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Temperature probes, Powered landing sim

Temperature probes

March 7, 2004 notes

 

Temperature probes

 

We finally started logging thermocouple data to help us get to the bottom of our engine warmup issues.  What we did was drill the swagelok fittings on the engines all the way through, so a 1/8" pipe can pass completely through it instead of butting up against the internal stop.  We then took an immersion thermocouple with a 1/8" shaft, put the swagelok ferules around that, inserted it all the way into the motor, and tightened the swaging nut to lock the ferules on the TC shaft.

 

We can now interchangeably put either a pressure transducer or a thermocouple at any port, which is very handy.  Conveniently, a 1000C thermocouple (K type) and amplifier exactly fits our engine operating temperatures.  We get up to about 1800F under well mixed conditions, but it does go higher under the flameholders where the water isn't mixed well with the flame.

 

Our data acquisition system is now logging eight channels: load cell, tank pressure, between catalyst pack pressure, chamber pressure, valve position, flowmeter current (which we still have never gotten working), upper thermocouple, and lower thermocouple.  None of our engines have ports for both the pressure and temperature simultaneously, but they would be easy to add.

 

We have learned some good stuff with the temperature logs.  When the spark hasn't caught anything in the flameholder, the exhaust is a steady 100C, exactly what you expect from decomposed 50% peroxide.  This answers one question we were always a bit unsure about – the methanol is all completely vaporized at this flow rate, but the water is still almost all liquid.  At higher flow rates the temperature probably goes down somewhat.  When we do get a flame started, the temperature starts going up fast, but there are some interesting changes of slope during the progress.  When the thermocouple is below the final cat pack, it sometimes shows a rapid rise due to flameholding below that bottom plate.  We have seen this in many of our warm-ups, where we get a popping flame out the bottom of the engine, but it doesn’t actually heat the pack very rapidly.  When the flame is held above the catalyst, the top probe warms very quickly, with the bottom lagging by several seconds.

 

Once the flame catches, it takes about 20 seconds at 15% throttle for the bottom of the pack to reach about 950C.  If we let it get that high, throttling up has no clouds at all, it just makes perfect thrust.  However, the flame still doesn’t catch reliably from the spark plug when the engine is fully assembled.  When we increased the plug gap to 0.125”, it seemed like we got perfectly consistent ignition in open pack tests, but with a catalyst and nozzle below it, we had more difficulties.

 

Several times now, we have seen a situation where we make a change that seems to result in reliable ignition, only to have it fail on the second starting of the engine.  We are theorizing that this may be due to different behavior after the actual flameholder has reached a higher temperature.  Because we are right at the boiling point of water, if the engine is cold, the liquid water hits the flameholder angles and slides off the side, while the oxygen and methanol vapor more rapidly curl up underneath, effectively being a gas /liquid separator.  When the engine is hot, the flameholder may be quite a lot hotter, so the incoming water may vaporize on hitting it, which then lets the water vapor curl around with the oxygen and methanol, making it difficult to light.  We have found that if the throttle is briefly opened higher, that the large inrush of propellant initiates a temperature rise, and if it can be backed off to 12% - 17% throttle, it will then continue warming reliably.  This is tricky to do with manual control, but we can easily do it with the computer if necessary.  Our MSD-10 ignition system has also just arrived, so the 5x stronger spark may let us bypass the procedure.

 

I had a theory that our high thrust instability might be due to the fact that our flameholder engines have directly supported the foil monoliths, which act as excellent flow straighteners, possibly giving more laminar flow past the flameholder bars that would be more susceptible to disruption.  All of our previous engines (that didn’t have stability problems)  had supported the monoliths with a perforated metal plate, which induced lots of turbulence.  We built up another production engine with the catalyst over the perforated metal plate, and a flameholder cross in the 7” section below that.

 

The combination of the perf plate, the angle cross, and the 5.5” to 7” step section made a SPECTACULAR flameholder in open pack tests, and with the wider plug gap we were able to light it every time we tried.  However, when we welded it into an engine, it was difficult to get the flame started again, and once we got it going and were waiting for the bottom temperature to reach fully warmed, we saw some of the dreaded sparks coming out the nozzle that usually mean we are melting something internally.  We went ahead and ran the engine, which still demonstrated the high thrust instability.  In later tests we found that it would run stable with our cavitating venturi inline, but that only produced 480 lbf.

