CEV Revolutionary Gimballed Thrusters

simonbp writes "A Tennessee Tech Professor has proposed an innovative gimbal mount for 'inclusion to the design of [NASA's] CEV (Crew Exploration Vehicle), revolutionizing the vehicle's RCS (Reaction Control System) and solar panel orientation capabilities.' This will allow for nimble maneuvering and for the solar cells to maximize power production."

Comments

Re:Gimbal?

[interiot]

Better is wikipedia's Gimbaled thrust and the NASA pages it points to.

(funny, I was just reading that before coming to slashdot... even better is the semi-related water rocket page.... awesome stuff)

Link to actual animation

[Animats]

If you don't want to plow through all the blogodreck and registration, here's the animation of the Canfield joint (quicktime).

As a rocket engine gimbal, this doesn't look promising. It's a rather bulky mechanism; the linkage is much larger than the engine bell. It requires fifteen bearings, not including the three motors. The standard solution, a gimbal ring arrangement, only requires four. The bearings also have to handle off-center loads, never a good thing. Bearings in space are headaches; lubrication is tough and temperature changes can jam them.

The motors are in a weak position from a leverage standpoint; the engine thrust is applied directly to the motor shafts, so they (and their gear trains) must be strong enough to overpower the thruster. In a gimbal ring arrangement, the bearings are usually placed so that the center of thrust is at the center of the gimbal, so that the bearings, not the actuators, take almost all the thrust. Very large engines, like the Space Shuttle and Saturn V main engines, have been successfully gimballed that way.

The three motors don't seem to add redundancy; it looks like they all have to be working.

For comparison, here's a simple gimbal from Amadillo Aerospace, Carmack's rocket program.

In reality, having many fixed reaction thrusters is probably more reliable than have a few steerable ones. Fewer moving parts.

Design is a disaster

[O2H2]

The design is interesting in a sophomoric way. It is clearly designed by someone who has never had to qualify a device for spaceflight. Highly cantilevered, indeterminate stuctures with eccentric centroids and active mechanisms in the loads paths are terrible. I would give that thing about 60 seconds on a vibe table before it came to pieces. The use of flex hoses with such large motions is also a really bad idea- expecially when they have to flex in multiple axes at once- there is no practical solution to make such a fluid connection last more than a few thousand cycles under cryogenic conditions under the 300 psig pressure conditions that are planned for CEV propulsion. As someone who DOES design such things on a daily basis I give it a D- for practicality, cost and reliability/redundancy.

Also the control valving is highly decoupled from the combustion chamber which means high dribble volume and terrible min Ibit. Those simple stepper motors also have to operate at 165R for prolonged periods- this denies you most lubricants and requires special resolvers and the like. There is also no way that such a mechanism can deliver the frequency response of multiple small thrusters pointed in multiple directions. There is also the need to interface either a fiber optic or high voltage spark igniter lead to the thruster across large motions- could be a problem for the non-optical approach.

The issue is : just what problem are we trying to solve? is it cost of the combustion chamber? Number of valves? Weight? Overall complexity? Or is this just an interesting exercise for a kinematics class? The vehicle attitude control function can be performed two active and two standby modules- not four fully active as was used on Apollo. This is highly optimal for cryogenic thrusters since it minimizes the number of lines which must be chilled and pure 6 DOF operations are rare as opposed to simple maneuvers with coupled rotations and translations. This solution was proposed to NASA and rejected as being "just too different from what Apollo did". I cannot imagine them actually flying this contraption.