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Venturi injector, TZM chamber, vehicle work

August 6 and 10, 2002 Meeting Notes

August 6 and 10, 2002 Meeting Notes

 

Venturi Injector

 

On Tuesday, we tested a new biprop injector design.  There was some interesting discussion on aRocket about it:

 

 

From:         John Carmack <johnc@IDSOFTWARE.COM>
Subject: [AR] venturi injector
To:           AROCKET@HOME.EASE.LSOFT.COM

We are going to be trying a new injector with our peroxide / kerosene engine tomorrow. Our 2" cat pack engine has so far achieved good combustion efficiency with ethane (when it lights...), but while the kerosene lights instantly, the Isp has only been slightly better than monoprop mode, indicating very poor combustion.

 

The two injectors that we have tried so far are a single point stream that shoots directly across below the catalyst pack with about a 0.030" orifice, and a 2" annular ring with a 0.005" gap (orifice is upstream).  The ring was mostly for gaseous ethane injection, because the required tolerance for decent kerosene velocity would be difficult to machine, and would probably change too much with thermal expansion anyway.

 

The single point injection delivered better results with a smaller jet on our initial 1" cat pack motors than on the 2" motors, so we were sort of resigned to having to make more injection points with smaller holes.  Some recent discussion about the virtues of fuel mixing in high speed gas streams made me consider that perhaps we just need to make the cat pack discharge faster moving when we are injecting the fuel.

 

Our current biprop engines have a 16:1 contraction ratio from the cat pack to the nozzle throat.  Our monoprop engines use 9:1. but we shrank the throats on the biprops to increase L* in a given length motor.

 

At 16:1, the gas moving past the injector isn't moving all that fast.  I have made a new injector that necks down underneath the cat pack from 2" to 1", which will have the gas moving four times as fast at the single injection point, and have the opposite wall closer, which may also have a positive effect.

 

There are two things I am pondering:

 

Should I care about smoothly transitioning back to the 2" chamber diameter?  On this injector, I just left it an abrupt transition.  The turbulence after the transition will probably be good for mixing, but there may be some absolute pressure loss, although with the temperature tripling after the transition with the resulting volume expansion, I don't have a good intuitive feel for pressure losses in that situation.

 

How fast is fast enough?  In theory, I suppose you could neck down at the injection point to a diameter even smaller than the nozzle throat, and have sonic velocity gas flowing by, because combustion will increase the gas volume before the real nozzle.  I suppose you would want to stay below the speed where compressible flow starts giving irreversible losses.  The cutoff profile probably ties into this, where a smooth profile with nothing but subsonic flow might be better suited to extreme contractions.

 

Making a venturi does increase motor length, but if it allows big motors to be made with only a small number of large injection points, it could be a notable benefit.  I am distinctly leery about tons of tiny injector holes, for several reasons.

 

John Carmack

 

 

From: "Hydrazine" <ramjet@dslextreme.com>
To: <AROCKET@HOME.EASE.LSOFT.COM>
Subject: Re: [AR] venturi injector

John,

 

When I worked at Marquardt I worked on a very similar project.  It was a 3"ID, 1000 lfb 85% Peroxide / JP8 thruster.  After reading your description of the low C* efficiency, it brought back memories of how difficult development of our thruster was.

 

There are many things you can do to increase the combustion efficiency. Some of which you already mentioned.

 

> The two injectors that we have tried so far are a single point

> stream that shoots directly across below the catalyst pack

 

We also found this method of injection results in very poor mixing.

 

 >   the required tolerance for decent kerosene velocity would be

> difficult to machine, and would probably change too much with

> thermal expansion anyway.

 

Thermal expansion won't be a problem.  Constant fuel flow will keep the injector passages at a reasonable temperature.

 

>

> The single point injection delivered better results with a smaller jet

> on our initial 1" cat pack motors than on the 2" motors, so we

> were sort of resigned to having to make more injection

> points with smaller holes.

 

If the jets are too large, they don't break up enough.  If the jets are too small, they don't have enough penetration. We found that impinging the larger fuel streams dramatically increased C* efficiency.  This is much more effective than simply using more, smaller diameter jets.

