We at El Reg's Special Projects Bureau are, as ever, grateful to all those readers who offered suggestions for the low-cost, garden-shed vacuum pump rig we need for our Rocketry Experimental High Altitude Barosimulator (REHAB) experiment. Click here for a bigger version of the LOHAN graphic Our bold plan is to stick the …
That's not a vacuum!
15 mm Hg? Any water stream pump will do 12 easily. Don't these things get used in labs anymore ?
It's just a perfume spray pump in reverse: instead of a stream of air sucking liquid up a tube and out the nozzle, a high-speed stream of water from a common or garden tap is run through a venturi with a side tube to let air in. The water sucks the air in through the tube and expels it out into the sink. You don't need to go to a lab supply shop: I got one with a waterbed (you use it to empty the bag) decades ago. Surely those are still around.
They want an absolute pressure of 15mm Hg inside the chamber, not a 15mm reduction from atmospheric pressure.
"not a 15mm reduction from atmospheric pressure."
A lab water pump will get to ~~15mm Hg absolute .
"Don't these things get used in labs anymore"
Less and less - they use an enormous amount of water unless a pump recirculates in a closed system.
but needs more flange.
"the top plate will be pulled firmly down onto the seal "
Er, pushed by atmospheric pressure actually - but these things are all relative.
Good test rig though.
With LOHAN, one is never sure whether it is sucking or blowing ... or, in this case, pulling or pushing.
Will there be a Special Projects Burro used with LOHAN? Perhaps as transportation, or some sort of launch platform, perhaps a helium counterweight before launch? Perhaps you could record a short video with LOHAN and said burro.
Re: Professor Haines...
I reckon that's a fine idea. I'll give it some thought...
Will there be ...
... an eventual full write-up of all the engineering involved in this project and how to build it?
A sort of Haines Manual?
Re: Will there be ...
Yup, we'll have all the details available as and when.
Sod the 'Joke Alert' icon. I think this is a fantastic idea. It would be fascinating to read the final experiment methodology.
Explosion because, well, it could.
Re: How much volume of gas will the rocket test produce ??1
Just to clarify again - this is an ignition test chamber, so the lid is designed to pop off sharpish. We really don't want this thing going bang. If you know a responsible adult, send him or her round.
My mother-in-law's a big girl.
Couldn't she pop round to sit on the lid?
Fridge compressor should be fine
My father, who was in refrigeration during the days when CFCs weren't bad, used to make vacuum pumps out of fridge compressors, which were needed when they used to pull a vacuum when charging a fridge with gas. If it's any help, for some reason he favoured using air conditioner sealed units - from those air conditioners that fit into a window. Using refrigeration fittings may be a cheap way of connecting it all too.
"local fridge-fixing chaps..."
"...have promised a couple of units for free, having immediately grasped just what the mad Englishman intended to do with them"
I'm impressed. Hat off!
Cold batteries won't start rockets
I could be entirely wrong, but I think the cold at 80,000 ft will provide more of an ignition problem than the low pressure and that, while freezing the rocket motor may be an issue, keeping the ignition battery warm enough to deliver enough amps may be the biggie.
Sacrifice an ignitor in order to measure the current it needs if the supplier can't tell you this, and then set up a second cold soak test to make sure you can keep the ignitor battery warm enough for long enough to deliver the required current after LOHAN reaches maximum altitude.
Size the battery to have enough capacity to keep itself warm inside its insulation for, say, twice the estimated time needed to get to 80,000 ft and still retain more than enough kick to goose the ignitor real good. I think you'll find that keeping itself warm needs more capacity than firing the ignitor and that you'll need a thermostat in the heating circuit to minimise power drain at lower altitudes. The PARIS time-to-altitude numbers should be a useful input to this capacity calculation.
Li-poly batteries don't work at all well below 25 degrees C and most other types get discouraged below zero.
Maybe a supercap, kept topped up by a lower current battery with chemistry that can deal better with lower temperatures, would work as well as a Li-poly kept at 25C. A look at the specs hints that you could charge it on the ground and forget about the top-up battery, though a top-up battery might be useful to counter any current leakage if you get condensation on the supercap and wiring as the rig gets cold. Supercaps may be lighter than Li-poly batteries (I can't find any weights), are certainly cheaper and are unlikely to be bigger.
I'll second that!
Don't have your battery sitting on a nice warm bench (I guess it'll be away from the action), wrap in some more dry ice for double the time the balloon takes to reach 80,000 feet or whatever height it's firing at... then you'll have a test closer to the real thing.
Beer, as you can chill it with the spare dry ice....
Shouldn't be a problem
If you are using cheap model rocketry igniters, then yes you will need a fair current for a decent fraction of a second to get it to fire. Professional ematches, as used in display pyrotechnics, however, are cheap, shouldn't be *that* hard to get and will fire in a split second from very low current. If people can get cameras to work up there, then ematches should be fine.
Maybe wrap the battery and electronics in chemical handwarmers, like in this project: http://www.bhaldi.org/1st-launch
Yup, we used chemical handwarmers on the PARIS project. They worked a treat.
