Monday, November 03, 2008

Lessons Unlearned

In 2007 we tried to build a 4 engine Lox Ethanoal rocket. When we were done it had 39 valves and was just wayyyy too complicated to work reliably. My present 180 second fantasy goes back in that direction a tiny little bit. If youmake the following assumptions:



  • Electrically driven peroxide and RP-1 pumps.

  • Catalyst decomposition with lots of pressure drop for relability

  • 1000 to 1200 PSI feed pressure

  • 700–800 PSI chamber presure.

  • Four engines.

  • 80% of theoretical ISP.

Then what drops out is



  • Off the shelf small aluminum frame hydralic pumps (Possibly with one section redone with stainless gears)

  • Off the shelf  Pump motors,batteries and gearing from  high end 600 sized RC helicopters.

  • Hover times of 240 seconds+ with payload.

  • A rocket with two manually operated valves (Vent and drain)

  • Tanks with a pressure safety factor >4 so humans can be near the presurized rocket.

  • A rocket with no active valves. (All control is from the pump speed control)

  • Precise Mixture control.

  • Safety system is all electrical, power stops to motors and down it comes.

I’ve ordered a pump to evaluate its peroxide compatibility and life time running unlubricated.  The pump was only $72.

12 comments:

Timothy J. Massey said...

What are you thinking about for the catalyst pack? AA had *nothing* but problems with catalysts: stripping, cracking, burn-through, or just plain ineffective...

Paul Breed said...

Actually Johnc, claims the silver packs were good until they switched to mixed monoprop work...
The Jury is still out on cat packs.

Anonymous said...

factor of safety fo 4 is too heavy

Paul Breed said...

Not for a low pressure tank, can't go much below that for minimum gage issues.

MisterQED said...

I think this is a great idea, simplicity is the key, but I have a few questions:
Won’t gear pumps give a pulsed output that will cause problems? Can that be solved with an accumulator or can you switch to screw drive pumps?
Aren’t twin electric motors adding a lot of complexity that could be avoided? Speed sensing, feedback loops, etc. Isn’t the mixture known or knowable thru testing and thus should be preset by pump gearing? This would drop you to only one motor with much looser constrictions on its motion.
MIsterQED

Paul Breed said...

Two possible configurations...
One large motor with one electric motor and two pumps, mixture ratio set by gearing.
Control would be by gimbal and roll thrustor.

Four Motors with one electric motors each driving two small pumps with mixture ratio set by gearing. Control would be by differential throttling.

Anonymous said...

I like your idea...I hope the testing proves it feasible for long endurance:

(1) Did your model include the extra weight for slosh baffles or would your new configuration not require them?
(2) Do you gain an endurance/range/max-altitude advantage by choosing a 4 engine configuration over your single engine configuration (from a system-level standpoint of building a space-capable vehicle)?

MisterQED said...

I think I have a better idea, tell me what you think:
http://groups.google.com/group/n-prize/browse_thread/thread/179b03d1ca4f9b16

Sage said...

You can 'probably' get it to work. You may get cavitation in the pumps(depending on the pumps), and you're likely going to need to drive the motors real hot to get enough performance to justify the added weight. I did an analysis using electric pumps for rocket engines several years back; if you both optimize and implement correctly (of which I'm not sure you can do with off the shelf components), you can possibly reach open-cycle gas generator performance via cutting-edge equipment. ~Sage

David said...

Thank you for all the additional information. For me it was very exciting leading up to the event, though now its over my appetite for information has not settled. So my thanks for being the only one still currently posting about your thoughts and steps taken. Armadillo was very good in the past but RRL slowed it down and with the new agreement with them its likely we will not hear much at all. So this is the blog I will be monitoring you have my best wishes.

Paul Breed said...

There are two parts of building a vehicle, designing the parts that are not off the shelf and then gathering the off the shelf parts and assembling fabricating what you designed.

The lure of the 4 engine vehicle is that you have a much simpler object to design, then you duplicate it 4 times. One has the potential to have a vehicle where the only active controls are 4 motor drives. The safety system is simple power interruption, and attitude/trajectory control just modulates the thrust control you already need to do a hovering vehicle.

The 70 second vehicle we flew had
a main throttle valve, a catalyst valve and 4 jet vanes in the main flight operations/safety loop.

We had 6 separate custom items to ddesign, the main valve, the cat valve, a Jet vane duplicated 4 times, the main chamber and the main tank.

If we go to a bi-propellant peroxide vehicle the jetvanes are most likely to melt so we need another control method.

The conceptually simplest vehicle is a blow down biprop with gimbals and roll control.

This has two tanks, two different throttle valves, a chamber, a gimbal a roll thruster and pressurization system. 6 things to design and debug.

If I can get off the shelf motors and pumps (looks like I can its only a $$ issue) Then I only need to design two items, the thrust chamber and gearbox/transmission to transfer from the motor to the two or three pumps per side.
Tanks would be off the shelf chemical jugs/drums.

Two pumps would be Peroxide and Kerosene, three pumps would be peroxide, kerosene and liquid catalyst. Building the chamber gets harder at pumped chamber pressures, but with pumped peroxide you can loose a lot of
preessure in the chamber jacket for high cooling velocities and the peroxide cooling has the highest cooling mass flow of any engine combination.

Too many choices.....

Anonymous said...

It sounds like, IMHO, that some more engine testing is required BEFORE a final system config is chosen. Mainly b/c you are unsure of which is better performance: cat packs or liquid cat. I'd tackle that first, even though it will take more time, $$$, infrastructure. Having an answer on the long-term performance of cat-packs is worth the investment. If they are a scrub, then I think the liquid-cat system is the better system, for all of the reasons you have previously mentioned (simplified safety system via power cut-off to pumps, increased regen-cooling with peroxide, reduced mass in tankage, etc). My two cents...