 

The first thing that melted was one of the thermocouples, but when we cut the engine open we found that there were various other parts of the hot pack damaged.  From the discoloration, it was clear that the center of the angle cross flameholder got the hotesst, and that the 3” flameholding section was nowhere near long enough for a flame that size to evenly mix with the surrounding flow, resulting in a very hot cross impinging on the hot pack and damaging things.  We can’t afford to make the engines longer, so we are going to have to deal with smaller flameholders.  It may turn out that the best thing to do is just reliably get the flame started on the catalyst support plates that we have always had, since those never showed any high thrust instability like the cross bars seem to produce.

 

Our big order of new ring catalyst just shipped, so we will thankfully be able to replace the parts we burned before that holds us up.

 

Matt has been too busy at Fountainhead the last couple weeks to work at Armadillo, so we don’t have any pics or video of the recent work.  We have also had a new face around the past two weekends -- James Bauer, a local welding instructor, has been giving us some tips and doing some welding for us.  We have never had a failure due to welding, but since Russ is self-taught, it is nice to have someone with more experience point some things out to us.

 

 

Powered Landing Sim

 

I finally got the right algorithm for the powered landing from altitude on the simulator.  It is trivial if you have sensors with no noise or latency, and you have engines that respond instantly, but I planted quite a few simulated vehicles while experimenting with realistic constraints.

 

Decision #1 is what altitude to transition from stabilized falling mode to landing mode.  Eventually, the decent won’t have the engines on at all (or just barely at idle), but our early tests will have them operating at 25% throttle to maintain attitude stabilization after burnout all the way to landing.  The conservative point for this is based on what altitude you exited boost mode, with the important caveat that "exited boost mode" is when the throttle reached stabilize level, not when you started to throttle down.  I kept smacking into the ground too hard until I figured that little bit out.  Eventually, when we are going fast enough that drag at the end of the boost time is significant and our available acceleration at landing is significantly higher than at launch due to propellant weight, the landing altitude will get much lower than the exit-boost altitude, but we will have to conservatively work our way towards those numbers with test flights.

 

The constant decision during landing mode is whether to throttle the engines up or down.  I tried having it mimick the boost acceleration curve or aim for a constant decelleration, but the finite movement speed of the valves made it always lag what you wanted, and hence hit the ground harder than you wanted.    What worked well was to take the current altitude, velocity, and acceleration, and calculate what the velocity at ground level will be (if it is even going to hit the ground under those conditions), and throttle up or down based on if that velocity is above or below our target 1.5 m/s landing rate.  Works great.

 

I still have to do a little work to make it aim for the landing rate 3 meters above the ground to account for gps inaccuracies.  The 1.5 m/s landing rate still gives the vehicle a pretty good bounce, because even though the engines start to throttle down to sustain level as soon as they detect a ground bounce, the vehicle is nearly hovering when the isolators kick back.  The GPS velocity is so accurate that we could aim for a feather-soft touchdown if we had a more accurate altitude signal, but we need to trade it off against what the GPS can be off by.  A laser or rader altimeter, even with a fairly low update rate, could give us the necessary data, but I still have concerns about lasers and landing engine dust.  Differential GPS, with the vehicle basically landing very near the differential sensor might also do the trick, with no vehicle modifications.

 

On long burns, the vehicle can pick up a fair amount of horizontal velocity, landing off horizontally by maybe 5-10% of the altitude.  We can use the GPS to aim for zero horizontal velocity, but to do this we will need to align the vehicle with true north at the launch pad, so it knows which way to tilt to modify velocity.  There will still be some drift, because it isn't aiming for an absolute position, but it should come out very close.  Aiming for zero horizontal velocity on landing will also allow us to use lighter landing gear in the future.

 

With the sub-one-G acceleration that the current engines are going to give, the vehicle isn't going to go very high at all.  A single drum of propellant will not even get it to 1000 feet in a 30+ second total flight, and if we are conservative on the warmup propellant and boost times, it is only going to go a few hundred feet up.  We could creep off the pad with two drums of propellant and still stay within the 3500' box at Burns Flatt.  We will need the big engines to go high and fast, but we will probably still want to reduce their nozzle size somewhat so we don't have ultra-low chamber pressures and possible flow separation during final landing, or 4G accelerations at launch.  I haven't done sims yet, but with the big engines and a launch-license-limit propellant load, it should go in the neighborhood of 20,000' altitude.

 

 





 






 
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