After spending a few days at XPC it's very clear that our system is too complex.
KISS Keep It Simple Stupid.
The temptation is very great to start over from scratch and redesign everything. If I were to start again I would design something that looks a lot like pixel or the Armadillo Quad, a Single engine. The counter to this is that the only thing we really need to finish our vehicle is fast robust reliable valves. Its always tempting to trade the devil you know for the invisible devil lying in the green grass on the other side of the fence. Everyone I've talked to in Alt space has had valve problems of one kind or another.
My core problem is that I need very fast (50 to 100msec) control of the engine thrust to maintain reliable attitude control. Our vehicle is relatively small so that it's overall rotational inertia is low, and the motors are on the very outside of the vehicle this combines to require fast/precise control of the motor thrust.
We can do some freshman physics and figure that out.
Lets assume that the vehicle is 2m diameter sphere.
(This is a slightly pessimistic view of our inertia the vehicle will be more stable than this)
I=2 mr^2/5 =2m/5
We are hovering with 4 motors. The total required lift is mg
So the force for each motor is mg/4
The motors are thrusting on a 1 meter lever arm from the Cg.
Now assume we can very the thrust in 1% steps around nominal.
Assume we increase the thrust in one motor by 1% and decrease the opposite motor by 1%
torque=1 * 2%mg/4
angular acceleration alpha=torque /I =(1*0.02mg/4)/(2m/5)
alpha = 0.0125g (notice m drops out)
or assuming g=9.8m/sec
alpha = .1225 radians/sec ^2
If we want 1 degree control of the vehicle 1 degree = 0.01745 radians.
The rotation is just like linear acceleration angle = (alpha*t^2)/2
So for 1% motor change a 1 degree error occurs in t=.53 seconds.
We would like our control to be about 5x to 10x the speed hence we need a 50msec valve.
How do we get such fast control:
Monster actuators and normal ball valves.
95% Off the shelf.
Lots of hysteresis (3 to 5%), Seals wear out quickly.
Hysteresis really complicates the control system.
Voice coil driven piloted spool valves.
Such things exist for Hydraulic systems.
Control response as fast as 15msec 0 to 100%
Hysteresis 0.1% Basically a perfect valve.
No off the shelf valves will work for LOX
Serious engineering effort to build such a valve in a Lox compatible way.
More complicated Odd combinations.
One can start doing things that add significant complexity:
Adding modulated water injection.
Modulating just the fuel supply while leaving the LOX on a slow actuator.
Using a pile of fast solenoid valves
One fast Solenoid valve as trim with Slow ball valve following.
Potentially the lowest cost option.
Requires a lot of development testing.
I'm sure I'll post more on this in the coming days and weeks.