Level 2 is hard

A lot of people have advised me to ignore the 180 until the 90 is in the bag. Its probably good advice, but I’d like to start doing some 180 second development tests when we go out to the site to test the 90 second vehicle. The trip to the test site is a significant portion of the time involved with a test and so I’d like to use that time for testing both if possible.  I did a very detailed model last night. I started with the following assumptions:

• It needs to stay as an amateur vehicle IE total impulse < 200K lb/sec


• I’ll use an off the shelf LR-101 as the motor


• I’ll use the same AMS industries Spun 5086 aluminum hemispheres for tankage that we used for the 90 second vehicle.


• I’ll assume that the LR-101 ISP starts at 200 at design Cp and degrades along the same slope as Cpropep down to the minimum Cp (Cpropep says LR-101 should give 230 Isp at design pressure) This discrepancy is probably because the LR101 is significantly over-expanded at sea level.

I included in the model:

• Pressure Drops in the LR-101 Jackets, and Injectors


• Isentropic Gas expansion in blow down mode.

Given these assumptions I could not build a 180 second vehicle. so I modified the assumptions:

• Add one Carbon fiber SCI 602 presurization bottle and Regulator to use on the LOX side. Allowing us to  fill the Lox sphere  past the point where blow down mode would run out of pressure.

Given these assumptions the model says we can hover for 200 seconds.

This probably won’t work for the following reasons:

• The motor is WAAAY over-expanded for the entire flight. We could use all the LR101 dimensions and injector and build a motor that has the same internal dimensions and lower expansion ratio. this would likely give us back some ISP, but we are no longer off the shelf.


• The motor needs to throttle 4:1 and as the pressure drop in the injectors goes down that low then we probably don’t get good mixing.


• The outcome is really sensitive to the initial loading conditions over/under filling the Lox tank by 5% causes a 10 second change in the hover period.


• We need to get the same hydro test performance out the tanks as Armadillo is getting and our first tank was 25% low, we have not yet been brave enough to test the 2^nd tank to the level we need.


• It would be really hard to keep the mixture ratio matched exactly over a 4:1 throttling ratio without really good closed loop controls.


• The LR 101 is hard to get lit.See the SDSU rocket at 1:42 into http://www.youtube.com/watch?v=Wzl_IaSJx28 

Some additional thoughts on the 180 second problem:

If I use different sphere sizes and thicknesses and put on lots of pressurant bottles I can get my detailed model to say we hover for 230 seconds, but the same issues identified above apply.

The 180 second level 2 is all about getting good mass ratio and ISP. For non-pumped systems the density of the propellants really matters. I’m using 1.1 as my Lox density 0.8 as my RP-1 density. I really think Peroxide would be better, 90% peroxide has a higher density than Lox 1.36, and it also makes up a higher percentage of the total propellant load so the portion of your propellant at 0.8 is lower. I’ve seen Density * Isp ^2 as a figure of merit (FOM) in SSTO studies.

Examples :

• Peroxide RP1 at 200 PSI running at best ISP has ISP 215.6 and Density 1.246 FOM: 58K


• Lox RP1 running at LR-101 mixture ratios and 200 PSI has ISP 219 and Density 0.993 FOM: 48K


• Lox RP1 running at best ISP and 200 psi  has ISP 230 and density 1 FOM: 53K

You are much more likely to get the peroxide motor running at peak ISP to cool in regen mode as there is much more cooling fluid available with a much higher heat capacity. Its clear the LR-101 designers with an infinite budget did not choose to give up 10 points of ISP with out trying.

One LLC competitor that had a static display at the 2006 xprize cup, had a system with electrically driven positive displacement pumps using peroxide, liquid catalyst and a fuel. this insures constant mixture ratios across the entire throttling range and makes your FAA safety system really easy as you just have a power relay that drops power to the pump and all propellant flow stops. Your tank thicknesses  are set by minimum gage issues not pressure requirements.  If one developed this it would also be easily transfered to vehicles with more aerodynamic tankage than the big spheres. (Space here we come….)

You know rockets have taken over your life when...

