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January 22 and 26, 2002 meeting notes

January 22 and 26, 2002 meeting notes


In attendance:


John Carmack

Phil Eaton

Russ Blink

Neil Milburn

Bob Norwood

Joseph LaGrave


Flight tests


After coming down tilted last Saturday, we decided to shorten the legs to make the stronger, and it seems to have helped out.


The one thing that we noticed after the hop on Saturday was that one of the engines wasn’t tilted very much, so it didn’t have as much roll control as it should have had. I played around with parameters on the simulator, and I was able to get it to do the same kind of roll-and-tip behavior that we saw if I offset the CG and actually put two of the engines canted the wrong way. On Tuesday I double checked, and while the didn’t have a whole lot of cant, the engines were all pointing the correct directions (we have arrows on the frame, but this was a brand new base, so I was afraid I might have drawn them wrong). We increased the cant on the engines, and made sure that they were uniform all around.


We did two more hops, and it still showed the same roll-and-tip behavior, although in comparing the telemetry with Saturday, the roll was now only going to five degrees a second instead of ten degrees a second in the first second after liftoff, so the increased cant was fighting the behavior.


The roll-and-tip is due to the way the attitude control uses the same engines for roll control as for tipping control. The roll control correction is prioritized at 25% of the tipping control, so it initially lifts off with the engines keeping it level even though it is starting to roll. After a half second or so, the roll has gotten to the point that it looks four times as bad as the tip, so the engines are prioritized to fighting the roll, which means that one tip axis begins to loose its control authority. If the CG is offset at all, this will cause it to always start to tip towards the CG if it is off the ground. I have tested some changes to the flight control software that will cause it to never allow it to tip (if it can help it) much, even if it starts spinning like a top, which is probably the desired behavior, but I didn’t put the code on the flight computer, because we wanted to change one thing at a time.


It looked like we could get the roll under control if we canted the engines a huge amount, like 40 degrees or so, but I’m not sure the flight control software would like that. We decided that the problem was due to the banged up nozzle on the main engine. When we had the “splat” crash, the motor exit cone hit the ground hard enough to deform it a fair amount. Russ touched it back up on the lathe, but there is a distinct bell shape to it now that isn’t completely symmetric. The CG is about four inches rearward from the center when we don’t have any pilot ballast on it, so if the nozzle was giving any thrust vector to the pilot’s right, it would cause a continuous positive roll thrust.


We shimmed the engine away from that direction for the tests on Saturday, and it looks like we were correct, because the roll is now properly controlled.


The tether attach points inside the attitude engine are structurally a lot stronger, but tugs on them are causing a lot more problems for the vehicle than when we had them on the outside of the legs. The next time we fly, we will probably connect the two tether points on the vehicle together with chain a few feet under the main engine, and just use a single chain from the lander to the ground. The tether is mostly to prevent a fly-away-and-land-on-someone’s-car scenario, not really to try and keep it upright.


The engines aren’t getting full decomposition. Even though there isn’t a lot of cloudiness, there are a lot of peroxide fumes when it is operating, and the total performance isn’t as good as it should be. We will be making new engines when we have the silver / stainless catalyst packs worked out.


The laser altimeter should be here within a week or so, so we will probably do a set of manually controlled flights to test the interfacing on that before trying to use it for active guidance.





High Temp Engines


Russ chucked up the TZM bar and faced it off to get a little feel for machining it. It isn’t very pleasant, with a narrow range of speeds that cut well.


NASA SP-8124 has a lot of relevant information for our next test engine:




Some highlights:


It recommends columbium alloys over TZM, even though TZM has a higher melting point, better strength at temperature, less thermal expansion, and a higher thermal conductivity.  The primary reason is that after thermal cycling, TZM has a ductile to brittle transition at 60 degrees, which means that it can be brittle at room temperature.  Solenoid thrusters can easily have significant overpressures on initial startup, but if we use a throttled valve, or a sequence of warmup pulses, we should be able to guarantee that we have warmed the chamber above 60 before opening up the throttle.


TZM has an ultimate tensile strength of 14,000 psi at 3000 F, and 120,000 psi at 70 F.


They recommend silicide coatings over aluminide coatings.  Slurry application followed by vacuum furnace curing is recommended over flame spraying. I have two quote requests in for various coating work on our test chambers.


They recommend using clamped flanges instead of bolted flanges for high temperature chambers. As well as reducing stress risers, that also has the benefit of removing some machining steps from the expensive, hard to work materials. There is probably some normal industrial flange clamp that we can use for testing, but I haven’t found anything appropriate yet.


There was an interesting concept discussed in the context of heat sink engines: "interegen cooling", a term which, amazingly, turns up zero hits on all the search engines I tried.


The basic idea is that instead of spraying the fuel into the center of the chamber, it is sprayed directly onto a (thick) chamber wall.  The heat of vaporization is then taken directly out of the wall, instead of out of the chamber gasses, then it functions like normal film cooling.


They talk about it allowing some heat sink engines to run at a steady state with lower pressures, but it seems like it should also be of some benefit to thick walled radiatively cooled engines.


A possible modification of that idea would be to submerge the nozzle throat inside the main chamber, and have the fuel sprayed directly at the back sides of the nozzle wall.  There would be a danger of puddling fuel in the submerged space, but it would direct lots of cooling at the area with high convective thermal transfer loads, while the rest of the chamber just used radiation to cool the lower loads.


In thinking about it more, while I have been looking around for low temperature / low performance biprop combinations, it might not be all that hard to make a radiation cooled peroxide biprop with normal fuels and moderate pressures.  Almost all of the radiation cooled thrusters are nitrogen tetroxide / hydrazine based, which burns quite a bit hotter than peroxide combinations.  They are low pressure, and have to use a fair amount of film cooling, but they also tend to be very small, and radiation cooling benefits with scale up.


In discussing low temperature biprop options, Jeff Greason of XCOR mentioned that there are a lot of alcohols that are water soluble, and they probably aren't all miscable with peroxide (and hence capable of making dangerous explosives).


Furfuryl alcohol and Propargyl alcohol have been noted as "potentially hypergolic" with peroxide, which sounds rather non-miscable :-)  I have just ordered lab quantities of them from http://www.citychemical.com , so we are going to find out. Propargyl alcohol is supposed to have somewhat superior performance to kerosene in both Isp and density, but it is a suspected carcinogen, so we probably wouldn’t use it in production, but we can at least get a definitive answer on the hypergolic question.


We will probably try firing ethane first in our heat sink test motor, because the gas will have quicker combustion and won’t have a danger of pooling.


We had been planning on making just nozzles out of the TZM for hybrid testing, under the assumption that we would run the grain all the way down to the nozzle, intentionally sacrificing some combustion quality for simplicity and a lot of film cooling. Research has shown that the combustion efficiency would probably be a LOT worse than we want. A 2” diameter hybrid test motor with a 1” core needed a 4” post-grain combustion area to achieve good C* efficiency, which would have been 16” at the core diameter. If we need to have chamber space for the hybrid testing, we might as well set it up so we can do both biprop and hybrid testing on the same chamber / nozzle setup. This does mean that we are only going to get two test articles out of our bar of TZM, so if the coatings don’t work well and we melt them, it could take an extra month (and another $1000) to get more TZM.


Rotor Vehicle


The big rotor blades are ordered, and should be here in three to four weeks.


This line of linear actuators looks fast and powerful enough for our gimbal control needs, at about $1700 each:




We are considering possibly using an ejectable fin can to keep stability on the ascent and descent.




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