HERPES (Hypobaric Experimental Rocket Performance Evaluation System).
Our piece earlier this week on possible power plants for our Low Orbit Helium Assisted Navigator (LOHAN) rocket-powered spaceplane got you lot thinking about how one would go about testing a rocket motor which needs to fire at altitude and -60°C. Click here for a bigger version of the LOHAN graphic To recap, experts gathered …
Pick the right day - higher temperature means the atmosphere can carry more water vapour before it condenses. As the pressure drops with altitude, the boiling point of any condensation will lower and most should evaporate before firing if it hasn't frozen out already.
Things happen gradually as the balloon rises - this rig seems to be designed to place the rocket 'at altitude' quite quickly unless the dry ice pellets and vacuum pump are applied over a longer time to simulate the ascent.
Once you ignite the motor, pressure in the chamber will increase very quickly, so unless you have a monster vacuum pump (or a fairly large hypobaric chamber), you won't be able to reliably assess the burn (e.g. does the motor burn properly, or does it fizzle enough to raise the pressure in the chamber and only then burn properly).
Two things spring to mind:
- a large-ish tank of some variety, connected to the test chamber via a pipe to effectively increase the available vacuum (similar to the expansion tank on your central heating system)
- a pressure sensor under the motor to measure the engine thrust, particularly during the first one or two seconds - to be compared against a similar burn performed with the chamber at normal pressure.
The air inside the rocket motor would be at ambient pressure; the test would not tell you anything.
How about just putting a hole in the top plate for the exhaust and losing the vacuum pump?
The exhaust rushing out might cause enough of a venturi effect to suck out the air in the main cylinder.
No idea how effective this would be and it still would not tell you if the rocket fired under low pressure, but it might tell you if it runs properly.
Note: "It doesn't really matter if the rig does blow its lid, as we're only testing the motor's ability to fire, not its performance across the entire burn."
*IF* it fizzles at first, there will be little exhaust and a a consequence only a un-energetic lid-blowing. So that aspect of the ignition sequence will be noticed.
But once such a solid fuel motor ignites, its resulting internal pressure will make the outside ambient pressure totally irrelevant.
Put the vacuum pump tube near the top, then you don't need to go through the outer tube.
Get the temperature low before pumping out the air - the thermal conductivity of a vacuum is pretty low.
And as mentioned above, make sure your lid doesn't get frozen on. I've seen rigs like this blow their top due to frozen relief valves and it gets pretty violent.
I wish to complain in the strongest terms about the misleading nature of this article.
You can imagine my disappointment when I learned that the content related to some mildly interesting space technology instead of the article I was led by the headline to expect.
I.M. Disgusted Col. (Rtd)
Thanks to the every more fiendish wordplay of the sub eds, I am now in a state or semi-perpetual confusion.
I am no longer able to tell if the stirrings I feel upon reading the headlines are excitement at the anticipation of a technological revelation, or of a baser nature provoked by the lewd punning.
yours (on the bus, at the back, near the engine, about to spill more than his coffee)
D. E. Viant
It would be a good idea to tether the metal top to prevent unwanted damages.
I expect the motor will not ignite at that altitude. You might be able to retain enough air in the rocket housing by surrounding the rocket tube in a couple of layers of some sort of sheathing material. Latex might be best - provided you can find something that would fit your tiny rocket.
1. Sit the motor on top of an electronic set of scales, logged appropriately so that you get a record of actual thrust over time.
2. Make sure that the rocket stays cold, and at low pressure, for an appropriate length of time, based on how long the balloon will take to reach altitude. Half that time if you are in a hurry, equal if you want to be certain.
what about using some of your low temperature grease to form a seal? I've seen glass vacuum jars sealed with this method.. as long as both surfaces are flat, you should get a good seal.
caveat: I could be completely wrong and you end up being sprayed by expensive ice cold grease, at which point I will absolve myself of all responsibility and claim it was someone elses idea
of course if it works, I shall expect kudos in the form of free beer and much back patting :-)
Given that any rocket will rapidly pressurise any sensible size vacuum chamber unless you have a vacuum pump the size of a shed, you are only really testing ignition at the temperature and pressure required. As such, using the full size monster engine wouldn’t be necessary, the smallest one you can get would test ignition just as well, provided it uses the same fuel composition and ignition method. This would mean a much smaller chamber and pump could be used.