You know that rockets have taken over your life wihen the Google street view of your house has the truck parked in front with a rocket test stand and an empty LOX dewar in it. We never parked there with a full dewar. The combination of things in the truck puts the picture between May and Sep of 2007 The real estate sign is not for our house.

Performance Dreams and Spread sheets....

According to Propep 85% peroxide has the following “frozen” ISPs at our altitude.

• 250 PSI :125


• 162.5 PSI 118


• 75 PSI : 103

Our realized ISP was about 83.

If we had gotten 90% of the lowest number: or 92.7 we would have hovered for 99 Seconds.

Checking Cpropep  against the FMC peroxide manual we get 130.32 (propep) compared to their 129.0 comparing like to like. (250 PSI 90%, 1ATM Pe)

So one possibility is that we switch to a solid catalyst. This sheds some weight, but lowers chamber pressure for the same tank pressure…. crunchingg the ISPs second by second as the chamber pressure goes from 220 down to 72  assuming 80% of theoretical performance we get 100 seconds. Assuming 100% we get  124

If we bump from 85% to 90% HTP and switch from N2 to He for our presurant (Our back up reserve boosters) still assuming 80% of theoretical we get 109 seconds.

One of the problems is that if you set the expansion ration so you are good at your lowest presure you are kind of stuck with that ISP for the whole flight. So you plan on running underexpanded for 50% or so of the flight. I’ve not found a good way to model performance underexpanded.

 

 

 

 

 

 

 

Condolences to True Zero and Congratulations to Armadillo.

Condolences to True Zero and Congratulations to Armadillo.

I have mixed feelings about TrueZero, I know the agony they are feeling, yet I'm a little bit personally relieved that I made the right choice to not compete this year.

Armadillo's third years the charm! Armadillo did 1/2 the task at the contest at least 5 times (I've lost count) before winning, they really deserved the win.
Now on to Level 2.....

Reproducing.....

2nd thoughts about a rational choice.

From a development standpoint we are at about the same technical level as True Zero, they decided to go to the contest we did not. Seeing them go has caused a lot introspection on my part for the last few days. Emotionally the decision was hard, rationally it was and is the right decision. 

First off we promised both the FAA and the Xprize foundation that we would demonstrate both a 90 second hover and a free flight. The 90 second hover  requirement was in our FAA application and in the LLC rules.  We could not accomplish that before the go/stay decision needed to be made. While I could have asked to waive the requirements, personally I don’t like waiving safety based rules. As a general principal establish your safety rules before you go out to the field and don’t violate them with out a pow-wow of all involved to think about what you are violating. I can’t say were 100% perfect on this front, but we try.

Secondly I saw no reason to risk the vehicle. When I was my sons age my Dad and I designed built and raced outboard hydro planes. We were successful with what we did. We placed at the nationals with our first joint boat design.  This taught me one thing  things never go as well at the competition as they do in practice. If you have an issue every tenth practice it will appear in every competition, thats just the way it happens.  We aren’t afraid to loose the vehicle, we just don’t think it makes sense to risk it until we have worked out more of the bugs. We have complete spare airframe all fabricated, it passed hydro last week.  I have a spare of every single part on the vehicle with two exceptions: I have not welded up the spare motor because we need to work on the mixing problems. I haven’t built a new vane assembly because I want to upgrade that to Rene 41 for possible biprop use. 

Third in order to go to the competition I would have had to write a 22K check for insurance. I would say without getting it done in practice we would have a 10% chance of success. All this for the possibility of taking 2^nd place. I have no illusions after watching the Armadillo team center on the pad with their camera  in real time, we could not beat that for the tie breaker.

If Armadillo takes the two first prizes that leaves 2 second prizes to be won. True Zero could take it, but given my own estimation the odds of that are low.

If the first prizes are won then its possible that the rules will change to first to demonstrate rather than a central exhibition.  This would allow me to demonstrate at my “home field” under my waiver without having to buy insurance. This could happen as soon as midyear. So I wait 6 or 7 months, get to do a lot more practicing, save 22K and improve my chances.  

A rational decision…. emotionally its still hard to stay home and watch others toss Hail Mary's.

We came so very close...

