attitude control rockets for model spacecraft 6

[KarlChwe]

This is a little outside my expertise, so I am asking you, the excellecnt spacecraft engineering community (or one of them.) Sorry if I use incorrect terminology and such, or if this is the wrong kind of post for this forum.

I am a volunteer at the Denver Museum of Nature and Science, in the astronomy and space exploration section called "Space Odyssey". We have an exhibit that is meant to allow visitors to experience maneuvering a spacecraft in zero-g (in 2 dimentions.) A 1/60 or so model of the space shuttle moves frictionlessly on a flat surface within a plexiglas enclosure. Visitors control the yaw and forward thrust of the model with a joystick. They have to "dock" the model with painted targets on the sides of the en

The problem is "fuel" for attitude control and forward thrust rockets. Previously we had used compressed air piped in by an umbilical suspended vertically overhead. The joystick controlled valves that would allow jets of air to shoot out of tiny nozzles mounted in the sides and back of the model. That approach caused lots of problems. The umbilical resisted twisting, and the weight of it created a constant pull toward the center, where the umbilical could become vertical (like a hanging rope.)

So we are trying to come up with a way to create self-contained reaction jets on the model.

Here are the design contstraints. The materials should be lightweight (say, less than five pounds for fuel, valves, nozzles, etc.) The motors should be able to operate for about 12 hours, on and off, on the onboard fuel. Everything should be cheap and readily available, or improvisable. The materials and their end products should be safe to breathe, touch and preferrably, to eat. The exhaust stream should be relatively cool. The amount of thrust required is small (I don't know how small), since it is moving an object on a relatively frictionless surface. In fact, we don't want too high thrust, because we don't want the craft to end up bouncing rapidly around the exhibit area. We can't really use sophisticated computer logic.

I thought about using cans of compressed CO2, but I decided that they would be depleted too quickly. I wonder if there is some way to continuously create CO2, perhaps chemically inside a pressurized vessel. Or perhaps a small compressor could pressurize air stored in a vessel. We could have a pressure regulator maintain steady pressure to the nozzles.

So 1. How do I create pressurized reaction gas? 2. How do I even figure out how much pressure is required, and how much gas? 3. What kind of nozzles, valves, tubing, etc. do I use?

Any tips or hints of where to go is much appreciated.

Thanks!

Karl

 

[KarlChwe]

Thanks, rburton. As you say, now I would just need forward thrust. (In effect, I would have a craft like that in the video game Asteroids. You can only point it and thrust forward.) CO2 might be good. The question I had about CO2 is how big 1 kg of CO2 would be at an achievable compression, and about how heavy the vessel would be?

Thanks scooter911. The playing field is round, about 10' in diameter. The dimensions of the shuttle model are a little flexible, but it should be something like 2'long.

If you turn the momentum wheel (thanks rburton!) one way, the model will turn the other way, so there is no net gain in angular momentum. However, as you say the motor will have to fight the bearing friction, which will add momentum to the system. When the wheel stops, that additional momentum will still be there, so yes, the model would continue to turn slightly. I would try to minimize that with some nice low-friction bearings.

But the problem for me is still about slowing a turn in such a way that the controls act like a rocket. Say the model is at a stop. To turn it left, you hit a button or move a joystick left (right?) to send power to a motor that will turn the momentum wheel. Now both the model and momentum wheel are turning. To slow the turn if you were using rockets, you would goose the correct motor a little by moving the joystick right a few times.

How do you do that electrically and mechanically so that when the joystick is moved right, the motor slows the wheel without stopping it, or starting it moving in the other direction? Further, if you keep goosing the joystick to the right, the wheel should stop spinning, then as you give it more juice, begin spinning in the opposite direction.

The best I could come up with is some kind of torque transfer mechanism with two paddle wheels, like propellers but with flat blades mounted coplanar with the rotation axis (so sighting down the axle, you would see the paddles or blades edge on.) You spin one paddle with a motor, which moves some air, which then moves the other paddle slowly. When you reverse the first paddle, the second doesn't immediately reverse, it just slows, stops, then starts moving in the opposite direction.

But this seems like a big awkward kludge of a solution. Maybe I could couple the wheels magnetically instead of aerodynamically.

Also, I would still need an unusual motor. When you give it power, instead of reaching a steady speed, it should ideally accelerate forever. If it reaches a steady speed, that will set an upper limit on the rotation rate of the model. Maybe that is a good thing, but it wouldn't simulate rockets very well.

Maybe I could use a regular motor, but use a heavy enough flywheel that in practice the motor never has to reach maximum speed. So it is always near stall. Maybe that would burn it out... How do they do it in satellites?

If a motor is spinning a frictionless wheel at a steady speed, does it draw any power? Why?

 

[rburton]

CO2 stores as a liquid at a pressure of about 60 atm and a density slightly greater than water, so 1 kg would be about 1 liter. A 2.5 lb CO2 cylinder is used for dispensing beer and is fairly common. Smaller ones are also available. You will need a hand shutoff valve and one solenoid valve for each thruster. CO2 systems use a regulator, but you might be able to dispense with that if you can find small 1000 psi solenoid valves. This is a safety question.

A steady motor speed is fine because you don't want a very high rotation rate. On satellites it is necessary to periodically dump the wheel momentum using thrusters. For your situation you merely need to turn everything (wheel and air table) off occasionally.

I am not an expert on momentum wheel drives, but all the coupling is electromagnetic. When the power is off the wheel coasts.

 

[Jetwax]

Hey, what a great and interesting discussion.

My 2 cents.

-Air hockey table for lift

-fly wheel with DC motor powered thru Thyresistors controlled by a simple PWM capable micro controller. The DC could be picked up off table with a grid of wires and a simple diode system to keep from shorting. The speed and direction can be controlled via the uC.


-RC Control. hopefully the EMI from the thyresistors switching doesnt interfere.

-and C02 thrusters as mentioned above.

Just thought I would share my thoughts,
Cheers,

 

[KarlChwe]

Thanks for the reply, Jetwax.

Okay, we have pretty much gone with the air hockey idea and RC.

I pretty much don't understand the thyristors and PWM thing. I am pretty sure we are going to go with batteries, though. The flywheel and motor shouldn't draw much juice, and building a table with a grid of wires was difficult to imagine.

And what would the CO2 thrusters be used for? I had imagined using the flywheel to control attitude, in the manner of a momentum wheel or reaction wheel, whatever the correct term is.

Cheers!

Karl