I can't bloody wait...
Seriously good fun. Wish I worked as a Vulture. Bet Sarah's not having as much fun now...
The Special Project Bureau's elite Hypobaric Evaluation And Design (HEAD) team has spent the last couple of days contemplating your suggestions as to how we might get LOHAN to suck more more effectively. Click here for a bigger version of the LOHAN graphic To rewind a bit for Low Orbit Helium Assisted Navigator virgins, we' …
1) Ensure that the tether's both short and strong enough that the rapidly ascending top plate stands no chance of smashing the camera(!)
2) Another sheet of perspex to protect the camera once the top plate's gone and it's sat in the rocket exhaust(!)
3) Use a high-speed camera. That might return valuable information as to what went right / wrong at the point of ignition.
You'll need to make a few perspex top covers then, because the top plate will be damaged by the exhaust and, unlike a metal plate, a perspex one won't be easy to clean up. If you expect a camera to look through it, then it will almost certainly be a single use item.
Re photography: if the spacing between motor and top plate is small, say 1-2cm, a camera mounted as much as 45 degrees off axis should get a good view. Just make sure that the light sources are behind the camera to avoid reflections. What about using a key fob camera? They give adequate resolution and are very cheap, so it doesn't matter it gets damaged, though a small piece of clean glass in front of it should do no harm optically. I don't know if the frame rate would be high enough, but that would apply to any domestic video camera too.
Polycarbonate would be better - perspex might shatter, while PC won't.
Polycarb is also really, really easy to work with as it won't crack when you're cutting/drilling etc - you can use normal woodworking tools like a jigsaw to cut it without affecting the strength.
I love the stuff.
For the size you're after, you can probably pop down to your local plastic dealer and get some out of the scrap bins for nowt.
Maybe I'm just being slow, but once the motor ignites (assuming it does) the pressure inside goes up. I don't think that happens in space, does it? At least not measurably, anyway. So is this a fair test? Or are you only interested in ignition behaviour and not the full burn?
And yes, I realise that the lid will lift to release excess pressure, but isn't that a safety thing? internal pressure still rises to a value greater than space, thus not giving an accurate idea of the full burn.
Re. Perspex lid, need to be careful you don't melt the lid with the rocket exhaust. I also wouldn't imagine you would see much when the motor was going.
I'll note that the percentage difference in chamber pressure at sea level and at umpteen thousand metres might not be so much, once the motor is burning.
The SRB apparently ran at 900psi at liftoff, so the effects of losing the vacuum would be less than 2%. I don't expect you're using a motor achieving quite so much pressure, so the pressure difference might matter. On the other hand, it might mean that you don't need the vacuum for a useful ignition test.
There's Burning and then there's Burning Properly! It might be worth wile testing with a Burst Plug to get chamber pressure up, and if it does no harm then use it! You cannot know if the low pressure will prevent chamber pressure from building up suitably such that the burn is just a burn out without developing any thrust. Without a massive pressure chamber (such that the exhaust gas does not alter pressure) it is impossible to test if the rocket chamber develops enough internal pressure to produce thrust during burn, or if the rocket will just combust gently without producing thrust.
Something I've been thinking about is the cables that you have stuck into the thing. If I remember my school sciece, most metals tend to get brittle when they are very cold and I believe they contract too. You may want to make sure they are long enough to avoid breaking with contraction.
Of course, I'm going from something I did over 30 years ago and my memory is deteriorating so I could be a little off bat here ...
How about a load cell under the motor or even under the whole rig? I guess you will be interested in not only whether the motor fires, but also whether maximum thrust is achieved quickly or if the fuel pellets have to warm up first. Graphing downforce against time might well give interesting data.
It looks like you've just built yourself a rather odd thermos flask.
I'm no expert, but it strikes me that you won't affect on the internal temperature much once you evacuate the conductor (air) from within the container.
Therefore, I suggest a great simplification - leave the temperature probe in the flask until the correct temperature is reached, then withdraw the probe, seal the top and then evacuate the air. The end temperature will not vary much during this process.
The advantage will be fewer holes to plug, truer temperature readings and generally follows the KISS principle better.
BTW, how brittle will the metal container be at these low temperatures? Is this a safety or otherwise concern?
One final point worthy of consideration - the dry ice will create copious amounts of a mist-like low-lying cloud, which will hamper finding and reading valves and gauges unless this is taken into account (e.g. raise the unit on a stool, lengthen the hose between unit and gauge, add a fan to the mix, ...)
I should point out that you do not need to fire/burn the entire rocket motor, just the ing head and the less than an ounce of the rocket motor solid fuel. The question is: what is the effect of warm moist air going to extreme cold them do the ing spacing to solid fuel surface and moisture freezing between them and how much does this effect on the surfaces by increasing the heat needed(ele.current) to ing. A real fine/thin thickest of a simple glue or caulk material for covering and sealing surface ing device and solid fuel should work. Remember the cold will affect the material by shrinking and increase icing. Nitrogen should be use to shush the final model, no moisture inside, so it does not freeze inside model, creating an off-center of gravity problem......
A lesson I learned from my Uni life when working on a vacuum chamber was that the gauge should always be directly connected to the volume it is measuring.
