Old air compressor?
Perhaps an old air compressor with the feeds reversed? (i.e. sucking the air out of the receiver tank). Might be fun adapting the air inlet though.
Got a few minutes to help LOHAN suck?
As we recently reported, El Reg's Low Orbit Helium Assisted Navigator (LOHAN) team has taken delivery of an AeroTech RC 32/60-100NS rocket motor – the proposed power plant for our Vulture 2 spaceplane. Click here for a bigger version of the LOHAN graphic However, no one – including AeroTech – is sure whether the thing will …
-
Tuesday 22nd November 2011 14:43 GMT Chris Long
Newcomen
Maybe you don't need a pump... fill the tube with steam, seal the top, then stick it in the dry ice etc. I've no idea how low a vacuum this will generate, and the humidity may not help matters. I suppose you could use the same approach with a large separate vessel - reduce the pressure via condensation of steam, then connect the low-pressure vessel to REHAB in the same way that you currently propose to connect the vacumm pump.
-
Tuesday 22nd November 2011 22:02 GMT Ahgoo
No Pump needed Use Charles' Gas Law
I agree you shouldn't need a vacuum pump. In a sealed environment as the outside air temperature decreases so to will the volume of air in the sealed environment. If simply cooling REHAB with dry ice doesn't decrease your air volume to meat your criteria then plum a preassure cooker in place of your air compressor. Remember the steam created will need to be vented to normal air as the preassure cooker comes up to temperature. Once the required temp is reached shut off the heat, close the vent to air and cool the preasure cooker. This will draw air out of REHAB to equalize the preassure between the two enclosed containers creating a vacuum in both the preassure cooker and REHAB.
-
Tuesday 22nd November 2011 22:08 GMT Stoneshop
After condensing the steam
you're still left with quite a lot of air, which was inbetween the steam before it condensed
Better to let the steam condense in a cylinder with a piston, which pulls on another piston (with a smaller diameter) which pulls air out of the test vessel via a one-way valve. Repeatedly introducing fresh steam into the first cylinder, then condensing it will gradually lower the air pressure in the test vessel, simultaneously enlightening you as to why James Watt is credited as the inventor of the modern (reciprocating) steam engine, instead of Thomas Newcomen.
-
-
Tuesday 22nd November 2011 15:05 GMT Alexander 3
Cooling even necessary?
Looks like fun. I'm wondering if you need to cool the environment though - my expectation would be that at that altitude, you'll have such a rarified environment that a dark-coloured rocket body might get quite toasty. Not done the calc, but my guess is that 99% of your "coldness" will escape REHAB altogether..
Be interesting to see about the pressure effects. I assume your rocket doesn't carry oxidiser, so it'll be fun to see if you can get ignition or not!
-
Tuesday 22nd November 2011 15:49 GMT Richard Ball
Freezer
Get the compresser out of an old freezer.
Don't know 'how low it will go', but they're basically free and suck pretty hard so worth a pop.
(for the sake of respecting the law, I believe you'll want a freezer that has already lost its refrigerant - that way you're not responsible for its release.)
-
Tuesday 22nd November 2011 15:51 GMT Vic
An engine.
Go to the nearest scrapyard and grab a small engine.
Plumb one of the cylinders inlet to the chamber, put some sort of exhaust on the manifold, and find something to turn the crankshaft (an old electric motor, for example).
If the cylinder you chose leaks - pick another one. Scrap engines have a habit of being knackered...
Don't run it for too long unless you've also sorted the cooling system. It will get hot...
Vic.
-
Tuesday 22nd November 2011 22:29 GMT Steven Raith
Micra lump
I'm reliably informed the the K11 Micra 1.0 and 1.3 litre lumps are so smooth that it's tricky to notice when they are running on three cylinders, unless you're fairly mechanically minded.
So just pop the injector feed and HT coil from one cyl, run your vacuum 'feed' from the tank into the intake manifold chamber having banked it off as best you can with JB weld and bits of washing machine bodywork, and wang that legendary powerhouse of an engine up to it's sweet, sweet peak power point of 6000rpm.
If nothing else, the resulting explosion/implosion/fire/injury would look cool in high framerate, high definition video.
