September 25 – October 6, 2001 notes

 

Fiber Optic Gyros

 

It turns out that we did have one major component die in the crash – one of the FOGs is no longer functioning.  I should have immediately tested all of them, but I had given them to Bob to use in fabricating the new mounting bracket for them, so I didn’t find out that one of them was dead until last Saturday.  I got it Fed-Exed off to KVH for repair on Monday, but I haven’t heard back from them yet.

 

We are grounded until we get this fixed, which is unfortunate.  I have been looking into buying another tri-axial FOG unit so we could improve a few things, and leave the current units as a backup in the future.

 

While I understand that various micro-machined mechanical gyros can be calibrated sufficiently to use in high acceleration environments, the true G insensitivity of laser gyros seems to be a significant benefit to me.  Ring laser gyros offer the greatest accuracy, but they all seem to be up in the six figure price range, so fiber optic gyros are the way to go.

 

Comparing the different FOGs is difficult, because the figures of merit aren’t all quoted the same way.  In general, the lower the sensed rate range, the more precise they are.  100 deg/sec is about the lowest I am comfortable with.  You can easily swivel a gyro faster than this in one hand, but vehicles should be able to be kept below this rate if you don’t need fighter plane quick snap turns.

 

KVH

 

http://www.kvh.com/

 

E-Core 1000 (single axis), $1500 each

E-Core 2100 (single axis), $2500 each

 

Range: +- 100 deg/second

Scale factor linearity, constant temperature: < 0.5%

Full temperature: <2% RMS, <1% RMS on digital output versions

Output: 2.5 V +- 2.0 V, digital output is offered at 10 Hz, and optionally 100 Hz on the 2100

100 Hz instantaneous bandwidth

 

The difference between the two is the bias stability and resolution:

 

1000: Angle random walk: 20 deg / hr / rt-Hz, 0.33 deg / rt-hr

2100: Angle random walk: 5 deg / hr / rt-Hz, 0.08 deg / rt-hr

 

KVH offers a dual axis mil-spec unit for turret and antenna pointing, but no three axis units.  I sent a request for information about reduced filtering for higher bandwidth, but didn’t receive a reply.  Delivery of our FOGs took six weeks from the time of order.

 

Our three E-Core 1000 gyros have worked fine, but a smaller, more integrated three axis package would be nice, and a digital output would allow us to get good enough inertial accuracy for navigation as well as stabilization, and let us continue to use our crappy PC104 A/D system for the less precise sensors (pressure, temp, throttle, etc).  If we stay with an analog output device, we will need to move to an external A/D board to get good enough accuracy for navigation.

 

 

Crossbow

 

http://www.xbow.com/

 

They only offer FOGs in integrated systems with accelerometers, and they range from $8500 to $11000, depending on the options.

 

The most appropriate FOG system, the IMU600CA-201 has the minor annoyance that it requires a 15-30V power source.

 

The vertical gyro (VG) systems have additional software to calculate absolute pitch and roll values from the basic sensors, using the assumption that planes or vehicles are going to be going “mostly” level, allowing the accelerometer to slowly correct long term gyro drift.  That isn’t of any value to us, because we could do the same thing in software.  They offer one version designed for automotive use that has a built in 12V power converter, but doesn’t offer the same 10G/100Hz accelerometer output that we would get from the CA-201.

 

The Attitude Heading Reference Systems (AHRS) also add a magnetometer so it can perform similar long-term gyro corrections on the yaw angle.

 

Range: +- 200 deg/s

Resolution: < 0.025 deg/sec

Scale factor accuracy: < 1%

Non-linearity: < 0.2%

Random walk: < 1.25 deg/sqrt(hr)

Size: 127x152x101mm, 1.6 Kg

 

They offer analog output, but it is really all digital, with the analog output being created by a DAC.

 

I almost purchased one of these last week, but I’m afraid the system latency is a little longer than I would like.  It spends about 8ms internally dealing with the gyros, then it takes another 6ms to pump the 18 byte update packet over a 33Kbaud serial link, which is a noticeable amount of latency on top of the smoothing already performed by the 100 Hz sensors.

