Sunday, December 02, 2007

Helicopter testing an a new notional vehicle.

I reviewd the telemetry from the last helicopter flight and I think I have finally resolved all my issues with telemetry and hardware. No glitches in either control or data , my telemetry logger logged correctly to SD media so I will never again loose telemetry data from a failure to save, or a dead laptop battery. I got no GPS data, but it's probably because I did not give ehough time for the GPS to lock prior to flight and it did not acquire in the high vib helicopter environement. I'm planning to swith GPS receivers before the next flight in any case.

A new notional vehicle. And a plan forward.
The big question with insufficient data is:
Are our rocket motors really robust for multiple firings and throttling?
We have fired multiple times for long runs within an hour, we have throttled, but we have also had failures. We need to run the motor through several simulated missions. That is follow the LLC flight throttle profile and restart in less than 1/2 hour. Until we have done that several times we do not know if we have a workable design or not.

If our motors work then we will probably continue with a 4 motor vehicle. If they do not we will probably develop a single larger motor. The notional design for the vehicle branches on that question.

Assuming we have a working motor the the current notional design is as shown above. The key is that the weight is supported by a simple pad under the tank and the landing gear only keeps the vehicle from tipping over.

If the motors don't work then we are probably going to build a single engine vehicle with spherical tanks. There are really only two configurations that seem to make sens, the Quad and the Pixel design. Pixel has balance feed problems, but is structurally more efficient. The one drawback I see to the armadillo module design is the mass of the landing gear and the fact that the motor is very close to the ground. I've ordered some hemispheres from AMS industries and they should be here some time in January.


Carl Tedesco said...

Regarding your landing gear concept... this presumes you land perfectly vertical each time. If the vehicle has any yaw it may hit a leg first. If the legs are lightly constructed to save weight you could damage them. In an AR thread Henry S. noted that the Apollo lunar lander was designed to take the full brunt of the landing loads on one landing pad. I'm sure you'll find out during your tether flights if this is an issue.

--- Carl T.

Anonymous said...

You wrote:

[quote]There are really only two configurations that seem to make sens, the Quad and the Pixel design. [/quote]

Pixel is a specific example of the quad design, isn't it? Did you mean Quad and Modular? Or am I missing something?

In addition to Carl's concern regarding damage to the legs on an uneven landing, it seems like the extra weight you would need to design tanks and plumbing that would with stand a rough landing would weigh more than sturdy landing gear.

Has anyone considered a vehicle with only 3 legs instead of 4, in order to save weight?

Paul Breed said...

A pressurized tank is insanely strong. Nothing other than a simple
pad would be necessary to keep pebbles/sharp pointy rocks from puncturing the tank.
A rubber block like the Pixel design would be fine.

And yes I meant the mod and quad design.

Ambivalent Engineer said...

The thing about landing gear is that the unsprung mass takes a really high landing shock acceleration, and the sprung portion spreads that acceleration out over time. If you are using your tank as an unsprung mass, the sphere will survive the shock with no problem.

But do you expect the sphere to deflect between the impact point and the mount points to the rest of your rocket? If the mount points are around the waist of the sphere, they will bend outwards if they deflect at all. I would expect little or no deflection -- it's not a rubber ball.

Doesn't the sphere usually have a liquid pickup line at the bottom somewhere? That structure will have to tolerate landing shocks. The plumbing from there to the sprung portion of the rocket will have to be compliant. I smell problems.

Is the entire vehicle going to take the high acceleration landing shock, or will you have a compliant mount between the tank and the rest of the vehicle? If so, is the tank going to move relative to the vehicle when you apply launch force?

Intuitively, I like the idea of a pressurized sphere just smacking into the ground. But I don't think the sphere itself is the problem here.

Paul Breed said...

Armadillo has published some commends on shock loading and landing gear. They determined that rubber blocks had a lower peak shock value than a traditional spring system. The next "notional" vehicle they are discussing is a modular setup with the engines mounted on the sides of the tanks and a rubber donut. (Think small tire) under the sperical tank as a pad. I agree that something has to go under the tank, but not very much.

Carl Tedesco said...

No plumbing needs to emanate from the bottom sphere. The propellant lines can go into the sphere radially, say at the belt line, where they meet at the sphere axis to a tube which then goes to the bottom of the sphere to scavenge the last bit of propellant.

--- Carl T.