Gimballing the motor tends to have one goal: steer the rocket by moving the
thrust line relative to the CG. The relatively small, light, and distal nature
of the object being gimballed relative to the rest of the rocket means that
deflecting the engine in one direction causes a relatively small deflection of
the rest of the rocket in the other direction.
The BPS system appears to have two effects.
Since the pivot between the motor and the rocket is so far up and the entire
motor is being moved, not only is the thrust displaced relative to the CG, the
body of the rocket moves non-trivially opposite the movement of the motor
assembly, causing some aerodynamic effects.
Which leads me to an idle daydream about multi-stage solids:
The first (atmospheric) would be fin-stabilized as per standard hobbyist / hpr
design. It pushes the stack out to thin air.
The second stage is connected to the third stage by a movable interstage
coupler (covered by a fairing). Once the stack is out of the atmosphere, the
fairing is separated, and the stack is controlled by swiveling the joint
between the second and third stages, wiggling the mass of the third stage
relative to the thrust from the second stage for steering.
A similar joint would be placed between the third stage and the payload, or any
subsequent stages. Significant despin would likely be necessary to make this
work.
This would allow solids to be used without the inconvenience of a gimballed
nozzle that has to seal and pivot in the middle of the hot section.
On Mar 3, 2018, at 9:16 PM, DH Barr <dhbarr@xxxxxxxxx<mailto:dhbarr@xxxxxxxxx>>
wrote:
For commercial high end hobby solids it's probably more useful to think about
it as wiggling the nose rather than gimballing the motor. I'm not aware of any
write-ups in this regard.
On Mar 3, 2018 8:43 PM, "Charlie Garcia"
<dragonrider.hhcc@xxxxxxxxx<mailto:dragonrider.hhcc@xxxxxxxxx>> wrote:
Robert,
Joe is building low power rockets for a reason. The design doesn't scale and
Joe will tell you the same. It actually has nothing to do with speed though.
You won't hit Mach 3 if you have a motor less than 1/3 of your airframe
diameter.
Any University using solids in a rocket like you propose would have a wide set
of engineering solutions to choose for control. A BPS style gimbal would not be
one of them. Since you're so inclined to call University groups and hobbyists
green, perhaps you'd regale us with tales of your launches.
Charlie Garcia
On Sat, Mar 3, 2018, 8:35 PM Robert Clark
<rgregoryclark@xxxxxxxxx<mailto:rgregoryclark@xxxxxxxxx>> wrote:
What I was wondering is if the methods used by BPS.Space would still work when
the rocket is traveling at supersonic speeds. They are rather green:
https://www.youtube.com/watch?v=u-86tsFJ7lE<https://m.youtube.com/watch?v=u-86tsFJ7lE>
About the gimbled nozzles, university teams trying to produce one would have
the advantage of knowing what was already done and trying to reproduce what has
already worked.
Bob Clark
On Saturday, March 3, 2018, Henry Spencer
<hspencer@xxxxxxxxxxxxx<mailto:hspencer@xxxxxxxxxxxxx>> wrote:
On Sat, 3 Mar 2018, Robert Clark wrote:
The method likely could be modified to gimbal a movable nozzle only as is
done with large solids.
Not easily or cheaply. The hot-gas seals used in the nozzle gimbals for the
big solids were a considerable technological achievement, and took quite a
while to develop, which is why a bunch of other vectoring methods were used in
the early days of big solids.
A big question though is whether the method would
work for rockets at supersonic speeds.
Since it's been used for ICBMs and orbital launchers for many years, there is
little doubt of that.
Henry