I just reviewed the data from our successful and unsuccessful flights.

By backing the realized thrust coefficient out of the successful flights and the chamber pressure from the unsuccessful heavy flight we are within  6%. If we had 6% more  Tank Pressure or throat area we would be flying.   Argggggg!  This would mean Hydro+ing the tank to ~ 400 PSI. I’m almost tempted to go out in the garage and pump it up to that point.

Our realized ISP was only 65% of theoretical(Adjusted second by second for chamber pressure ISP changes) If we could fix the catalyst mixing or had chosen a solid cat instead of the hated purple menace we would be golden! If we could hit 75% of theoretical ISP we could reduce the fuel load enough to do it.

If wishes were horses beggars would ride.

Best luck to Armadillo and TrueZero

Were Done for the Year.

Keven met us at FAR and we tried a 90 second hover. We just did not have enough thrust to lift the vehicle with the fuel necessary for a 90 second flight. We had margin in the system design, but not enough.

We never did get the mixing to work well enough to get proper decomposition. This lowered ISP meant more fuel to carry, higher liftoff pressures and higher flow rates on the flow orfices, we had some margin in each area, but the low ISP and poor mixing ovewelmed the margin. We can fly for 60 seconds, maybe even 75, but not 90 seconds with 25Kg of payload. We will try again in 2009 if there are prizes remaining.

 I assume that at least the 180 second the 2^nd place prize will be unwon.

I’m going to take some time off and relax, I want to thank all the people that have wished us well and folowed our journey. I also want to wish the Armadillo and truezero teams good luck.

Poor Mans Metrology

Many months ago we hydro tested our Sphere (370lbs) , Yesterday we tested our Backup vehicle sphere (385lbs) ,today I tested our new catalyst tank (475lbs) . These pressure tests set the upper limit (with some safety factor ) of what pressure we can operate the tanks at. In a traditional aerospace company everything involved would be “calibrated” and traceable to NIST. We can’t afford that so we do the next best thing we calibrate the pressure transducers used on the vehicle against the beat up old pressure gage on our hydro tester.

So I pumped up the transducers while hooked up to the vehicle electronics and compared what the telemetry said against what the gage said. I’m happy to report that they match exactly.  

The new Cat tank is installed, the repositioned catalyst injectors on the motor are done and the valves and vehicle all check out. My son is on the way back with a Swageloc fitting we needed to hook up the last new line to the new cat tank.  We are still on track to leave tonight and we will try a short translational flight under tether and then a long 90 second hover under tether at daybreak Wednesday morning.

Still trying......

We will keep you posted on any new tests. Today we receive our new 316 Stainless Cat tank. If all goes well and we can arrange for an early morning meeting with Kevin we may try a long hover Wedensday.

Status uncertain....

We flew a 59.5 second tethered hover with payload today. For the amount of fuel loaded we needed to get to 65 seconds. We thought we ran out of peroxide, but on more examination we ran out of catalyst, and the last few seconds of flight were in water rocket mode with the remaining peroxide.

We ran out of catalyst because our catalyst tank sprang a pinhole leak in one of the welds. That ended todays testing. This looks like it might end our year as well. We drove home to try and adapt one of our 14” spherical tanks to replace our leaking 10” tank and it just wont fit.  In driving home we had a long discussion, I’m for calling it done for this year and trying to pick up the 2^nd place prizes next year. My Son does not want it to drag on beyond this month, so he wants to make a hail mary attempt for this year.

I don’t know if it is even possible. To make it this year we would have to:

Fix the tank problem and go out and test with Kevin for a 90 second tethered flight on Wedensday. Then go do the free flight next Sunday, 5 days before the contest. Can we still satisfy the Xprize and the FAA with less than 5 days between the possible required test and the contest?

We just don’t know. We will keep you posted.

True Zero is close, but they are just about to discover that their performance is not quite what they expect…. (see the comment at the end of their last post) We are struggling with that. It looks like it may be an Armadillo only show this year again,  we gave it all we had and it looks like we might come up just short….

 

Update...

