mam@charm.UUCP (Matthew Marcus) (04/04/84)
[Orbiting laser]------------------> [space bug]
Something I've been wondering about for a long time is why the
technology of Oberth wheels has not been much used for attitude/spin
control in space. The idea goes like this:
Inside your ship, you have a flywheel spun by a small motor.
If you start with both ship and wheel not spinning, then you can spin the
ship by turning the wheel the other way. When you want to stop spinning,
you brake the wheel. If you use regenerative braking, you recover most of
the energy you used. Use three wheels on orthogonal axes, and you get
complete attitude control. For satallites, you spin the wheel, launch
into correct orbit, then stop the wheel. For torque used to stop the wheel
spins the bird. To despin the bird, you simply start the wheel again.
If the wheel is mounted in a gimbal with magnetic bearings and a magnetic
clutch to decouple the wheel from the bird at will, you can decouple,
bring in the bird, work on it without worrying about the gyroscopic effect
of the still-rotating wheel, then recouple and despin the wheel.
You could imagine a super-cheap (as these things go) version of the
MMU in which attitude control is done by wheels worked by hand-cranks.
If you want to make a left roll, you turn the "roll" crank until you are
faced as you want to be, then hit the "thrust" button. To compensate
for the center of thrust not being in line with your center of gravity,
you could have booms with masses on the ends which could be extended.
This adjustment could be done manually or under control of a simple
autopilot.
There must be something I'm missing which makes these ideas impractical,
since I know of no present uses of the Oberth wheel technology. Could it be
that since NASA didn't invent the wheel, they won't use it :-)? BTW, I
read a lot of Heinlein stories and I know it shows.
{BTL}!charm!mamkarn@allegra.UUCP (Phil Karn) (04/06/84)
Reaction wheels are indeed widely used in spacecraft that require three-axis stabilization. Examples include Skylab, the NOAA TIROS-N series of polar weather satellites, and the RCA Satcom geostationary communications satellites. They are combined with gas thrusters or magnetic torquing systems to allow for momentum dumping when necessary. Phil
David.Smith@cmu-cs-ius.arpa (04/09/84)
Oberth wheels were used for attitude control in Skylab.
Lynn.es@Xerox.ARPA (04/09/84)
Information I recently saw says that the Amateur Space Telescope, going up in a couple of years, and being built mostly by Rensselaer (RPI), will use reaction (Oberth) wheels. They concluded that it was the only way to build it for kilobucks instead of megabucks. It also has the advantage of running off electric power from a rather small solar panel, and so needs to carry no fuel. It will have four wheels, one a spare to stand in for any failure. They are using magnetic bearings and expect very small fraction of a second of arc pointing error. I think the bearings are the unproven technology in the design, but they will be tested in space before being used on the telescope. /Don Lynn
kcarroll@utzoo.UUCP (Kieran A. Carroll) (04/09/84)
* The devices which you refer to as "Oberth Wheels" are in fact being employed on satellites right now, going under the name of "momentum wheels" or "reaction wheels". They are used in much the way that you describe, as angular-momentum sinks to allow for the maneouvering of the satellite about its three axes without the need for reaction jets. They are also used to "soak up" the angular momentum generated by the various torques found on orbiting satellites: gravity-gradient, solar, atmospheric, magnetic, what-have-you. The cyclic components of these torques are absorbed temporarily by the wheels, and are later vancelled when the torque reverses direction. The secular component (or DC, as opposed to AC) is absorbed until the wheel is spinning at its rated maximum speed, at which time the momentum is dumped by simultaneously despinning the wheel, and firing the satellites attitude-control thrusters to provide a balancing torque. Neat, huh? The problem with these wheels ( as with all hardware) is that they possess mass; thus, they may not be used on some satellites, if it is determined that a set of thrusters with fuel supply for the length of the mission would weigh less than the wheel system -Kieran A. Carroll ...decvax!utzoo!kcarroll