Friday, November 09, 2018

Starting with the basics...
What is the tank layout?
For control authority you really want the center of gravity (CG) as far forward as possible.

So I sketched up the 5 tank layouts shown above...
I fully accounted for the weights and moments for all the fittings down tubes, up tubes etc...
for the full rocket...I estimated engine and gimbal weights base don stuff I've already fired...

I wrote a C++ program where I could instantiate parts, add them to the rocket, then run the tank to depletion and  calculate the CG....

I modeled #1, #4 and #5  #1 has the farthest forward CG, but was heaviest. If I ballast #4 by adding nose weight until it matches #1 then the CG is farther forward with #4 at the same weight at the extremes of  the flight. 

#1 goes from  152 to 255 cm from the front.
#4 ballasted to the same weight goes from  233 to 251 cm.
Its much simpler to build and the nose ballast can become more recovery gear bigger batteries, better telemetry etc...

As an added benefit #4 is much simpler to build and cleaner aerodynamically with no external feed tubes. I could do #4 with internal feed tubes, but then all the bulkheads become 5 axis machining projects, where for #4 they are all basic lathe parts.


I've simulated this rocket 3 ways...
My own written from scratch  simulator in C/C++.
Using RAS Aero II
Using OpenRocket...


All of the results agree and all say I have enough margin to cross the von-carmen line with a class 2 rocket... It will be interesting to see if  I can build it as light as I think I can.

That's all for now...

4 comments:

Jimmy Beal said...

For the rocket class system, is it just based on total impulse? Specifically, does having active control change the waiver you have to get?

Paul Breed said...

That is a complicated question...

At FAR the standing waiver specifies max impulse and max altitude. So to start with it makes no difference.

Once you have to get a new waiver from the FAA of any class, it exposes you to the FAA may request more information to insure the operation is done safely clause. Assuming the FAA/AST is competent (and all I've met are) they should ask more questions of a guided rocket than an unguided rocket. In this case the "Class" is really just a stand in for how big of a mess can we make on the ground if things go badly.

Lars Osborne said...

Paul,

I am a little shocked at how much pressurant gas would be needed for this system. How does the extra tank age weigh against having a higher pressure bottle for the gas and a regulator?

Paul Breed said...

Its blow down....
Blow down is often filled 50% (This is what these drawings show)
So no regulator, no bottle, nothing.... I can fill it more, but then the pressure and gravity losses at the end of the run mount... With the aluminum walled tank for this vehicle, it might make sense to add a bottle/regulator, but remember this is a stand in for my super light carbon tanks. The super light tanks I developed two years ago. In a vehicle where performance really mattered the tank would be the super light carbon, and in that case the Tank figure of merit is actually better than the very best carbon fiber wrapped high pressure bottle, that is assuming regulator, valves etc weight nothing at all....