 

If this took a 12V power source and could output packets continuously at 115Kbaud, it would be my perfect sensor.

 

 

iMAR

 

http://www.imar-navigation.de/englishside/imar.htm

 

IMAR iTGC-F

DM 28.000 ($12,795) for just FOGs, DM 33.000 ($15,080) with accelerometers

 

Range:  +- 200 deg/s (50 to 800 deg/s optional)

Output: +- 4.75 V

Biad stability: <160 uV

Resolution: < 50 uV (0.002 deg/s with +- 200 deg/s range, that’s 18 bits of resolution!)

Noise (0-100 Hz): <100 uV/sqrt(Hz)

Bandwidth: 0-100 Hz (optional up to 300 Hz)

Linearity error: < 0.2% (compensated)

Power: +5 V in small package, or 9-18 V or 18-34 V in larger package

Shock: 200 g, 6 ms

Size: 80x80x80 mm or 80x80x118 mm, 900 grams (light weight version on request)

 

This is higher end in all specs than the others, but more costly.

 

 

JAE

 

http://www.jae.co.jp/eproduct/p_menu.htm?GENNGO=1&BUNNRUI1SEQ=05

 

Range: +-100 deg/s

Resolution: 0.05 deg / sec

Bias stability: 360 deg / hour (probably a misprint)

 Linearity: 1%

Dimension: 80x80x45 mm single axis, 170x170x137.5 mm tri-axial system

 

I have requested a quote for these.

 

Other Reconstruction Notes

 

Since I was pulling the PC104 computer apart for several tests, I finally bought one of the little plastic Parvus PC104 board separating tools, which I highly recommend.  No more bent pins.

 

I have replaced all the nylon standoffs with metal ones now.  My one gripe with WinSystems is that they only ship two nylon standoffs with each PC104 board.  Very early on, we had some boards pull apart on a tip-over, so we went to four standoffs per board.  We had some metal ones, but they weren’t exactly the right size, and they caused some problems for one of the boards in the stack, so we still had several nylon ones in use.  During this crash, one of the boards broke off all four heads of the nylon standoffs.  I finally got the right size standoffs, so we all metal now.

 

The batteries were secured in the box with epoxied on angle brackets, which broke loose during the crash, allowing the battery to damage some other things.  I have now cut some slots in the bottom of the box to allow a big hose clamp to be wrapped completely around both batteries and through the bottom of the box.  They aren’t going anywhere now unless they tear the box apart.

 

We are going to rotate the entire electronics box on the lander so it is in a more natural position and can be secured with ratchet straps.  The current orientation has the gyros aligned with the engines, which is convenient, but it isn’t a secure mounting positions.  I will have to correct for the new position in software.

 

We are going to statically balance the big lander with ballast so that if the attitude engines are shut down, it should fall straight back down instead of pitching over.  That is going to require about 30 pounds out by the engines opposite the pilot.

 

 

High Energy Peroxide Fuels

 

Michael Carden of X-L Space Systems visited with us last Friday, and we got some very good historic peroxide documents from him that we are going over now.  So far, the most interesting point is that some unusual fuels can deliver extremely high Isp and density-impulse with peroxide:

 

98% H2O2 and BeH2 has a theoretical vacuum Isp (1000 psi chamber expanding to 0.2 psi) of 498 (!!!) at a chamber temperature of 3884 deg K.

 

98% H2O2 and AlH3 has an Isp of 424 Isp with chamber temperature of 3923 deg K and a density of 1.58 gm/cm3.

 

Lox / H2 under these conditions is listed at 470 Isp and a density of 0.227.

 

They didn't have a density listed for the BeH2 mixture, and it seems to be an extremely hazardous material, but the AlH3 sounds interesting.  There was no indication that these combinations had actually been fired, so it may well be too good to be true, but the possibility of making a pressure fed SSTO hybrid is sure interesting.