Steven R
-
-
Tuesday 22nd November 2011 15:51 GMT PC Paul
Do what the RC modellers do...
When making composite wings, RC modellers use an old fridge compressor which pulls and holds much more vacuum than 15mmHg if you want it. These things are often left running overnight while resin sets under pressure inside a vacuum bag.
For an example:
http://www.paragoncode.com/shop/vacuum_pump/
-
Tuesday 22nd November 2011 22:01 GMT Martin Gregorie
What to expect from a fridge compressor
According to this site: http://www.paragoncode.com/shop/vacuum_pump/
a fridge compressor maxes out at 100mb, or about 75 mm of mercury, so its probably not good enough.
@PC Paul: if you sucked down to 15mm Hg (0.98 bar) when vac-bagging a foam wing you'd end up with a very thin wing. I have an AutoVac II system I bought from ASP which was set to pull 5" (128mm) of mercury when I got it (0.16 bar): that's plenty for vac-bagging wings. I vac-form carbon shells on Dow blue foam male moulds and pull about 11" (0.36 bar) but this is much stiffer foam than anything you'd make a wing out of: its sold for under-floor insulation and is rated to support something like 260kG/m^2 load with a hard floor surface on top of it to spread the load.
-
-
Tuesday 22nd November 2011 15:51 GMT Stoneshop
Fridge compressor?
A bit of searching around the web suggests they're capable of sucking about 25 inches. That's not even in the same ballpark as the air pressure at 80k feet, but I doubt that lack of air pressure would affect the firing; temperature would be the prime concern. And once the motor fires it's a self-sustaining reaction, it doesn't need outside air (that'd be quite a problem with pressures inside the motor casing being a tad unfavorable in letting air _in_). So there's probably no need to create near-absolute vacuum, and a fridge compressor would be good enough.
-
Tuesday 22nd November 2011 15:53 GMT Anonymous Coward
Gauge on wrong side of valve?
Physical engineering isn't my thing but don't you risk false positives if you have the valve and the gauge in their current positions? I would prefer see the gauge reading conditions *inside* the container, not conditions between the valve and the pump.
I am assuming the case where the required vacuum is reached and the valve closed. Either way, its better to move it, in my opinion.
-
Tuesday 22nd November 2011 15:53 GMT Roy T-H
Vacume Pump
I use an old compressor from a Freezer as a vacume pump for doing composite layups for rocketry, I got it from my local scrap yard ony cost a Fiver, then a bit of pipe and a pressure gauge and it will do what you want. If you want more details on what I have then let me know and Ill help you out with a few photos of the vacume pump.
The freezer compressor is just a pump that normally pumps refrigerant gas from one side to the other, they are quite good pumps too. so using it to pull a vacume is no problems for it. If I can find any links on the web on how to use the compressor as a vacume pump ill let you know.
Roy
-
Tuesday 22nd November 2011 15:55 GMT andreas koch
geoff, you're half way there
Now Geoff's idea will work with almost every car, put a t-junction into the brake servo line and you got a vacuum pump. Just don't forget to seal it up again afterwards!
As another idea: Your test rig isn't all that big. Take one of these £10 electric tyre inflator compressors, put it in an airtight sturdy container and lead the supply cable and pressure hose outside. seal around them. now drill one more hole, fix nipple to it and attach vacuum hose.
The air that the compressor pumps outside has to come from somewhere, so you got yourself a vacuum pump. As they all do loads of pressure and little volume, it might take a few minutes, but I'm sure it'll do the trick under £25. It's easier to move than a car and might, if you put a bit of effort in, even look vaguely scientific.
Good suck!
-
Tuesday 22nd November 2011 15:55 GMT K. Adams
Local Garage or HVAC Contractor?
Well-stocked auto mechanic shops qualified to do air conditioning repairs have vacuum pumps that are used to evacuate the air conditioning system prior to refilling the system with refrigerant.
You may also be able to convince a local HVAC (Heating, Ventilation, and Air Conditioning) contractor to come out with an A/C service truck, and use some engineering tomfoolery to connect its A/C evacuation pump up to REHAB.
You may even be able to pay for the job by offering to run an advert for the company providing the equipment on LOHAN's side...
This topic is closed for new posts.
Biting the hand that feeds IT
