Lightweight solid rocket motors
A rocket motor burns propellants under high pressure and expells the
burned gases through a nozzle. The ideal rocket motor today uses
liquid propellants. This allows to store the propellants in lightweight
reservoirs, under low pressure. The rocket motor uses pumps to inject
the propellants under high pressure. The pumps and the motor form a
little and lightweight device. The propellants are commonly made of two
liquids: an oxidizer and a reductor. The best common oxidizer
propellant is liquid oxygen. The best reductor is hydrogen, though
other reductors like petrol are excellent. Inside the rocket motor the
oxidizer and the reductor are mixed and ignited.
A solid rocket motor trades in two characteristics mentioned above.
First of all, it is a giant rocket motor: it contains all the
propellant it will burn and must keep the whole under high pressure.
Hence the weight of that motor/reservoir is much more important.
Second, a solid rocket motor contains no liquid oxygen. Instead it uses
solid powders or plastics that can release oxygen. This is a
far less efficient way to use oxygen. On the other hand, a solid rocket
motor is much simpler and reliable than a liquid rocket motor. It
contains no moving parts. It doesn't need to mix the propellants since
the propellants are factory-mixed in the solid propellant block. One
more advantage of solid rocket motors is
they can burn up quickly, releasing all their push force in a short
time. This is useful when trying to escape Earth gravitation.
A common way to build space-bound rockets is to use a big first stage
using low-cost and heavy techniques. The last stage on the contrary
uses expensive high technology and is lightweight and very efficient.
Best example is the Space Shuttle. It takes off relying mostly on its
two giant solid rocket motors as a first stage, then ends the flight
relying on its liquid hydrogen-oxygen motors.
So, a solid rocket motor is made of a block of solid propellant
surrounded by a heavy reservoir.
Maybe a way to make the motor/reservoir more lightweight would be to
the solid propellant itself to hold the pressure. The reservoir would
be a light steel skin, like the reservoirs of the Atlas rocket. At the
beginning of the flight the center part of the block burns at high
speed and creates a high pressure. That pressure is contained by the
outer layers of the block. Steadily, as the center of the block burns
away, the block burns slower and creates less pressure. The outer
of the block would burn slowly and create no more pressure than the
steel reservoir can hold.
The fact the pressure decreases is compatible with a travel towards
Space. First, a high motor inside pressure is especially needed when
the outside pressure is high, that is at sea level. When the rocket
travels towards space and the outside pressure decreases, a lower motor
pressure can be used. Second, the optimal nozzle end diameter depends
on the outside pressure. The lower the pressure, the wider the optimal
nozzle output diameter. The fact the burn rate and pressure decreases
will decrease the amount of gases expelled. Hence a same nozzle output
diameter can be optimal at all altitudes.
When the center chimney of a solid propellant block burns, its surface
increases steadily, hence the amount of gases produced. A common trick
to circumvent this is to give a star section to the chimney. This makes
the chimney has a wide surface from start on. For the current purpose
this may not be enough, as the chimney must produce much more gasses at
the start than at the end. Making the outer layers burn slower is quite
easy. On the other side, very quick burning speed at the inside may be
difficult. More precisely, it may be difficult to get a reliable high
speed burn speed together with a high energy yield. A simply solution
is to carve chimneys inside the block. Another solution is to
put fast burning fuzes through slow burning high yield solid propellant.
Liquid oxygen can be used inside a solid rocket. That is a hybrid
rocket motor. The solid propellant is depleted in oxygen-rich
compounds. Liquid oxygen is contained in a reservoir above and injected
inside the burning motor. In this case the oxygen flow should decrease
with the time. I don't know if it is better to inject the
liquid oxygen at the top of the motor, to spray it near the nozzle
convergent or to spray it all along the chimney. An interesting point
with this solution is the oxygen-depleted solid propellant will burn at
lower temperature, hence sparing the lightweight steel envelope at the
end of the flight. Also, the oxygen-depleted propellant is less
dangerous to store. And an explosion of the rocket during take off
would be less destructive since the solid propellant will yield few
What about the weight of the liquid oxygen reservoir? It can be
lightweight and under low pressure, with a pump to inject it under high
pressure inside the motor. This would be a little sorry since it
compromises the advantages of solid rocket motors. Another way is to
use a heavy reservoir under high pressure. This is interesting since
the volume of liquid oxygen needed is not very high. Anyway, are there
ways to improve on this? A first way can be to use two reservoirs. The
main reservoir would be heavy, yet it would be dropped at
high altitude and the motor would switch to the second oxygen
reservoir, at lower pressure and more lightweight. A second way is to
place the oxygen reservoir inside the motor. Either in the center of
the chimney or above the chimney and surrounded by slow burning solid
propellant that is lit at the end of the flight. This approach
seems risky to me but it may be sound anyway. A third approach
could be to put the liquid oxygen in chimneys or little burnable
resrevoirs inside the solid propellant. This is straightforward. A
fourth solution is to make the liquid oxygen reservoir be heavy by
being made of solid propellant surrounded by a lightweight sheet of
steel, just like the motor
itself. Once the motor and the oxygen are burned up, the reservoir
would become a second stage solid rocket motor, possibly with a second
little liquid oxygen reservoir above.
A problem with liquid propellants rocket motors is the propellants must
be mixed and lit very quickly inside the motor and yield a stable
flame. That's one reason why such motors need a high internal pressure
and are very expensive. Inside the hybrid motor mentioned above I
expect things to be simpler. The oxygen is gradualy mixed with partly
burned hot gases. It should ignite immediately, with no instabilities.
Maybe this can be used to burn hydrogen too. The rocket motor can
transport a lightweight reservoir of hydrogen and spray that hydrogen
during the end of the motor flight to gain some specific impulse.
March 31 2005