<--- Says it all
Well done lads!
We're delighted – and somewhat relieved – to announce that last weekend we finally persuaded a solid rocket motor to fire at a simulated altitude of 76,500ft (23,300m). Click here for a bigger version of the LOHAN graphic Followers of our Low Orbit Helium Assisted Navigator (LOHAN) mission will need no introduction to the …
What about the 'and return him safely to earth" bit?
a) Include some bacteria inside his head, that is known to live quite well under vacuum and lots of radiation.
b) Include some simple "You are here - home is back here" drawings of the moon and earth.
c) We can include an extra rocket and a box of matches....
And in a few zillion years he can grow an organic brain, figure it all out and return to earth to personally greet his long dead makers....
Looks like the fuse is generating quite a lot of gas, judging by the buildup on the inside of the perspex lid.
Is the vacuum pump capable of keeping the pressure inside the chamber low enough during the period between the fuse ignition and main engine start? If not, then the pressure inside will build up and render the test unrepresentative.
I am curious if the low pressure in the vessel was maintained after the initial firing of the igniter, before the engine fired. The igniter carries a significant amount of oxidizer which enables it to burn in an oxygen-poor environment (like a vacuum). But this also means that when it fires, it releases a lot of gas, which would reduce your vacuum pressure. So while the igniter might burn, it still might not indicate whether the engine would fire at low pressure. The pressure might not be as low as you expect, at actual ignition time.
I watched your REHAB test today with extreme interest. As per some of the other comments regarding the pressure inside the REHAB chamber I question the ability to maintain a high vacuum after the igniter fired. Both the Aerotech and Cessaroni motors use Ammonium Perchlorate Composite Propellant (APCP) which has a positive coefficient of burning with respect to pressure. In other words the higher the pressure inside the motor the faster the propellant burns.
This would explain both the failure of the Aerotech motor to remain lit and the long delay between igniter activation and the full power burn of the Cessaroni reload. The Aerotech apparently burned, but not fast enough to bring the pressure inside the casing up to normal operating pressure before the heat leached away into the motor's casing. The Cessaroni motor on the other hand uses a thick plastic, instead of a cardboard, liner so the heat was retained until the pressure in the chamber increased enough for the motor to start operating at normal internal pressure.
I'd suggest adding a large pressure tank between REHAB and the vacuum pump to maintain the low pressure inside the chamber once the pump is isolated. In addition I would look into adding a burst diaphram to the motors. A simple disk of copper or aluminum made from 0.002" shim stock, with a hole just large enough for the igniter, could be added to the Aerotech motor between the nozzle and the propellant grain for example. You could also experiment with a smaller nozzle on the Aerotech motor. I would contact Aerotech and ask Gary Rosenfield if you could obtain a number of un-drilled nozzels for the 32/60 reload casing. I'd start by reducing the nozzel's exit area by about 50% and then proceed based upon tests in REHAB.
Kenneth G. Holloway
I'd agree, adding a large pressure (vacuum!) tank to the system will act as a buffer to suck the gasses given off by the igniter and therefore minimize the added pressure.
Where you would get a large-ish container capable of withstanding that amount of vacuum i am not sure. Perhaps bodge it from welded steel plate - it doesn't have to be complex, just a hollow box with an outlet pipe.
However I would go against the idea of changing the rocket motor nozzle characteristics. I think once lit, the internal pressure of the motor will not be dependent on the ambient low pressure outside.
Shrinking the nozzle diameter could lead to a rather nasty bang.
In terms of a burst disk with a hole just large enough for the igniter to poke through into the motor, I think that may be over-complicating it. The fuses are designed to be a rather tight fit in the nozzle already, so I doubt an additional diaphragm/disk would make much difference to be honest.
There have been many rockets launched from altitude. While most solid fueled, air-launched missiles fit into this category, their details are mostly classified by the respective country that designed them. However, there are been quite a few rockets launched from balloons ("rockoons"):
I remember that Bill Brown's team, out of Huntsville, AL (USA) launched one not too many years ago, where they were trying to reach space (x miles/km above the ground). Although, I think that the rocket that they ended up using was a hybrid, based on solid Asphalt (Yeah, the same stuff you pave roads with!) and liquid Nitrous Oxide. Someone ought to be able to find it easily enough, given those keywords. :*)
I share my fellow readers concerns about the chamber pressure rising too much before the motor really ignites for this to be a representative test. Is the vacuum gauge linked to the chamber side of the isolation valve or on the pump side? (If it's already on the chamber side a readout there could give us an answer)
I also somewhat agree with Cagey. A different nozzle design might be needed. I would suggest to the LOHAN team to contact the university lads and see if they have someone that knows how to do transsonic and possibly hypersonic gas-flow analysis and make some "back of a floppydisk" FEM calculations to see if changing the nozzle and throat design is needed.
@Prof, rocket engines are designed to work for a certain pressure differential between the chamber and the exit. While the exit pressure is tiny compared to the chamber pressure, and chamber pressure is preferably a constant, it does affect the expansion of the exhaust gasses in the nozzle. A different nozzle design can improve trust, cooling and engine stability when working in a vacuum over one designed for working at sealevel to moderate altitude. (The "big boys" engines usually also come in atmosphere and vacuum application flavours, with slight variations in throat and nozzle design to keep optimum gas flow)
I'd like to echo the concerns about the pressure increase (plus build-up of flammable volatiles) during the igniter burn, facilitating ignition of the rocket motor. You will not have that assist at altitude.
A large tank under low pressure can be used as a vacuum buffer, but, to make a meaningful difference (PV=RT), it needs to be awfully large.
The alternative, once you have dry ice available, is to use a cold trap: put a small, copper or metal vessel or coil between the vacuum pump and the REHAB chamber, and chill down the vessel with dry ice plus a bit of ethanol added to the dry ice to improve thermal transfer (what to do with the surplus ethanol?!). Most of the products of the igniter should condense out well above the -72C temperature of the cold trap. You can now keep the pump running throughout the ignition and burn sequence. If you are paranoid about any residues making its way through into the vacuum pump, a quick oil change will take care of it. Also, it is a good idea to clean out the cold trap between runs.
The buffer tank would indeed ensure a more consistent level of vacuum in the initial ignition stage, but it wouldn't need to be *that* huge, given that it only needs to compensate for the volume of the reaction products of just the igniter, not the rocket as well, since REHAB is actually designed to "blow it's top" on successful ignition.
(not very "scientific" , but it has got that lovely VultureTech touch.. ;) )
A cold trap could be used in the interconnect between the REHAB chamber and the buffertank to try and maintain vacuum as long as possible through the extra condensation of reaction products once the temperature variable is introduced with the Cold Start.
I would like to see a separate test of a frozen engine at atmospheric pressure first to see if it will fire at all even with the improved igniter before the double whopper of low pressure and low temperature is tried.
Fiddling with the actual nozzle might work, and worth having a peek at, but I wonder if simply plugging the remaining space between the igniter cord with something like rice (cigarette) paper would suffice to trap the pressure and heat long enough to ensure ignition.
Ah well.. I guess that's why they call it rocket *science* ;)
*I* believe the first playmonaut on the moon should not be bound by silly "return" prerequisites but should go for a record of some sort, and viewing footage of brave British attempts to beat the Americans at their own game at Woomera gives me the inspiration needed.
The playmonaut's LM should free fall toward the lunar surface. At the same height that the Eagle wimpily cut it's engines, the PLM should ignite a truly awe-inspiring cluster of boosters for the attempt at both the hardest moon landing and deepest penetration of the said heavenly body.
We can pick him up on the second mission.
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