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 …
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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.
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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.
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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.
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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!
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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.)
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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.
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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
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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/
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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.
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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.
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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.
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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
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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!
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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...
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Tuesday 22nd November 2011 15:55 GMT BristolBachelor
Easy
Just use the compressor motor out of an old fridge / freezer (nevera / congelador).
Have used this in the past for vacuum applications; vacuum casting, testing Li-Ion cells at low pressure and also thermal-vacuum testing (Keeping your electronics cool when there is no air around is hard work!)
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Tuesday 22nd November 2011 15:55 GMT Dan Paul
Try an "Eductor"
Folks,
You need one of these http://www.1877eductors.com/pdf/Pumping_Liquids_V2010.pdf
This will pull 27 Feet of water on the suction side. Use a garden hose to provide the force required along with a hand operated control valve. Use a small needle valve to control the suction "air side".
Or you could just use an old refrigerator compressor and reverse the piping. Recycling the compressor is easy if you don't tell anyone you are dumping refrigerant to atmosphere.
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Tuesday 22nd November 2011 22:34 GMT securityfiend
Charles Henry Boyle
*ahem*
As pressure drops, temperature falls too. The chamber will be cold enough for the experiment due to this effect. Conduction to the sides of the vessel will be significant but not by much. Convection to the glass plate may be significant until the low pressure puts an end to that.
Careful about letting the glass plate get frosted up. I want HD quality video (slomo of course) of the
ignition and H2O crystals are going to get in the way of my viewing pleasure...
Icon seems appropriate...!
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Tuesday 22nd November 2011 22:32 GMT BristolBachelor
Sort of, but what happens is that the rocket motor will still radiate quite well (unless the surface is polished shinny metal). On the other hand, the inside jacket may be below -70°C, so will radiate a lot less. This would help to cool down the motor, although only very slowly.
However, yes it would be a lot easier to cool the rocket motor using the cold air in the chamber first, and then evacuate. It would also be a good idea to put the thermoucple on the side of the motor itself to measure its temperature, not the dry ice.
If the experiment was a bit nearer to Madrid, I could possibly supply some liquid N2, but there is no way that you are setting off a rocket motor inside my nice thermal vacuum chamber.
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Tuesday 22nd November 2011 15:56 GMT Markus Imhof
How dedicated are you?
If the schools or the other offers above don't pan out - 15 mbar isn't that much. Pros apparently use a simple membrane pump for that. So a modified bicycle floor pump in the hands of a strong lad should do it, if your metal botherers are up to the job of a) finding and b) reversing the air valves (or at least reverse the one on the piston, remove the one in the foot and have a second person open and close an/the external valve in time with the pumper).
No, I refuse to make up any jokes or acronyms about pumping and LOHAN.
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Tuesday 22nd November 2011 20:19 GMT Dr. Mouse
Plenty of ideas here, but my personal favorite (for it's simplicity) is using condensation to lower the pressure. I am not sure how low you could get with this setup, but I would suggest the following:
* Fill the chamber with steam. This is probably easiest to accomplish by pouring in a small amount of boiling water.
* With lid still in place, cool with cold water to condense the steam.
* Remove the cold water and add the dry ice. This should remove any remaining water vapour and freeze the water at the bottom, so the humidity should not be too different.
To increase the effectiveness, you could add a small vac pump (e.g. vaccuum cleaner or from a diesel) to reduce the pressure before chilling, but this will also remove some steam... It'd take some experimentation to ensure it would get the pressure low enough. You may need to try a different refrigerant, too, but if it works it's definitely the simplest option...
One other option I can see... Take the chamber, with a one-way valve installed (in place of the vac pump) to a hypobaric chamber. They take the pressure down, then when you take it out it's all set at the required pressure, ready to chill and fire.
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Wednesday 23rd November 2011 10:03 GMT Stoneshop
With a bit of calculation, this may work
but you have to make sure the volume of the hose (and excluding those 10m that you want your remaining water column to be, ideally) is way larger than the volume of the test vessel.
Getting most of the air out using a fridge pump first, then this trick, would be most the effective way.
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Tuesday 22nd November 2011 22:01 GMT an it guy
given you're in spain ...
try a local uni. They usually have a couple kicking about and it would be easy to get those down to 1mm Hg (done that lost). The air aspirators all depend on the quality of the seals, and at 15mm Hg might be a bit hard to get to.
