We know you lot do wonder just what's afoot down at the Low Orbit Helium Assisted Navigator (LOHAN) fabrication bunker, so we thought we've give you an update on progress on our garden-shed hypobaric chamber experiment. Click here for a bigger version of the LOHAN graphic If you're a LOHAN newbie there's background info on the …
All good fun!
We might be on the ropes financially here in Spain, and Rajoy may be wringing us dry, but it's great to see there is still a sense of fun and adventure!
Sing it LOHAN!
They tried to make me go to REHAB but I said no, no, no.
LOHAN to REHAB
Oh you guys!
I just love ...
... how this project (and the previous one) is such a mix of proper science and advanced engineering on one hand and just a bunch of guys who turn up with a plastic sheet and a couple of carrier bags on the other. I guess that just makes it "engineering" - getting it done with what you need to get it done and no more.
Beer, and lots of it!
Proper garden-shed science can't be done without the input of generous amounts of ethanol
Oil recuperator for the compressor
It would be good to put a nice long vertical pipe at the exit of the compressor, to allow the oil to condense and slide to the bottom and a capilar at the bottom recirculating said oil to the compressor admission pipe.
Else, you may add some oil to the admission pipe before connecting the compressor to REHAB, so it won't run dry too quickly.
As the compressor is for R134a, best use non mineral oil, and in a pinch, 5WD20 oil will do fine for a while.
I know that probably will be a 1 run system, but would be a pity to be unable to do a re-run because of a dead compressor.
Re: Oil recuperator for the compressor
Dead compressor? In a world with a massive shortage of scrap fridges, that might be an issue......
Just how much have you thought about this? How far is that rocket motor actually going to get you? So far the thinking seems to be, "Ah, but gravity is much less at 100,000ft so it'll get us a lot further." I've got news for you.
The radius of the earth is 6,387km. Add 100,000ft to that, and you get 6,409km - the difference could be politely described as "bugger all". Because gravity is proportional to the inverse square of distance, the acceleration due to gravity is about 9.7 m.s^-2, a whole 1% change from the usual 9.81.
Based on the test firing, that 100Ns seems to actually be about 16N for 8 s. Suppose that your rocket weighs 500g, then the force from gravity will be 0.5 * 9.7 = 4.85 N, so the net force on the rocket is ~ 11N, producing an acceleration of 11 / 0.5 = 22 m.s^-2.
That acceleration over 8s will produce a distance of 713m. It'll then be doing 178 m.s^-1, and before gravity overhauls it that will carry it another 1640m or so. So you gain a total extra 2350m. It's not trivial, I guess, but it's not exactly a big gain when you've already reached 30,000m. Less than 10% extra. And if the thing actually weighs 1kg, you're down to 333m additional height. If you can get the thing down to 200g then it starts to get respectable, an extra 18.5km of height.
All this assumes no drag losses, which may or may not be reasonable at 100,000ft, I'm not sure. It also hasn't taken into account how the weight of the rocket changes as the fuel is used up, which may be quite unrealistic. But how realistic is a goal weight for the rocket of 200g?
Re: Basic Physics
Yes, that's right. Fun, innit?
Is this not just a low temperature/condensation ignition type problem?
Assuming the rocket supplies its own oxidant, which it will, then I would have thought sucking the air out would not matter too much. Condensate, low temperature, will be worst case at ground level so if your rocket ignites at the expected temperature at ground level it should be good to go at altitude.
Have a word with your 'local' mil-spec power supply developer/supplier, or indeed UoS. They should be able to loan you an environmental test box. They come with holes including the top where you can mount your rocket so it vents to atmosphere and controllers to dial an appropriate time/temperature curve. They are also 'portable' so you can stick things in the car park for testing.
Plus, and this might be a biggy, you can place the ignition box inside the chamber to check its performance out as well. At the moment you have it sitting outside of your present contraption.
Failing that just stick the whole thing in a bucket of dry ice for an appropriate amount of time and give it a wang. As I say... if it works at ground level it will/should go at altitude.
Beer because you can buy some with the money saved.
1) "...mount your rocket so it vents to atmosphere...". As that would be the end that needs to be proven to light in the test conditions, that's probably not a great idea.
2) As these off-the-shelf solid motors are invariably lit on the ground to propel something to altitude, assuming that they'll light in a low pressure environment ain't good enough. Fine in theory, but a quick test will prevent the possibility of a massive balls-up come the glorious day.
As I say... low temperature/condensate problem. Assuming a 'sealed unit', which it probably will be, just wait a bit longer for the business end to get down to temperature/[worse]condensate at ground level. Once again since the thing supplies its own oxidant it is not going to be bothered about the lack of it at altitude... if you do it in a bucket of dry ice then it is likely that it will not be seeing much atmospheric oxygen anyway... they probably come 'pre-sealed' anyway. Make the test worse, rip the seal out.
Yadda Yadda Yadda Whatever.
Why is the compressor inside the device?
Er, it's not. It's attached to the device by a tube.
Take Hi-Vac 101 Course Before Beginning
This test chamber will not work as shown for several reasons. Please do some reading about high vacuum chamber construction before you commit to purchasing materials.
As an example, the "gasket" seal (as shown) will get sucked into the chamber - need a hi-vac O-ring in a capture channel.
Leads from measuring devices need a proper feedthrough (not across the chamber opening seal).
Partly for the garden shed engineering angle, partly because it would make a good seal, and partly you're mixing LOHAN, REHAB and a lot of lube.
High vacuum grease is better: better seal (high viscocity), low volatiles.
It's relatively inexpensive, available in small quantities. Or, head to the nearest university chemistry department and cadge some.
Maintain thrust, blow its load early, or fall flaccid
There's no air filling the gaps and defects in the rocket that would otherwise slow and cool hot exhaust gasses, so it could explode. There's also less maintained exhaust gas pressure against the propellent so it could fizzle. Either way,wear goggles and be prepared to tell the rocket that it could happen to anyone.
A lot about nothing.
You mention a vacuum for test purposes. I think the best your fridge compressor (compressor!) can do will be about plus 5psi. No vacuum I'm afraid.
If you are serious about a cheapo vacuum producer, get you hands on an old, very small, air compressor, reverse the pipe connections, and hey presto you've a vacuum maker that will really suck!
Use a plastic tube water column for measuring your "vacuum", looks extremely Heath Robinsonish but it will give you an indisputably accurate measurement. First principles, you see.
Good luck chaps.