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Official Armadillo Q&A thread

Posted by: John Carmack - Tue Jun 15, 2004 8:01 am
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Official Armadillo Q&A thread 
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Post    Posted on: Sat Nov 24, 2007 11:24 am
On the subject of pumps, i understand that Armadillos position for now and foreseeable future is to avoid any types of pumps at all, and not just deal with the complexities.
I keep wondering if biting the bullet at some point would not pay off in opening up engine configuration trade space. Or then again, maybe not.

I does not seem that options for low cost pumps have been very thoroughly explored in this industry. There are turbopumps which are by the very nature very expensive, then XCOR has their piston pumps and Flometrics has the pressure pump and thats it. That sounds awful limited to me.

For example, what about the middle ground between the turbines and piston machines, the rotary engine/pump configuration ? Mazda has practically perfected the rotary engine in RX8, i wonder if anyone ever has worked on a similar pump principle ? Rotary engines regularly have way better power to weight ratio than piston engines, at the expense of more difficult-to-manufacture chamber geometry and construction, but they are nowhere nearly as difficult as turbines. Maybe this middle ground could hold true for pump configurations as well.

Just as an aside, i saw two guys with Flometrics teeshirts walking around at XPC this year, but they did not seem to have an exhibit out. Their last news update on the website is from september last year. I wonder if they are making any progress with that at all.


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Post    Posted on: Mon Nov 26, 2007 9:27 pm
Andrew -

No I was not talking about a bypass back to the tank; but rather a bypass to the engine. And (as a separate issue) the comment about no pressure change through the jacket isn't true - because the whole purpose of a cooling jacket is to absorb and transfer heat. Remember, increased temperature = increased pressure.

-----
But that does lead me to ponder something:
-----
I wonder if a purge-valve just upstream of the engine is worth installing, to "burp" out any gas or heat-soaked liquid prior to a warm engine start/restart (so that you don't have to flow this stuff through the engine itself and have a potentially explosive mix in the chamber when you go for actual start).
-----

no_way -

Rotary pumps are smooth and functional (look at all of the hand-crank fuel pumps available out there for simple working examples). But on a rocket you're going to have to _drive_ that pump somehow. Good luck coming up with a simple, lightweight, robust, and failure-proof system that can produce the kind of pressure needed (batteries, electric motors, a whole extra high-pressure-gas system? If you go for some kind of combustion or gas-generator cycle you might as well have a full turbopump).

Furthermore, rotary motions / systems are not well suited to compressing a large volume into a small one to produce high pressure. This is why the Mazda RX motors produce great horsepower but very little torque (and, as a result, why you rev 'em so high in order to accelerate - I used to race RX-7's *grin*). The RX motor has a relatively low compression ratio, compared to high-output reciprocating-cylinder-based engines. And you're going to have to ensure that the pump can keep up with the outflow when the motor is at full throttle.

Don't get me wrong, I love rotary engines for situations that they're well-suited to... but I am not sure this is one of them.

Take care,

--Noel
(Adding former race-car driver & mechanic to the amateur aircraft designer tag)


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Post    Posted on: Tue Nov 27, 2007 9:34 pm
NWade wrote:
system that can produce the kind of pressure needed (batteries, electric motors, a whole extra high-pressure-gas system? If you go for some kind of combustion or gas-generator cycle you might as well have a full turbopump).

Aha, here is the catch i think. From what i gather, turbopumps are that expensive because of the materials involved and machining precision do manufacture them. It might just make sense to use a rotary configuration instead. And, by the way, have you heard of Mazda's RX8 prototype running on hydrogen ? Sounds like a good fit to certain rockets ...
I get the impression that while cheap, piston pumps ala XCOR dont scale well, so just maybe there is room for rotary configuration too. Its something worth at least doing some further research on.
By the way, after all these years, Moller Inc ( yeah, i know ) has finally done something useful: Freedom Motors and the Rotapower engine. Apart from Mazda, they are the only ones i know of still seriously working on rotary engines.

Of course, i am biased towards rotary stuff myself as well. I love my RX8 ;)


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Post    Posted on: Wed Dec 05, 2007 12:05 am
Rotaries are also interesting in certain aircraft applications (Google to find the Van's Aircraft "RV" kit-planes running them, for example). I've looked at some of the Freedom Motors prototypes for self-launching sailplanes, too...

But even if they *are* less trouble than turbopumps, they are still heavy and complicated to machine/fit (according to Mazda, a big part of the cost of the old RX-7s was the manual labor required to properly assemble and fine-tune the 12A and 13B motors).

Wankel-Rotaries are small and provide high power-to-weight, but the inherent geometry of the design entails a lot of surface area and heat in the combustion chambers - this is why you have heavy iron/steel in the RX series of motors. Its not something you can build out of lightweight materials - even with advanced techniques and the $$ of a company like Mazda, the RX8 "Renesis" fell far short of its original expectations in terms of power and weight (though it is a neat engine).

To support a Rotary engine on a rocket, you must have extra plumbing, extra cooling (well *some* Freedom Motors prototypes can be run with pure air cooling if they are going full throttle most of the time - but have issues at lower output levels), extra valves & servos, extra control-logic, extra sensors & fault-tests, etc etc etc.

I'm not saying that its impossible - just that it may not be an optimal solution, based on your goals. You have to require enough additional performance to justify the added weight and complexity on the vehicle; otherwise you're probably better-off with a less-efficient-but-scaled-up rocket.



I wonder what AA is up to these days?
(Hopefully they're preparing to enjoy the Holidays with their friends and families!!)