 

For comparison, an 85% H2O2 / Polyethylene hybrid has a chamber temperature of 2600 K, so these fuels would have a 50% higher chamber temperature.  Due to the metal content, they would probably also be much harsher on nozzles.

 

Long Duration Motor Firing

 

We fired one of the attitude engines for 60 seconds (three gallons of peroxide), with everyone a long distance away, because there was still some concern about the strength of the brass motors if they were allowed to completely heat soak and reach full chamber temperature.

 

The motor came through it fine, but the catalyst pack (which had been used on a few flights already) started tunneling during the last 15 seconds of firing.  On opening it up, there was evidence of both channeling around the outside, and tunneling through the center.  We still need to get better longevity out of our catalyst packs, because they seem to start wearing out after about 100 seconds of firing.  There are likely both flow and material issues.  We have some pure silver foam coming to try out, and we are going to try making anti-channeling rings (basically piston rings) to intersperse with the catalyst discs, and some anti-tunneling buttons to put in the middle.

 

Motor Latency Testing

 

We finally did some accurate timings of the onset and tailoff of thrust in our attitude engines.  The timings are done at 240 hz, and they are adjusted for the sampling latency (solenoid signals, which take effect instantly, versus serial A/D, which is latched and bit serialized).

 

It takes about 28 msec from the energizing the solenoid for thrust to begin.  I believe that about 5 msec of that is the valve opening time, based on the fact that that is the minimum pulse time that will let anything out of the solenoid when it is under pressure.  It is possible that the total time is larger for complete valve movement.  These are already pretty quick valves that draw eight amps of current and get pretty hot, but that might be possible to juice up a little bit by changing to an open-sustain split current approach at a higher voltage.

 

We theorized that some latency could be saved by building a motor that connected directly on the bottom of the solenoid, and packing catalyst right up to the top, without a spreading plate, and we tested that today.  This does let a lot of heat transfer back into the valve, so it would probably require some form of insulator for production use.

 

When we first fired the motor on a continuous run for reference, we got behavior that we have seen at one time or another on all of our motors, but in an extreme case:  it started off fairly smooth, then abruptly got wildly rough.  We attribute this to a compression and settling of the catalyst pack, which opens up an area that can have things thrash around in when there isn’t a significant pressure drop somewhere upstream of it.  It is possible to have an unchoked engine run smoothly (in which case the power drops in nearly direct proportion to the tank pressure), but it is very finicky, and it is almost always easier to get it running well if it has a significant pressure drop.

 

We opened it back up and packed a couple more discs in, and set up a restrictor jet between the solenoid and the feed line.  We didn’t have any jets in this size, so we first tried one of the jets from our really old small engine tests.  It did run perfectly smooth, but at only ten pounds of thrust.  We then adapted it to have a –3 fitting in the line ahead of the engine, which is a pretty tight restriction all by itself.  It remained perfectly smooth, starting at about 50 pounds of thrust, and decaying with the sqrt a tank pressure, a sure sign we are adequately choking it.

 

We then ran the pulse timings, which came out fairly rough.    When I first looked at the data, I thought we were getting a really big improvement in latency, but I was comparing the wrong data.  On proper comparison, it seems to be consistently between 4-8 msec faster.  This is significant, but not enough to bother making new motors and worrying about insulation over.

 

We wondered if the roughness was permanent (another cat pack problem), or if it was related to the pulsing, so we ran another continuous run, which turned out smooth again.  The intermittent pulsing is definitely causing rough feed flow.  The feed hose was vibrating around pretty visibly during the testing, so something we might want to try next is to test with the tank rigidly connected to the motor with no flexible link at all.

 

 

media.armadilloaerospace.com/2000_10_06/continuous.xls

The flush mount motor initial run, then the run with the –3 fitting restriction, and the final run (also with the –3) after the pulses tests.

 

media.armadilloaerospace.com/2000_10_06/pulses.xls

The original engine on top, new engine on bottom.