February 4 15 Notes
Sorry for skipping an update last weekend, ran out of time
We spent some time very cautiously investigating a potential
A Russian rocket engineer that had worked at Energomach sent
me a document with his thoughts on X-Prize vehicles. He covered a lot of ground, some that I agreed with, and some that
I didnt. One particular thing he
mentioned, however, was quite interesting:
Propellant: unitary fuel mixture of 50% water
solution of hydrogen peroxide (H2O2) and 8% of ethanol. It is well-forgotten
unitary fuel developed by Germans in WWII and containing the oxidizer (H2O2)
and fuel (ethanol) in one. When it burns, H2O2 is decomposed by catalyzer
producing the mixture of water steam and oxygen at temperature about 450C that
ignites ethanol. It was designed for gas-generators, but it can also be used
for rocket engines. It provides the theoretical specific impulse of I = 205 (kg
thrust/kg sec) at 30/1 ratio. Practically, counting all thrust chamber losses;
it could be about 180 190 (at 30/1 ratio). It is the steachiometric
combination, so the exhaust contains mostly water steam and just a few percents
of carbon dioxide. Gas temperature inside of thrust chamber is about 800
850C (about 1000 1100K). The fuel is stable, decomposing at 150C and
freezing at 30C. This fuel is much better than pure H2O2 that can provide
the specific impulse of 140 150 only. The propellant and exhaust are
environment friendly and do not contain poisonous and dangerous components.
One fact in here is clearly wrong: 50% peroxide decomposes
to a temperature of basically 100C, failing to boil all the water. Also, mixtures of high concentration peroxide
and alcohols are very very dangerous explosives (we have tested them, and the
detonations are truly something to give one pause), but with that much water to
de-sensitize it, it might be a viable propellant. A little looking around dug up this passage in Ignition!:
At any rate, peroxide is still used as a low-energy
monopropellant, and will probably continue to be used in applications where its
high freezing point isn't a disadvantage.
One such application is as a propellant for torpedoes.
(After all, the ocean is a pretty good thermostat!) Here it is decomposed to
oxygen and superheated steam, the hot gasses spin the turbines which operate
the propellers, and the torpedo is on its way. But here a little
complication sets in. If you're firing at a surface ship, the oxygen in
the turbine exhaust will bubble to the surface, leaving a nice visible wake,
which nor only gives the intended victim a chance to dodge, but also tells him
where you are. BECCO (Buffalo Electrochemical Co.) came up with an
ingenious solution in 1954. They added enough tetrahydrofuran or
diethylene glycol (other fuels could have been used) to the peroxide to use up
the oxygen, letting the reaction go stoichiometrically to water and carbon
dioxide. The water (steam) is naturally no problem, and CO2, as anybody
knows who's ever opened a can of beer, will dissolve in water with the help of
a little pressure. That solved the wake problem, but made the stuff
fearfully explosive, and brought the combustion temperatures up to a level
which would take out the turbine blades. So BECCO added enough water to
the mixture to bring the chamber temperature down to 1800F, which the turbine
blades could tolerate, and the water dilution reduced the explosion hazard to
an acceptable level.
We thought it was worth a try, so we prepped one of our tiny
1 cat pack engines and biprop test chambers for some very very cautious
It turns out that our catalyst packs cannot effectively
decompose 50% peroxide. Even with a
very low flow rate, most of the liquid comes out with very little
decomposition. Even with perfect
catalyzation, 50% would still leave the water content liquid, but there should
be quite a bit of obvious gas flow as well.
If a little shot of peroxide was pulsed into the engine, it would fully cook
off eventually, but any kind of a flow would quench it. This is rather to be expected with so much
water taking up valuable catalyst-contact surface area. Liquid catalysts can work much more
effectively with lower concentration peroxide, but then you have all the
plumbing of a biprop, with only the performance of a monoprop.