 

> recent discussion about the virtues of fuel mixing in high speed gas

> streams made me consider that perhaps we just need to make the

> cat pack discharge faster moving when we are injecting the fuel.

 

This is true, but try to keep the mach number below 0.4.  Otherwise there will be significant viscous losses.

 

> Our current biprop engines have a 16:1 contraction ratio from the

> cat pack to the nozzle throat.  Our monoprop engines use

> 9:1. but we shrank the throats on the biprops to increase

> L* in a given length motor.

>

> At 16:1, the gas moving past the injector isn't moving all that fast. 

> I have made a new injector that necks down underneath the cat

> pack from 2" to 1", which will have the gas moving four

> times as fast at the single injection point, and have the

> opposite wall closer, which may also have a positive effect.

 

Design to a chamber gas velocity of Mach 0.2 for optimum results.

 

> There are two things I am pondering:

>

> Should I care about smoothly transitioning back to the 2" chamber

> diameter?

 

Not if you are at Mach 0.4 or lower.

 

> On this injector, I just left it an abrupt transition.

 

This works as a boundary layer tripping device.  The pressure drop won't be significant if the Mach number is low enough but it significantly enhances mixing of the boundary layer.  OTOH your chamber will also experience a higher heat transfer rate.

 

> Making a venturi does increase motor length, but if it allows big

> motors to be made with only a small number of large

> injection points, it could be a notable benefit.

 

The benefits are not worth the added effort unless the gas velocity is very high. Mach 0.6+.

 

 > I am distinctly leery about tons of tiny injector holes,

> for several reasons.

 

 Use the larger diameter impinging fuel streams and you will obtain much greater efficiency.

 

Another alternative is to design the combustion chamber similar to that of a ramjet combustion chamber and use a fuel atomizer with a flame holder.  As built, it weighed more than the others, but we achieved nearly 96% C* efficiency with this combustion chamber design.

 

Tony

 

> John Carmack

>

 

 

From:         Henry Spencer <henry@SPSYSTEMS.NET>
Subject: Re: [AR] venturi injector
To:           AROCKET@HOME.EASE.LSOFT.COM

On Mon, 5 Aug 2002, John Carmack wrote:

> Should I care about smoothly transitioning back to the 2" chamber

> diameter?  On this injector, I just left it an abrupt transition.

 

That is essentially what the Brits did on their peroxide/kerosene engines: the fuel injector was a plate across the chamber, with big holes (they didn't even round the edges) for the gas from the catalyst pack to pass through, and a ring of small fuel orifices around each gas hole.  There were eight holes, each (by eye) no more than 1/8 the diameter of the plate, so the total gas-flow area was only a small fraction of the chamber cross-section, with no attempt at smooth transitions.

 

They did find it necessary to drill some small gas holes in the parts of the plate which didn't have fuel circulation, so that a small flow of decomposed peroxide would keep the downstream flame off the plate.  You might watch out for heating problems on your injector base.

 

> How fast is fast enough?  In theory, I suppose you could neck

> down at the injection point to a diameter even smaller than the

> nozzle throat, and have sonic velocity gas flowing by...

 

At least some of the non-peroxide work I referred to in earlier mail did that, I think, but it's probably serious overkill.

 

                                                          Henry Spencer

                                                       henry@spsystems.net

 

 

 

http://media.armadilloaerospace.com/2002_08_10/parts.JPG

 

From left to right: venturi injector, single point injector, annular injector (hard to see the ring), copper gasket, standard retaining plate, experimental retaining plate, clamp ring, 2” catalyst chamber, 2” nozzle, 2” aluminum biprop chamber (water flood cooled).

 

All tests with the one liter of peroxide, 1/4" ball valve on the peroxide side, temperature was over 95 F, and all tests were at roughly 250 psi regulated tank pressure.

 

We tried the ethane first, to see if the venturi injector would help it light more reliably, but still no luck with that:

 

With the old (recompressed loose pack) bench test motor:

 

0.060 peroxide

0.055 ethane

no light.

 

We switched to one of the new highly compressed catalyst packs for future tests.

On the first run, we forgot to put the ethane jet back in, so it was running with about 6x the ethane it was supposed to have.  Not surprisingly, it didn’t light.