The bonus is that some of the scientists there would probably want to come along for the ride, and debate writing a paper about it (or getting their students to do that for them)
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Tuesday 22nd November 2011 22:04 GMT Martin Gregorie
Don'r forget the acetone
Get yourself a liter or two of acetone to pour into the dry ice chamber before adding the dry ice. Without it there will be almost no thermal conductivity between the dry ice and the steel tube and cooling down will take forever. The advice to get things cooled down before pulling vacuum is good too, but you should also think about packing steel wool into the space between the chamber wall and the rocket motor for better thermal conductivity and faster cooling.
However, two things to watch with acetone:
(1) it dissolves styrofoam instantly, so don't use any styro insulation round the acetone jacket. Wrapping the dry ice container with 2-3 layers of corrugated cardboard, gluing on a ring of cardboard top and bottom and finishing with a coat of acrylic or enamel paint to keep the acetone out (both are immune to acetone) will provide plenty of insulation for the fairly short time you need to keep things cold.
Its possible that you could get away without any exterior insulation. Things will get cold enough inside the chamber without the insulation - you'll just use rather more dry ice, but its fairly cheap at around $US 2 - $US 4 a kilo and, as it sublimes at the rate of around 5 Kg / day if kept in a chilly bin, you'll probably need to buy 10 Kg or so in any case. When I used it in the lab, we always bought it as 5 lb or 10 lb chunks first thing on the day we needed it and didn't have much left at the end of the day.
(2) Acetone is very flammable, so drain it out before hitting the GO button on the rocket motor: fit a pluggable drain hole in the bottom of the dry ice compartment.
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Wednesday 23rd November 2011 14:28 GMT Anonymous Coward
Doh..
I can't tell if this is deliberate or not.. the scientard above speaks with what appears on the surface to be scienspeak...
" so drain it out before hitting the" drain it out of a low pressure vessel? without letting air in? just how exactly are you proposing that? using a vacuum pump on the drain container perhaps? and if you are using condensation you will only draw down to vapour pressure ie the vapour that is left is gas and therefore cant be drained... your vessel will be full of low pressure acetone gas.
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Tuesday 22nd November 2011 22:08 GMT Mike Moyle
On a separate note...
...not about the pump...
Are you getting a stainless steel camping mirror for your test stand...? I imagine that both it and your glass cover will get smudged with exhaust when you set your engine off. so I'm not sure how much visual you'll get, but at least it will mean one less piece of glass to shatter and go flying.
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Tuesday 22nd November 2011 22:33 GMT Chris Holford
Don't forget the collapsing can demonstration.
When the pressure in the test rig is reduced, atmospheric pressure will try to crush the container so it needs to be fairly strong.
BTW I liked the idea of taking it to the top of a building; the water only needs a head of about 10metres to give a pressure difference of 1 atmosphere.
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Tuesday 22nd November 2011 22:37 GMT rrevolverr
Hacking Your Water Supply Company In Five Easy Steps
Inspired by the theme:
1> Turn off the pipeline main valve, open the tap, let the xs of water out. Keep the tap open.
2> Open a water filter inlet in the pipeline.
3> Join your vacuum cleaner to a pipe that matches the diameter of a water filter inlet. Make an airtight junction.
4> Switch the cleaner on. Don't keep it working for too long, otherwise you will put yourself at risk of becoming a creditor.
5> Enjoy, the whole company sucks. You may rename it into LOHAN, just to decrease possible conscience-strike and increase pleasure.
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Wednesday 23rd November 2011 00:54 GMT AManFromEpsom
Vacuum pump
For 20mbar or so you need a rotary vane vacuum pump. If you look on Ebay you can pick up these for a couple of hundred quid. Once you've done with it you can sell it on and probably not lose too much money. Be aware that these pumps produce a fair amount of oil mist in their exhaust (especially when starting to pump down a volume) and that you don't want to breathe that in.
I assume that this pump isn't meant to continue to function once the rocket starts firing - you would need a much, much bigger pump to cope with that.
If you talk nicely to Edwards High Vacuum they might be willing to lend you a pump for a few days in exchange for a mention in the article
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Wednesday 23rd November 2011 09:58 GMT pete 22
quite simple
Its quite easily done. Similar to the cooling and distillation systems used on steam engines circa 1920. You can create a vacuum with either gravity, or sir pressure. In those days if they needed a high vacuum, they would fill a column part with mercury, open at the bottom into a pool. The mercury would of course try to fall out into the pool, thereby creating an almost perfect vacuum at the top of the column.