Wedensday was scrubbed by problems with our remote quick disconnects, So we spent Thursday redoing them from scratch… Much Much better. We were going to leave Thursday night, but th wind Gods in moajve did not think we should fly…. gusting winds, wind warning and blowing sand yuck.

So I spent the morning going over the vehicle with a eye for potential problems. I found an issue with the vane actuator wireing harness that was not quite right and fixed that. We’ll do some flights on Saturday, but FAR is going to have a lot of people out for various projects nearby so we may not get to fly saturday at all. My waiver requires that I clear the uninvolved from the area for flights over 14.999 seconds.

All in all were ready to go out testing  4 hours before we are due to depart! I’ll keep everyone posted when we return.

 

 

 

 

 

 

 

Two weeks from Today...

We need to leave for the XPC two weeks from Today.
We are going to try and test very early Wednesday morning (If Kevin can make it).
(Scrubbed, our repaired quick disconnect broke. Parts arrive Wednesday.
we will try for Friday morning.)

We are also going to try and test either Saturday or Sunday.
(Depends on people and Scheduling at FAR and the ability to clear the area of the uninvolved.)

If we have not completed the 90 second tether and the free flight by Monday we are realistically done for the year. Its been a real push to get everything done and the lack of sleep is starting to wear on me. I'm at the point where the push to complete is taking a toll across a broad spectrum of life, physical, emotional, relationships, work are all strained.

I've been pushing for 18 months so I can push for 16 more days, but I'm looking forward to Oct 26th one way or the other.

Status...

• We built tested and installed a new emergency vent valve to replace the one that got snapped off.


• We finished rewireing the top end of the vehicle.


• We repaired the bent vane mount.


• We modified the motor so we can change catalyst orifice sizes.

We are waiting on several next day parts orders but the tasks for today are:

• Reassemble and calibrate the vane actuators.


• Change the position feedback on the main valve to be more robust and visually verifiable.


• Redo the bottom end wireing


• Buy more Tether (clmbbing rope)


• Replace a damaged battery


• Replace all the battery tie down straps as they were damaged by the peroxide.


• Replace the 1” Peroxide check valve with a 1.5” checkvalve.  (One of the parts we are wating for)

 

 

 

 

Going for it...

We are going to keep trying for at least another week. I’ve confirmed that the main valve had sensor clocking issues.

 

 

Notes and Video

(This post won’t make sense witout the previous one) After reviewing the data I think we have two problems. We have some kind of issue with te Motor and consistant throttlings. It may be as simple as having the magnetic sensor that monitors the throttle valve position be wrong/loose.

The second problem is a heavy weights it vibrates a lot more this is causing problems measuring the GPS velocity, feeding back into the target pitch/roll to hold position loop causing the osscilation you see at increasing amplitudes on the video.

No decision on plans for this year. I need to look carefully at the damage.

Here is the video of the fateful flight. (I found it very difficult to watch) I’m still not sure how a vent on top of the vehicle can spray peroxide everywhere when the tank is less than 1/2 full.

Good news we got data right up the the end of the flight, the remote commanded shutdown of the vehicle works, the tethers and vehicle structure worked as designed , and the peroxide cleaned most of the spilled purple menace off of the pad. (It also damaged the tether ropes enough that they need to be replaced)

Murphy is a skilled and devious opponent

We tried a 90 second Tethered hover today. It did not go well. The vehicle would intially lift off then thrust would decay and it did not have enough lift. This may be a catalyst issue. At the same time we had dynamic stability issues at the heavy weight. We tried 5 times and on the 6^th flight we solved the stability issue and murphy struck.  The vehicle lifted off flew stably and  climbed too high. When we hit the abort it got tangled in the tether and with close to 600lbs of weight it crushed all four actuators on the control vanes then fliped over tearing off the emergency event, showirng everything on the rocket with peroxide. The peroxide melted or damaged almost all of the wireing on the vehicle. All in all its probably a “60 hours of work event.”

We need to depart for the contest in about 21 days. We would have to have zero additional “big events” to have a chance. Its been an emotionally trying day. I’ve been up for 18 hours and I’m going to make a probaly futile effort to get some sleep. I need to make a difficult decison in the next 24 hours.