 

We ran it again with the proper jet:

0.060 peroxide

0.055 ethane

no light.

 

We gave up on the ethane, and had much better luck with kerosene.  Quoted Isp figures here are “oxidizer Isp”, since we don’t have a flow meter on the kerosene.  True Isp will be 15% to 20% lower than this, given a 7:1 optimal O/F ratio.  Theoretical peak Isp for 90% peroxide and kerosene with a 200 psi chamber pressure expanding to atmospheric is around 215.

 

We increased the peroxide jet size, hoping that the highly compressed pack wouldn’t run rough when the peroxide flow is cut down in biprop mode.

 

0.080 peroxide

0.020 kerosene

34 lbf thrust, smooth

149 Isp

 

0.120 peroxide

0.030 kerosene

47 lbf thrust, a little rough in biprop mode

170 Isp

 

0.120 peroxide

0.036 kerosene

47 lbf thrust, a little rough in biprop mode

193 Isp

 

0.100 peroxide

0.036 kerosene

smoother

 

We wanted to see what it looked like leaning out the mixture:

 

0.100 peroxide

0.026 kerosene

179 Isp

 

0.100 peroxide

0.022 kerosene

169 Isp

 

0.100 peroxide

0.018 kerosene

39 lbf thrust

143 Isp

 

That was as lean as we could run the jet, and it was still burning smoothly, although it was obviously less powerful from the noise level.

 

We then started running it rich:

 

0.100 peroxide

0.040 kerosene

187 Isp

 

0.100 peroxide

0.044 kerosene

193 Isp

 

0.100 peroxide

0.050 kerosene

194 Isp

 

We were out of time, so we stopped there.  Given the relationship between these “oxidizer Isp” and real Isp figures, we were probably past the true Isp peak, at around 164 seconds.  This was somewhat better than we had gotten with the ethane on the rare occasion that we got it to ignite, and a drastic improvement over the previous kerosene injectors, so the high speed flow at the injection point is definitely a big improvement.

 

From 250 psi tankage, the best we can realistically hope for is a 200 Isp at sea level, and even that is a bit optimistic.  The low design pressure is based on light tanks and radiatively cooled engines, but if we wind up with a different cooling strategy and custom tanks, we could increase the chamber pressure and get another 10% or so.

 

 

TZM Chamber

 

Our radiatively cooled chamber is finally ready for testing.

 

http://media.armadilloaerospace.com/2002_08_10/chambers.JPG

 

From left to right: uncoated TZM chamber, silicide coated TZM, burned-through aluminum chamber, current aluminum chamber.

 

This has the classic R512 silicide coating used by NASA since the late 60’s for oxidation protection of refractory metals.  The standard material for radiative chambers is usually C-103 columbium, which we may switch to in future engines.  TZM (molybdenum) has somewhat better strength at temperature, but has a ductile-to-brittle transition temperature of only 60 degrees F after thermal cycling, so it isn’t considered a good material for thrusters used in space.  It shouldn’t be a problem for us.

 

Radiative engines trade expensive materials for simplicity of operation.  The raw material was $44 / lb from H.C. Stark, and we contracted the machining out, because it was a bitch to turn.  The coating was twice as expensive as the base fabrication, but it should only scale weakly with increasing size, being mostly labor and process based.  The coating looks like paint primer, and has a slightly rough texture to it.  We are being careful about scratching it, although it does have a metallic bond to the base metal, and is somewhat self healing.

 

Our target of 200 psi chamber pressure is on the high side for a radiatively cooled engine, so we added some additional drilled holes to the injector to let some unburned catalyst exhaust provide a film cooling blanket around the outside, and we made sure we were at a rich O/F ratio.

 

We had an interesting set of issues with our testing.

 

Things obviously got very hot when the engine had been running at an orange glow for ten or twenty seconds.  We started insulating our fuel solenoid after the first couple runs.

 

We were expecting to warped the brass retaining ring, which did happen, and contributed to bending the retaining bolts. We started putting the bolt heads (socket head) on the chamber side and the nuts on the catalyst pack side, which helps some with rigidity.  We need to make a stainless steel retaining ring.

 

The brass injector did not seem to suffer from the firings, but it is a really heavy hunk of metal.  The injector should generally be ok, being “cooled” by the catalyst gasses on the side opposite the flame.

