
and other numbers above have? Assuming they got caseless, nozzleless,
end burning rockets to actually work using such Fsps, would that allow
for SSTO?
>
> > 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
Well it seems that their reaction rates alone are fast enough to create
the supersonic velocities without having to do anything else.
> 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...
Interesting, but what if you were using their super-well-mixed nano
aluminium fuel. Could you get an effect like this?
>
> > 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.
Would this really be a problem if you are burning at the end rather than
inside a case with a nozzle? At any rate on the web page I cite above
they claim that their fuel has stable burn rates to over 1600 psi.
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