With a setup not disimilar to what you have proposed, my lecturer one day helpfully closed the valve and shut off the pump to get some quiet in the lab. He later switched the pump back on but didn't open the valve.
So I stroll in, read the pressure gauge, see that all is well and activate the second stage oil vacuum pump. Result? A coating of soot over every square inch of the internals of the chamber as the oil cooked in the air that had seeped in to the vessel.
Be prepared to sacrifice the gauge, and put the valve between it and the pump. Who knows, if you log the gauge's output, you may get some interesting data.
I respect the poster's point of view, but please consider the danger. There's going to be an ignition in the chamber and a rapid increase in pressure. If said pressure is greater than the gauge is rated for there's a risk of the gauge disintegrating explosively. That could cause injury to operators.
.... but would it be an idea to put the whole ignition system in the chamber along with the rocket? Granted there will be danger of damage to the equipment but it will test that EVERYTHING will work as planned when it gets to those sorts of temps and pressures. Alternatively test that the required ignition V/c is achived in a seperate test. It wouldnt take too much extra effort. This is probably all redundant and I should go back to my corner and be quiet...
I hope the batteries are included in that heated payload. Li-ion batteries start to lose power below -10C. Consumer batteries aren't much good below -20C. NASA uses batteries that work between -60C to -80C, but I don't think your budget allows for that level of performance.
On your drawing, the chamber is way too voluminous. Granted, if the lid is as lightweight as possible and held in place by atmospheric pressure alone, you probably won't get enough exhaust gas under the engine to eject it against the thrust (think spud gun). But your retainer might go flying. Or at least be dislodged.
Also, the closer the exhaust nozzle is to the end of the pipe, the lower the pressure inside during the burn.
So, make it as tight fit as possible. You could use some of those leftover paper straws to wedge the (I'm assuming cylindrical) engine into the pipe.
One problem with making the inner tube from perspex / polycarb for the camera is that it is an insulator and will slow the rate of cooling.
Why not make the lower part of the tube i.e. from the top of the CO2 level down, from metal & the upper part from plastic.
This would provide maximum heat transfer for the cooling & insulate the top part which is exposed to the ambient temperature.
The drawback to this is that you will have introduced another, potentially leaky, joint to reduce the obtainable vacuum.
Incidentally, will you need to top up the CO2 whilst it is cooling down?
If you're going to all the trouble to knock this up, you can add a second chamber connected by 1" pipe or similar whose sole purpose is to give you a bit more vacuum to play with. Nothing fancy, but a simple 1m sq box welded from mild steel, or some recycled hot water cylinders (if they're tough enough), or just about anything will give you a bit longer before the vacuum vanishes at ignition time, it may be worth it just to confirm that the ignition catches properly
Have a look at the Casio Exilim range for high-speed cameras - somewhat excellent for the consumer-level money. Pre-ignition you may need to add a heap more light for high-speed filming, but post-ignition it may be too bright - I doubt the electronics will adjust terribly well in the fraction of a second, but you could get a very cool little youtube vid for the project if it goes well
As others have said: You'll spend possibly _days_ trying to cool the setup with the engine in a vacuum and and a gap packed with dry ice.
Just drop the dry ice into the hypobaric chamber, cool the whole mass and then draw down the vacuum. You'll only need a couple of minutes of vacuum pump operation to achieve < 1% pressure (it's the last few millibars which are the hard part and even the best setups on the planet can't achieve/maintain the fractional pressures found at only 150 miles up.)
Granted you'll only keep the low temps for a few minutes but that's all you actually need. Embrittlement of gaskets, etc isn't something you need to worry about for this kind of duration.
Perspex lid: Good, but 8mm laminated glass will work just as well, be far stronger than anything you need and be reusable (your local glass merchant will probably have anything up to 50mm glass circles on hand. Anything larger will need to be cut to order). Personally I'd put a wire cage over the top to stop it flying, rather than relying on a retaining ring and chain (which could have the lid flying on a wild arc if pressures are high)
Mirror for camera protection: Good
Given the size of the motor, You could forgo the whole chamber manufacturing process and just put the whole setup in a chemistry lab gas jar. They're strong enough to withstand a couple of atmospheres of pressure both ways and have flanges (test it empty, of course.)
With suitably thick gasket(s) you can run the sensor wires and vacuum pipe between 'em (sandwich style) and not have to drill any holes.
Anon because I work in a space lab. Our hypobaric chambers are "somewhat larger" than the one envisaged and get "somewhat colder/hotter". A full space component validation takes several months....
WRT the comment about hauling liquid nitrogen around in thermos bottles: That's fine as long as there's a hole drilled in the lid. I've seen the results when someone didn't do that. (A 0.5 litre thermos bottle holds quite a bit of pressure and the results of failure would likely be fatal to anyone within 5 metres.) Bear in mind that it's illegal in the UK (possibly EU wide) to transport LN2 in this manner because of the risks of something like that happening (if you're going to insist on putting LN2 in a thermos, DON'T put a lid on it - even with a hole, there have been instances of that icing up!)
FWIW we pay about 10p/litre for LN2 but we do buy 4 tonnes at a time. 100 litres is about 12p/litre and the companies will rent you a suitable (large) dewar to hold it, but you need cryogenic handling training before they'll consider it.
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