 

The radiative chamber has an interesting benefit in that you can see where combustion is taking place by how quickly it heats up to orange heat.  Our injector is clearly getting a lot more fuel on the bottom side, opposite the injection point, even with the faster venturi mixing.  The next injector we make will have two opposite impinging streams, and an even smaller venturi.  It looks like we still have 15% combustion performance or so to pick up, and this may help.

 

The completely unexpected problem we had was that we couldn’t get a good seal between the injector and the TZM chamber.  All of our brass and aluminum chambers have either been bolted directly against a brass injector, or just had a copper flange gasket in between, and we have not had any leakage problems.  Every run we had resulted in various degrees of gas leaking out past the chamber, and eventually bursting into flames.  We tried one of our copper gaskets, then two different Cotronics high temperature sealants, but it never held a seal.  The high temp thread sealant worked best, holding a seal for the first five seconds or so of the burn, then letting things leak after that.  The combination of the rough surface texture of the coating, and the drastic thermal issues are making this challenging.  We are going to try some alumina-silica gasketing next, then move to an energized metal o-ring if necessary.

 

We started testing with the rich jetting, which should be nice and safe:

 

0.100 peroxide

0.050 kerosene

It ran rough in biprop mode, meaning that our highly compressed pack had lost its exceptional smoothness, and seemed to be reverting to the same characteristics as our previous packs.

 

0.090 peroxide

0.050 kerosene

A little rough on the first fire, but smooth on the second.

 

0.080 peroxide

0.047 kerosene

Acceptable burn, but thrust is down to 39 lbf, which means chamber pressure is down 25%.  We don’t mind at the moment, as it is less stress on the chamber.

 

We tried a few runs with the ethane, still with no ignition:

 

0.080 peroxide

0.070 ethane

 

0.080 peroxide

0.080 ethane

 

0.080 peroxide

0.090 ethane

 

We went back to kerosene (still fighting the sealing problems over all these runs):

 

0.080 peroxide

0.047 kerosene

Rough in biprop mode.

 

0.070 peroxide

0.042 kerosene

Still a bit rough.  The pack seems to be worse than the old one for smoothness now. (although this is very low flow rate)

 

0.060 peroxide

0.030 kerosene

Still moderately rough.  We had graduated our kerosene sightglass, so we could tell that this flowed about 600 ml of kerosene for the 2000 ml of peroxide, which, at 0.81 kerosene density and 1.40 peroxide density, is a 5.8 O/F ratio, moderately rich.

 

We did have over 60 seconds of hot fire on the chamber without any problems, but the sealing problems and roughness keep it from being a completely satisfactory day of tests.

 

http://media.armadilloaerospace.com/tzm.mpg

 

(amusing shot of us “blowing out the burning rocket engine” at the end…)

 

Vehicle Work

 

The seated lander is completely reassembled, and ready for testing.  We have moved away from aluminum AN fittings where possible, because we have started noticing pitting in the flares on fittings that have been exposed to peroxide for long periods.  We attempt to use 316 SS for everything now, but we still have a couple larger custom fabricated parts (tank thread adapters and the main distribution manifold) that are aluminum.

 

http://media.armadilloaerospace.com/2002_08_10/lander.JPG

 

I need to find a supplier of stainless steel male NPT to swivel female AN/JIC fittings, which we use between our valves and engines, mated with a ground down AN male fitting holding a restrictor jet.  All our other fittings are now stainless.

 

We added some additional foam bars underneath the wire rope isolators to give it a little more cushion on landing, but they may well get ripped off with any side speed.  I think the best bet is going to be getting some large wire rope isolators made with custom wire sizing, so we can get a larger stroke with the same basic weights.

 

Joseph built us a nice cradle for the tube vehicle.  We can use this for working on the vehicle in the shop, and for transporting it to launch sites.  It is large enough to handle our next two planned vehicles.

 

http://media.armadilloaerospace.com/2002_08_10/cradle.JPG

 

Our new parachute arrived, but the spun nosecones haven’t.  I am getting quotes for filament winding nose cones and tail cones for 2’ diameter vehicles

 





 






 
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