Lateral CG shifting for steering?

[Adrian A]

Another idea for steering out tilt on a multi-stage rocket is to have a servo move a mass laterally within the airframe to slightly shift the rocket CG off of the aerodynamic axis. During the burn you would want to use relatively low gains and it would steer the rocket in the direction of the mass shift, and then during coast, when drag is decelerating the rocket, the rocket would move away from the direction of the mass shift. I haven't done the math yet to determine how much mass you'd need to move in order to make a measurable difference, but I'm intrigued by the idea because it would be nearly drag-free and somewhat simpler implementation. One downside is that it could only be used to correct tilt, but would have no effect on roll.

 

[G_T]

Knocks a hole in your propellant mass fraction, and the lateral shift you can achieve is pretty small. I'm interested in perhaps a CO2 cylinder (they can be had up to much larger than most people realize, or even a small composite tank can suffice - say, from a match target air pistol), some micro solinoid valves... RCS! I started looking but haven't found where to purchase commercial micro or mini solinoid valves. They do exist as commercial products, specifically for RCS usage. But perhaps paintball ones would work.

 

[rocket_troy]

Knocks a hole in your propellant mass fraction, and the lateral shift you can achieve is pretty small. I'm interested in perhaps a CO2 cylinder (they can be had up to much larger than most people realize, or even a small composite tank can suffice - say, from a match target air pistol), some micro solinoid valves... RCS! I started looking but haven't found where to purchase commercial micro or mini solinoid valves. They do exist as commercial products, specifically for RCS usage. But perhaps paintball ones would work.

The problem there is that solenoid valves are heavy, solenoids are heavy. Yes, they are mighty quick acting which is what you want for a control system but I think a reasonable enough response can be had with custom designed butterfly style valves made from mini geared motors overdriven with higher voltage. They don't have to totally seal when closed allowing for non-contact (or light contact) metal-metal seating. Use a master valve to provide flow to the manifold that does seal which can be slow acting allowing for mass reductions.
It's one of those fast, light, 100% sealing - pick any 2.

TP

 

[G_T]

Just an example. There are several of these sorts of microsolinoids around that are designed, and used, for small RCS systems.

Gerald

 

[rocket_troy]

Impressive. But 200mN wouldn't exactly provide a high corrective response unless levered quite away from the CG.

Looking at the expansion cone, it wouldn't surprise me if that 200mN was vacuum based too?

Further to this: assuming that 200mN is the product of the thruster operating @50bar expanding to say 0.001 bar or whatever... let's assume a thrust coefficient of 1.8 (all safe assumptions). That equates to a throat area in the ballpark of 0.022mm^2 (!). Mannn... even a bee would be struggling to poke his pecker through that. No wonder the solenoid is so small.

TP

 

[G_T]

I just grabbed that one quickly. There are better commercial units, smaller or larger.

 

[rocket_troy]

I just grabbed that one quickly. There are better commercial units, smaller or larger.

Yeah, fair enough. No doubt the advances in metal additive tech has opened the door up to mass and volume efficiencies well beyond my understanding of what used to be the limits to this without going all exotic.
Certainly interested to follow that project if you decided to proceed with that.

TP