Hack in Space

MelloDawg writes: "From the press release: NASA's Far Ultraviolet Spectroscopic Explorer (FUSE) spacecraft, which some had given up for dead in December after critical guidance components failed, was returned to full operations when the team developed an innovative new guidance system. The system uses a complex new set of procedures that lets controllers use electromagnets in the satellite to push and pull on the Earth's magnetic field. Details of the mission are online."

Comments

Where to put angular momentum

[Animats]

This is indeed neat.

A basic problem in satellite stabilization is how to get rid of unwanted angular momentum. There are a few options.

You can throw away something. Usually this is reaction mass from a small rocket, or just compressed gas. Weights on the ends of cables that unwind and break free have been used to despin satellites.

You can store angular momentum in an inertia wheel, which is a flywheel on a motor. This doesn't get rid of angular momentum; it just stores it as long as you keep the wheel spinning. Eventually, you hit the maximum motor speed and can't do anything more in that axis. So it's also necessary to have some way to drain off angular momentum, even if very slowly.

You can couple to a gravity gradient. This is done with a long pole aimed towards a nearby planet. The difference between the gravity at the ends of the pole is tiny, but enough that if you get the thing pointed down and stable, it usually stays that way. Only good for one axis, of course.

You can couple to a planetary magnetic field, like these guys are doing. Again, only good for one axis, but it's a different one than the gravity gradient. It's a weak effect, but stronger than the gravity gradient.

You can put out sails and get reaction forces from solar energy. This gets talked about a lot, but isn't done much.

All of these are known techniques. It sounds like this satellite had four inertia wheels and an electromagnet for torquing against a planetary magnetic field. The plan was presumably to maneuver with the inertia wheels, and slowly drain off unwanted angular momentum with the magnetic torquer.

With two inertia wheels down, there are still three torquing devices available, so control of orientation is theoretically possible. Tough, but possible. It's impressive that they made it work.

Re:Where to put angular momentum

[fwc]

This technology is also being used (with great success) on the Amateur Radio satellite AO-40 which was on Slashdot a while back.

Specifically, it was used to de-spin the satellite from almost 18 RPM down to the desired 5 RPM.

Some more relevant quotes I found while looking around on the web about AO-40's system:

From http://www.amsat-dl.org/journal/adlj40ge.htm

Magnetorquer In the satellite, several electro-magnets, also named magnetorquer, are distributes that can be used in the interplay with the Earth's magnetic field close to perigee for the attitude-control of the satellite. The satellite acts as the rotor of an electric motor while the magnet-field of the Earth forms the stator. The process of this movement is named as magnetorquing. With the magnetorquing, the flight-attitude of the satellite and the spin-speed can be changed during perigee-passes.

From http://www.rac.ca/spacenws.htm:

The onboard magnetorquing system--which consists of solenoid coils--makes use of Earth's magnetic field to control the spacecraft's spin and orientation. Magnetorquing is most effective when Earth's magnetic field is strongest, so it typically only takes place at perigee--when the satellite is closest to the Earth. Ground controllers have been making incremental adjustments during each perigee.

I also remember someone saying that this was somewhat "experimental" on AO-40. I can't find a quote though...

I agree fully that it's good to see the NASA engineers thinking "Well it's broke, we can't send someone up to fix it, so what can we do to make it work?" What I would like to know is who came up with the original idea (pre AO-40, or this satellite). It sure doesn't seem like the type of thing which I would have thought about when trying to figure out how to control the attitude of a spacecraft.

Re:Where to put angular momentum

[Gino]

All of these are known techniques. It sounds like this satellite had four inertia wheels and an electromagnet for torquing against a planetary magnetic field. The plan was presumably to maneuver with the inertia wheels, and slowly drain off unwanted angular momentum with the magnetic torquer.

In most cases at least 3 single axis magneto-torquers are provided, one per axis and normally one extra for redundancy. They are indeed used to reduce body angular rates and to control the wheel angular momentum. The reaction wheels (or inertia wheels) of course are limited to a maximum speed and to avoid wheel saturation the magneto-torquers are used to 'dump' some excess angular momentum.

With two inertia wheels down, there are still three torquing devices available, so control of orientation is theoretically possible. Tough, but possible. It's impressive that they made it work.

Actually, in smaller LEO (Low Earth Orbit) satellites this technique has been used for years to facilitate orientation control. It is normally used to reduce the spacecraft angular rates directly after separation from the launch vehicle. Using the magneto-torquers and a magnetometer monitoring the earth's magnetic field, the spacecraft can be controlled into a stable state with the minimum of hardware and with relatively simple control algorithms.

The idea is to get the spacecraft into a stable, known state (either earth pointing or aligned with the earth's magnetic field) before the more complex systems are powered and tested. You need to be sure your star camera, reaction wheels, propulsion systems and all the other cool hardware is in a nominal state before you enable the complex control algorithms required for more accurate orientation control.

The first satellite I've worked on we could not afford the more expensive magneto-torquers (compact electro magnetic rods) and instead opted for a self made magnetic coil without the assistance of ferrite material. Basically just a very long copper wire rolled into a coil and fixed to the frame of the solar panel (one coil per axis). Crude, but it had the desired effect!

What is really impressive is that they've managed to achieve this level of pointing accuracy with a system intended to achieve only basic orientation control and for desaturation of the reaction wheels.

My favorite space hack

[Thagg]

About five years ago, Hughes launched a communication satellite. To be useful, these have to reach a geosynchronous orbit. It turns out that the most efficient way to get to from a low-earth orbit to a geosynchronous orbit is to fire a rocket twice [or so it was thought.] The first firing raises the apogee of the orbit to the geosynchronous altitude; so that the satellite is in a very elliptical orbit. Then, when the satellite is at apogee, you fire the rocket again to circularize the orbit. Usually this same motor changes the plane of the orbit as well. Most satellites are launched into orbits inclined to the equator somewhat, and geosynchronous satellites have to be over the equator. It is most efficient to make the orbital plane change at apogee, too.

There was a rash of apogee kick motor failures, and in this particular satellite the motor failed, leaving the satellite in a uselessly ellipitical orbit. There were small thrusters on the satellite which were to be used for station-keeping (small orbital adjustments) but it didn't have nearly enough propellant to raise the perigee. Hughes finally abandoned the satellite.

But one engineer refused to give up. It turns out that the transfer orbit paradigm above is the probably most efficient path in a single-planet system, but Earth has this anomolously large, close, Moon. And while there wasn't nearly enough fuel to get raise the perigee to geosynchronous altitude, there was more than enough fuel to raise the apogee out to lunar orbit. He was given permission to try to rescue the satellite.

In the end, two passes by the moon were made, each raising the perigee somewhat and lowering the inclination of the orbit. The remaining fuel in the satellite was used to lower the apogee back to the geosynchronous orbit altitude, but unfortunately the inclination couldn't be brought down quite to zero, so the satellite isn't in its desired orbit even today. Still, it's in an orbit where some use can be derived from it.

The satisfying conclusion to this story would be that all geosynchronous satellites are launched this way, now. Unfortunately, you can't mess with the status quo to that extent; and satellites are still, in the main, launched the old transfer-orbit way.

thad