What is the insulation on those wires? Looks awfully close to the rocket motor exhaust. I'm not sure how long even pyro would last in that environment.
All we want to know is
will the rocket ignite?
As soon as it has ignitied succesfully the experiment is over, and the wires and everything else are no longer needed - no matter how long the rocket burns for.
Re: All we want to know is
Thank you. That is exactly the way it works.
For your records
I found a US source of aspirators: http://www.capitolscientific.com
They sell a Brinkman pump with a 10litre tank of water that it recirculates with an electric pump for a mere $US1850, so far too expensive for the LOHAN test, but its the only one on their catalogue that can get down to an absolute pressure of 20mBar
At the other end of the scale are a couple of plastic ones that you can run from any water supply with enough pressure and capacity to squirt 12 litres/minute through the aspirator. These cost $12.50 to $14.50, so are affordable and can get down to 64mm Hg (85 mbar) and 37mm Hg (49 mbar) respectively.
So, the $12.50 aspirator is twice as good as a fridge compressor (lowest pressure around 100mbar) while the $15 one is three times as good. I'd say go for the fridge compressors but keep the $15 aspirator in mind if the compressor doesn't do the job for some reason.
Have the fridge compressor
do the bulk, and once you're down to whatever it manages, get the aspirator pump to do its thing. Two extra valves required, of course.
I'm not sure if this was covered last time around
but with the evacuation tube running through the CO2 at the bottom there, isn't there a risk of water vapour condensing within the chamber, freezing and blocking it entirely? Might be worth putting that connection up on the side of the chamber at ambient temperature where any water can't roll into it.
I had the same thought; if you are chilling the air and lowering it's pressure you will condense all the moisture in the chamber pretty rapidly and then potentially freeze it solid.
If this happens in the vacuum line itself you could get a nasty blockage (so to speak).
My initial thought was that the vacuum line would be better if it went through the lid itself (and easier to make the fitting to), but that might interfere with the plans for it to blast off as the motor ignites.. as a minimum you should leave the pipe protruding into the chamber so that if condensation pools at the bottom it does not enter the pipe.
And have you considered the effect on the vacuum pump as it suddenly goes from low negative pressure to high positive pressure at ignition? any seals and valves in it might get a pretty rough ride in the blowback.
"If you know a responsible adult"
What? Why do you need one of them?
"If you know a responsible adult"
Surely these "responsible adults" are nothing more than an urban legend, I for sure dont know any.
beer, cos this project is a direct result of beer, and I'll drink to that.
"...the second-hand fridge compressor,"
Probably sitting in your neighbour's garden, in true Viz's "8ace" mode...
I'd go with that!
What sort of pressure gauge is it?
If it's a cheap and cheerful Bourdon tube gauge then it might not be all that accurate; and the measurement will depend on atmospheric pressure at the time. To be certain that the required 15mm Hg has been achieved (measuring -745 mm or thereabouts) it would help to calibrate the gauge, if this is possible.
As expected, a completely vacuous article. Perhaps some suds sucking is in order.
Put a couple of stress guages under the rocket mount, and use them to measure the amount of thrust the engine produces over time?
A bodged, broken wii fit board might have what you need if you're looking for cheap.
It were much easier in my day...
You don't need fancy gauges for 15 torr pressure (that's the approved style for "mm Hg"). We used to make our own mercury manometers out of glass tube and yes, that awful systemic poison that is such fun to play with. We also bought them in when all else failed.
After a bit of thought...
Lester's 'country pile' seems to be on a hill.
Just wondering if a pipe (e.g., fire engine hose) full of water, one end connected to the chamber, the other end - plugged, at 35' down would work? Release the plug, the vacuum created (using a simple one-way flap valve) might do the trick.
I'm much less of a Rocket Scientist than is Lester - but - just an idea...
Actually, can't fire engines suck (to extract smoke)? Maybe a chat with the local fire dept. might bear fruit.
Suck too hard?
My experience suggests that the fridge compressor as a vac pump will work –maybe too well. Are you sure getting the pressure down so quickly will really simulate the balloon ascent? If your local hvac contacts can spare a couple of compressors then they could probably come up with some capillary tubing as well. Capillary tubing is used as a pressure reducing device in a circulating system and could probably work as a vacuum delay device for you.
If I was asked to run this simulation I would probably run it like this:
1. Make the test enclosure that you have already planned minus the evacuation tube but keeping a connection for the vac gauge.
2. Obtain a second vessel that will hold a vacuum, and fit the evacuation tube to this.
3. Connect the test enclosure to the second vessel with a length of capillary tubing; you should be able to run this through the seal in the same way as the wiring.
4. Once all set up add the dry ice and wait till the inside temp of the pressure vessel gets down to-at least- its dew point, then turn on the vac pump. By cycling the pump on and off you should be able to get a pretty good approximation -in the test enclosure- of the slow drop off (over about an hour?) on the way to your target pressure altitude.
I know you think the electronics will work OK but it might be an idea to test anything using capacitors like this as well.
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