Since auto-ignition didnt seem to be at all viable (which
we rather expected), we tried a couple solid propellant ignitor slugs to see if
we could get combustion going. We didnt
have any luck with a couple tests, and we are scared enough of the dangers that
we arent going to pursue it any farther.
Also on the peroxide front, just for completeness, we made
contact with Solvay about supplying high concentration peroxide. They used to produce 85%, but after our
local sales rep checked around, she found that they do not have the capability
at all anymore. FMC and Degussa seem to
be the only large scale producers in the world.
Hatch and Nose
We got the hatch reinforcement welded to the side of the
cabin, and the crush cone mounted. We
also have the hatch sealing lip and the hatch itself ready, but we probably wont
install those until we have the full sized tank to mount the cabin on for
pressure testing. The hatch
reinforcement needs to be strong enough that the hatch isnt a buckling point
in any axis, which basically means it has to weigh nearly three times as much
as whatever you cut out of the base cone for the hatch. We used ¼ thick plate for the reinforcement.
Working with the big vehicle is starting to get challenging
because of the size. We are moving into
a new facility in a couple weeks, which will allow us to set up some dedicated
areas with scaffolding and multiple hoists for manipulating the vehicle.
The crush cone is 0.050 thick aluminum (except for the 12
hemisphere nose, which is 0.125), secured to the 0.125 thick cabin cone with 31
¼-20 nutserts, a couple inches above the main pilot bulkhead. The cone weighs 37 pounds, and should be the
only thing discarded between vehicle flights.
Big Crush Test
We did our full size, full weight drop test this
Saturday. To drop a 2400+ lb load, we built
a custom electric release mechanism. We
needed to do this for drogue and main releases on the full size vehicle anyway,
so this was a good opportunity to test them out. We used the same trunk release mechanisms we are using for the small
vehicle drogue releases, but we built a mousetrap load amplifier that gives a
15:1 lever arm. The small releases were
able to handle 850 pounds without breaking (although they did need 24 volts to
release at that weight), so a pair of amplifiers like this are more than
adequate for the worst abort drogue loading on our vehicle design.
The design worked perfectly through all the tests. This one is set up to hang between two D-rings
for crane or helicopter drop tests, but the vehicle ones will be machined
integrally into the bottom tank flange closure.
We mounted the 50g accelerometer on a small piece of foam
with hot glue, and stuck it to the pilot bulkhead in the vehicle. The hope was that a little bit of isolation
would remove the large shock vibrations that we saw in the small vehicle drop
We had spent a while discussing ways to rig up something to
drop the vehicle from a sufficient height, but we finally decided to just rent
some heavy equipment for the day.
Joseph has experience working with it, and the several hundred dollar price
was well worth it.
We welded four very strong D-rings onto the inside of the
boilerplate tank end we have the cabin mounted on, and used high strength ½
chain for everything. We used 50 pound
sandbags for ballast, loaded into the tank end before the drop test, with the
total weight measured by a scale above the latch release.
The drop test was done with 2400 pounds, with the nose ten
feet above the ground, for a 0.79 second drop time and an impact speed of 25
fps. These are ballpark figures for our
X-Prize vehicles landing configuration.
Basically, everything worked perfectly, except for one thing
we didnt have enough pressure vents, so the cone acted like an air-spring as
well as crumpling. At about 1/3 volume
crushed, the inside pressure may have been as high as 7 psi, which equates to over
four tons on the back of the pilot bulkhead.
Our flox fillets popped off, and the entire bulkhead shot to the
bottom. You can see this in the video,
as the crumpling finally changes to a buckling, then the bulkhead pops up
visibly through the hatch. We had a
large area of epoxy bond holding the bulkhead in, but pushing it down towards
the wider part of the cone put it in tension, and it obviously wasnt
enough. Surprisingly, the bulkhead isnt
damaged at all, this 2 honeycomb paneling is some tough stuff. We are going to do a more aggressive surface
prep on the aluminum when we bond it back in, and we are also going to weld
some custom brackets above and below the bulkhead to mechanically hold it in
next time. We are going to add several
1 diameter vents in the crush cone, so it shouldnt see an overpressure
anyway, but we are going to triple-fix the problem.