--Noel
(...turning back to his IT & programming day-job, with a sigh...)


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Post Exotic engines   Posted on: Fri Dec 07, 2007 8:08 pm
In the area of exotic rotary engine/pump designs, here are the most interesting ones that I have found looking around on internet:

MYT Engine: Claims to have a power to weight ratio of 1:40, and claims to support pumping with both high flow and pressure at the same time.
http://www.angellabsllc.com/

Quasiturbine: Similar to wankel, but appears better balanced.
http://quasiturbine.promci.qc.ca/EIndex.htm

Also, I think the following design is pretty cool, although perhaps more useful for hybrid cars than rocket building:

http://www.lceproject.org/en/

I like the idea of a fully electronic engine with only one moving part (the piston).


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Post ?   Posted on: Fri Dec 14, 2007 9:32 pm
wondering what AA is doing these days! I imagine they took a (well-deserved) break from the rockets for a little while? I was hoping for a web update this past weekend, but that's their perogative.


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Post    Posted on: Mon Dec 17, 2007 9:45 am
By the time you get here, you'll have seen it. A new update:
http://www.armadilloaerospace.com/n.x/A ... ews_id=352


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Post    Posted on: Wed Jan 02, 2008 7:31 pm
Quote:
Rotary pumps are smooth and functional (look at all of the hand-crank fuel pumps available out there for simple working examples). But on a rocket you're going to have to _drive_ that pump somehow. ...

Furthermore, rotary motions / systems are not well suited to compressing a large volume into a small one to produce high pressure.


Actually, rotary pumps are the standard for high compression ratio GAS pumping (as in vacuum pumps with > 1000:1 pressure ratio). They are also the prefered method for LIQUID pumping, medium volume at high pressure, as in HYDRAULIC pumps.

Because of the high liquid density, THE PUMP is usually the lesser problem. Van Braun realized that what he needed for the V2 resembled a production "Fireboat" (Marine Firefighting) pump. The Turbo "Motor" (as with a gas turbocompressor) has to deal with a much lower fluid density and needs far higher speeds to be effective.


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Post    Posted on: Sat Jan 26, 2008 11:03 am
New Update Up!

Quote:
Once Spaceport America gets their final permit, we will take any remaining modules out there and see how high we can go. With the legs on, the modules don’t have a chance of getting to 100km, but if we learn all we need with normal flights, we might make a potentially-sacrificial flight of a module lifting off from a stand without legs.


This sounds brilliant! Would make a great video too! Was there any sort of date/timeframe for such a test?

Great update guys!Look forward to
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I’ll see if I can get Matt to make a rendering of the vehicle for the next update.


Rob G


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Post Differential throttling v.s. Engine Movement   Posted on: Mon Jan 28, 2008 4:10 am
Regarding John's recent blog update, I'm curious why you've chosen to go with fixed, differentially throttled engines v.s. your current design with steerable engines? Is the problem that on a multiple module vehicle the engine plumes would interact and cut down thrust or possibly damage the vehicle?

Also, regarding Paul Breed's comments on the valve actuator latency, this seems to me like a standard optimal control problem. If the latency can be characterized, then it should be possible to incorporate the latency into the on-board avonics in a optimal control model solver that would ensure the control signals are sent on time. This means it must be possible to know sufficiently ahead of time when the throttling is required ("sufficiently" here being the valve latency + processing time). I guess this may be a problem in emergencies. If the value actuator latency is too large, it won't work, of course. Or did Paul have another problem in mind?


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Post    Posted on: Mon Jan 28, 2008 9:03 pm
great to see updates! I'd love to see on-board footage of a 100km flight! I'm wondering what kind of launch cradle you could make that would also serve as a landing cradle?


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Post    Posted on: Thu Feb 14, 2008 5:01 pm
Been pretty empty here lately. Here's a question for the Armadillo folks...

Has there been any thought on building a spike-nozzle engine once you start making flights of significant altitude?

-Jim


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Post    Posted on: Sun Feb 17, 2008 5:46 pm
dmc6960 on Wed Nov 21, 2007 8:25 am Phil wrote this in this thread

Quote:
We have toyed with altitude compensation nozzles, but that was with peroxide monoprop. It looked cool and different, but it was a little harder to make and it didn't buy us anything because we weren't going very high. At that point, it didn't make sense to continue with that design.

In the not to distant future, we will be putting things to a much higher altitude, and we are again exploring ways to compensate for reduced pressure at altitude. The bigger factor is probably how to handle the expansion of multiple engines off of the base of a very wide vehicle rather than a single engine.


I am assuming that he is talking about a spike design


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Post Rocket Pumps   Posted on: Mon Feb 18, 2008 4:03 am
Just read the discussion on pumps, and I wonder if you couldn't build a small rotary rocket to power the pumps?

An impeller shaft with pumps on one end and offset symmetrical chambers with low ER nozzles pointing circumferentially and say 45 deg down to clear the vehicle and add some thrust.

Sort of a gas generator turbo pump without the turbines.


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Post    Posted on: Mon Feb 18, 2008 9:37 am
You'd still need the impeller and all the rotating parts though and it would be a damn side more complicated to feed the propellants to rotating chambers as opposed to a stationary gas generator. I still think the best option for pumps and rockets of the armadillo size are positive displacement piston pumps like they tested a few years back.

I was thinking if it would be possible to combine a compressor, staged combustion chamber and turbine into one unit a while ago, that might be something to think about (still means you need to have complex rotating parts, you get less control and have to have even more mass spinning around wildly).


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