Solid vs. liquid rocket motors

Am I correct that solid rocket motors are always running at full
thrust, because they have no valves to control the fuel flow, so you
can't throttle them up and down?

On Apr 24, 8:29�am, "F/32 Eurydice" wrote:
Am I correct that solid rocket motors are always running at full
thrust, because they have no valves to control the fuel flow, so you
can't throttle them up and down?

Pretty much light and thats it.

There is a design for a cease thrust but its not very practical

On 4/24/2010 4:29 AM, F/32 Eurydice wrote:

Am I correct that solid rocket motors are always running at full
thrust, because they have no valves to control the fuel flow, so you
can't throttle them up and down?

You can pre-throttle them by changing the shape or composition of the
solid fuel inside the motor so it gives you the thrust you want at any
particular time while the motor is firing; some examples he
http://www.nakka-rocketry.net/th_grain.html

Pat

"F/32 Eurydice" wrote in message
...

Am I correct that solid rocket motors are always running at full
thrust, because they have no valves to control the fuel flow, so you
can't throttle them up and down?

As Fred eluded to, picture drawings of a circle and a star - the star has
much more surface area. A star-shaped solid propellant segment (grain) will
have much greater thrust than one shaped like a circle.

On Apr 24, 7:29*am, "F/32 Eurydice" wrote:


Am I correct that solid rocket motors are always running at full
thrust, because they have no valves to control the fuel flow, so you
can't throttle them up and down?

What earlier posters have said is right, but there's a technique
called "generalized energy management" that can give some of the
effects of varying thrust by causing the rocket to waste energy by
steering a wavy trajectory.

On 4/24/2010 4:43 PM, wrote:


What earlier posters have said is right, but there's a technique
called "generalized energy management" that can give some of the
effects of varying thrust by causing the rocket to waste energy by
steering a wavy trajectory.

You can also vent the casing once a stage has reached the velocity you
intend, as is done with solid-fueled ICBMs and SLBMs.
These photos of a Minuteman III plume after sunset probably show that
going on: http://www.freqofnature.com/photos/mmiii/
This one in particular is interesting, as a stage tumbles back towards
Earth: http://www.freqofnature.com/photos/mmiii/P9192066.jpg


Pat

F/32 Eurydice wrote:
Am I correct that solid rocket motors are always running at full
thrust, because they have no valves to control the fuel flow, so you
can't throttle them up and down?

Generally yes, most are full-on all the time - though some vary the
thrust in a preplanned way by varying the shape of the propellent, some
do thrust termination by opening the case, and a very few motors do do
real-time thrust control.

This is done by (usually) using a propellent with a pronounced negative
differential regression characteristic - which means that above some
pressure the propellent burns much more slowly when the pressure in the
case rises, though a positive technique can also be used - and varying
the case pressure by changing nozzle size.


The last is star-wars stuff though, and not really for the amateur -
afaik it's only used in kinetic kill vehicle manouevering systems, which
change effective nozzle size by opening different nozzles.



-- Peter Fairbrother

Peter Fairbrother wrote:
F/32 Eurydice wrote:
Am I correct that solid rocket motors are always running at full
thrust, because they have no valves to control the fuel flow, so you
can't throttle them up and down?

Generally yes, most are full-on all the time - though some vary the
thrust in a preplanned way by varying the shape of the propellent, some
do thrust termination by opening the case, and a very few motors do do
real-time thrust control.

This is done by (usually) using a propellent with a pronounced negative
differential regression characteristic - which means that above some
pressure the propellent burns much more slowly when the pressure in the
case rises, though a positive technique can also be used - and varying
the case pressure by changing nozzle size.


The last is star-wars stuff though, and not really for the amateur -
afaik it's only used in kinetic kill vehicle manouevering systems, which
change effective nozzle size by opening different nozzles.

Oh, there were also some experiments in the 70's or thereby which tried
to blow out a solid by rapidly decreasing chamber pressure, while still
being able to relight the remains.

I think they managed to get it working, just, but not to a practical and
reliable state - at least I don't know of anyone who used the technique
in anger.

-- Peter Fairbrother

On 4/25/2010 7:11 AM, Peter Fairbrother wrote:


The last is star-wars stuff though, and not really for the amateur -
afaik it's only used in kinetic kill vehicle manouevering systems, which
change effective nozzle size by opening different nozzles.

Another interesting technique was used on the Dragon antitank missile
which used large numbers of small (265 pound thrust) solid fuel motors
to modify its trajectory and speed as it flew to its target:
http://www.designation-systems.net/dusrm/m-77.html

Pat

"F/32 Eurydice" wrote in message
...

Am I correct that solid rocket motors are always running at full
thrust, because they have no valves to control the fuel flow, so you
can't throttle them up and down?

You're not correct. Solids for launch vehicles are typically core burning,
which means their fuel has a hollow core, so when they are ignited, they
burn from all of that exposed surface area in the core. They stop burning
when their fuel is exhausted.

You can vary the thrust of a solid over its burn by carefully choosing the
shape and size of the hollow core. For example, a simple cylindrical hollow
core would start out with less surface area at the start of the burn and end
with more surface area at the end of the burn. This means that the thrust
will actually increase as the solid burns, which usually isn't very useful.

It's more useful to start out with more thrust and end with less thrust
because as the solid stage burns it gets lighter and lowering the thrust
over time helps to limit the acceleration of the vehicle. You can do this
by making the hollow core a star shape. The star shape will have more
surface area at the beginning of the burn, but as the stage continues to
burn, the star shape will turn to more of a circular shape with less surface
area. In this way, you can design a solid to have more thrust at ignition
and quite a bit less right before it burns out. This sort of star design
for the hollow core is commonly used in large solid launch vehicle stages.

Jeff
--
"Take heart amid the deepening gloom
that your dog is finally getting enough cheese" - Deteriorata - National
Lampoon