The accelerometer still had the back and forth vibrations, so
Im still not sure if they are a sensor issue or a true reading. If I scroll back in the data, there is a
nice, clear transition from 1G to 0G when the release is triggered, then the
drop time before the first impact. The
initial impact to squash the top dome and start the cone crumpling is higher,
then the cone crumples at a fairly constant rate, then the bulkhead pressurizes
up and pops, absorbing most of the remaining energy, then the cone tips over
and bounces once.
We are going to re-test in the same configuration without
the overpressure, as soon as a new crush cone is fabricated. It should continue the even crush quite a
bit farther without the air-spring effect, and we need to make sure it still
absorbs all the energy.
We have our drogue ejection system for the small vehicle
basically worked out. One of our
priorities is to be completely pyro free, and also minimize the unique consumables
for flights. After some discussion on the aRocket list, Rick Weber graciously
offered to fabricate a system for us that would use the little 12 gram CO2
cartridges. He did quite a bit of
design and experimentation work, but in the interest of getting us something
quickly, he built a simple adapter that punctures the CO2 cartridges and
converts to 1/8 NPT, which was plumbed through a solenoid. He tested it in a piston of the size we were
going to use, and sent it off to us.
When it arrived, we hastily put together a piston out of
some HPR components Phil had lying around, and tried it out. Our initial results were very poor the drogue
didnt get out of the tube at all. We tried
both orientations for the CO2 canister, to see if the liquid vs gas CO2 injection
mattered, but it didnt. We finally
hooked a nitrogen bottle up to the solenoid, and found that even at 600 psi, it
still didnt punt the drogue to our satisfaction. The issue was that Ricks test used a machined piston with an O-ring,
which sealed well enough to build up pressure.
Our slip-fit piston just let most of the gas leak out without building
We finally swapped out the little solenoid Rick had provided
for the largest one we had, and this time 200 psi nitrogen popped the drogue
out, and 600 psi did it with some authority.
It is possible that the large solenoid would have let the CO2 cartridge
pop the drogue, but we didnt test that combination. We are still going to make a good piston out of aluminum with
O-rings, but since we arent very weight constrained, we have decided to go
ahead and just use a small paintball tank filled with nitrogen for the
ejection. I have a quick connect on the
bottom bulkhead for this, so we can just pressurize the ejection tank before we
fill the main peroxide tank. There is a
pressure transducer under the little nitrogen tank, so we can monitor any
leakage, and I can have a warning light to check before launch.
The new electronics bulkhead is ready for hover-testing the
vehicle, but we are still going to move to more capable actuator boards and
power supplies before free flight testing.
Battery lugs are soldered on, and the main system has dual
batteries. We arent going to have the
same issue that crashed us last time.
I am still having some issues with the compact flash based
drive system. The Ampro bios sometimes
needs a second power cycle to get it to boot from the CF, and, more disturbing,
I seem to get an occasional error while doing disk access. My previous attempts to use CF would always
give errors to the point that I couldnt get my Linux system installed, but
moving to a Synchrotech CF card (instead of the Kingston cards I had) let me
get things running. However, I had some
corruption during my first installation, and now on the second installation
(after reformatting with a full check for bad blocks) I still get the compiler
crapping out every once in a while with a Signal 11. If anyone has any relevant advice, please let me know. I am probably going to go back to the
IDEFLASH drive that never gave me any problems, but it is a shame to stack
another board when the Ampro has the CF slot built in.
One issue I ran into this time was that using 50k ohm
resistors for the voltage divider that is necessary to monitor a 12v battery on
a +/- 10v ADC didnt give a strong enough signal, giving weird values that
changed with the connection of other channels.
Finding some 1k resistors fixed the problem.