One could calculate the weight of the mercury and substitute something like water or oil, and use about 30 feet (say 10 metres) of plain iron pipe to accomplish the same thing. Open at the bottom with a hose connection on top for the vacuum, partly filled with whatever liquid, standing vertically.
You can demonstrate this for yourself with a bowl of water and a drinking straw. Insert one end of the straw into the water partway, and close off the other end with a fingertip. As you withdraw the straw from the water, it begins to pull a vacuum equal to the weight of the water.
Cheap and effective. Hope that helps.
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Wednesday 23rd November 2011 13:13 GMT Anonymous Coward
Acronyms
I once invented an apparatus for producing almost any gas -- actually, any that could be made by reacting a liquid with a solid -- using the pressure built up when the exit valve was closed, to hold the reagents apart. (Solid, liquid, gas and energy -- because chemical reactions get hot -- representing earth, water, air and fire, respectively). The intended application was with hydrogen peroxide and just about anything to make oxygen to feed into a bong.
I called it the Demand Regulated Universal Gas generation and Induction equipment (D.R.U.G.G.I.E. for short).
Posted anonymously, as though that makes much difference .....
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Wednesday 23rd November 2011 20:12 GMT Retired Spy
A tank full of water, a pipe, and a 40 foot drop should be sufficient
You need an enclosed, rigid tank of water with a volume slightly larger than the amount of air you want to evacuate. This is connected to a pressure resistant (copper water) pipe/hose which provides a 40 foot drop. Fill the tank, pipe and hose with water and let the water run (you need 33+ feet of drop, 1/2 psi per foot plus some to provide an airtight seal) this creates a siphon and as the water pours out you create a partial vacuum above the water. You will need a valve to shut off the water when the momentum drops, otherwise bubbles of air will rise up the pipe. With the right size pipe/hose and a little luck, you should be able to drop the pressure in the tank down to a few PSI. The top few feet of hose has to be rigid or semi rigid to prevent collapse. The bottom can be garden hose. You will probably want a second valve at the top to keep water vapour from filling your test rig (think low pressure boiling). Note that the 40 foot drop doesn't have to be from the test rig, just from the water tank.
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Wednesday 23rd November 2011 21:29 GMT AManFromEpsom
A tank full of water - will produce a pipe full of water vapour
Can't help thinking that all the 'pipe full of water' solutions will merely produce a long hose with quite a high pressure of water vapour in - maybe not a large fraction of a bar but probably higher than the 10 mbar target. Putting a large cold trap in full of liquid nitrogen would take out all the water vapour and most of the gases but again probably wouldn't get down to 10 mbar. Pumping is the only true way!!
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Wednesday 23rd November 2011 23:31 GMT Mike Manes
LOHAN altitude chamber vacuum pump
Edge of Space Sciences (www.eoss.org) snagged a decent used Welch 1/2 HP vane vacuum pump from an industrial salvage outlet for US$75. It was rated new to pull down to 0.1 Torr (mmHg), but gets a 1/2 m^3 chamber to 2 Torr in about a minute. Or, you could just buy a new vacuum pump from Harbor Freight Tools for about US$75 and save yourselves plenty of sub-minimum-wage pro bono hours trying to kluge one.
Mike W5VSI, CTO EOSS
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Thursday 24th November 2011 09:44 GMT Mips
Short of geting hold of a mercury pump
and the many tens of kg of mercury needed. You remember the old school experiment with the tin can full of steam. You boil the water to drive out the air then close up the vessel and cool. In your case you need to freeze the item to simulate the cold at altitude. It will not be the same conditions but close to actual.
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Wednesday 30th November 2011 00:18 GMT anonmint12
Lohans box is not big enough.
Hi. The volume of the container doesnt seem sufficient for the expansion of gasses from the exhaust, unless the vacuum pump can move a very large amount of gas from this volume, the lid will be forced off by the gasses. Has this volume been calculated. Sory I cant help as I have no data.
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Wednesday 30th November 2011 11:54 GMT Andy The Hat
Whilst building one out of old stuff is all well and good, legally you shouldn't be removing compressors off fridges etc in scrap yards unless they are known to containg environmentally 'safe' refrigerants or they are removed correctly and the gas recovered - the scrappies' method of cut the pipe and let the gas bleed off to atmosphere *is not* the correct method!.
Go with the "borrow one"... lot less hassle and most good techies in a school would be more than willing to help out just for curiosity! We have two but then I'd be too far away to help :-(
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