Just make sure you have an adequate pressure release valve that will NOT freeze up ;)
Depending on the thermal output of the igniter, the volume of the ignition/combustion chamber, and the surface area of the fuel against which the igniter rests, a lot of the igniter's heat output could end up in effectively "empty space," or dissipated across too large a surface area (of the fuel grain), and therefore not be usable in triggering ignition.
Thus, in this case, the closer you can get to a full-scale test, the better.
Hmm, not sure what pressure you are looking to achieve but creating a vaccuum chamber of this nature is not as simple as it seems... For one thing, using copper tubing into your steel or aluminium vessel witll be difficult to plumb and may have tempurature expansion issues. Your tube and bottom cover will need to be welded on and that will require a very good internal tig weld. Any other type of weld will produce virtual leaks. You also need to be very careful when working with it since a fingerprint or any sort of contamination inside will outgass severely when the pressure drops and could require hours (or days) of pumping to get the pressure down unless you have a large capacity (expensive) vaccuum pump.
Mind you, my vacuum experience comes from working with my Fusor, so I'm thinking in the high vacuum range under 1 micron of pressure... That would test your rocket for ignition in outer space... Your vaccuum requirements are probably not quite so stringent.
I've done some work with both low and high vacuums as a physics lab demonstrator. I'm pretty sure outgassing won't be a problem at the pressure required. At 90,000 ft the pressure will be 2 kPa, or 12 mmHg, according to http://www.altitude.org/air_pressure.php . We did low vacuum experiments with an oil-filled single stage rotary vane pump, probably at around 0.1 kPa. At that pressure it was no problem to let students handle the test samples and equipment.
I'm concerned about the rubber seal in the proposed design, it may become brittle at the temperature required, and start leaking. Page 323 of this book
has a table of sealing temperatures, it looks like silicone rubber is the best choice.
You probably don't need to chill the whole chamber, just place the dry ice around the motor itself (sealed in a plastic bag) until it reaches temp then place it in the chamber. If you had a temp probe strapped to the side you could check the temp. You could overchill it, then wait til it reaches the correct temp before ignition.
It doesnt matter about the exhaust changing the pressure in the vessel as you only want to see if it ignites at that altitude and temp.
I use a chamber similar to this at work to test altitude sealed hi-voltage connectors for military aircraft, at altitudes of 70Kft and above and voltages of up to 60kv, my chamber has a vol of approx 3cubic ft and our edward series 8 vac pump can get it to 45mbar/70kft in about 30sec.
You could run any temp probe leads across the top seal under the top plate, as long at they are quite narrow (bellwire type dimensions) the pump will hardly notice the slight leak it'll cause, though it obviously wont hold altitude once the pump is switched off, ( i have trapped test leads under the lid of my chamber numerous times and not noticed til the pump was off!)
Chill before pumping out - you will find that an 80 K temperature drop will go a long way to getting the vacuum you need.
I would mount the motor the other way up and attached to the lid, for two reasons:
It will ensure the ignition mechanism is at the real launch attitude.
The thrust when the motor ignites will help automatically depressurize the test chamber.
I suggest a simple condom stretched over the vent holding the igniter in place and keeping moisture at bay. The gap where the ignition wires enter should allow any trapped air to vent.
For added strength, use corrugated cyclinders - smaller version of what you get in a steam engine. I'm sure you could find a group of grey beardy types loosely grouped as a modelling society to knock one up for you for the price of a few pints of real ale. Saw that on WDYTYA last week.
Making a suitable chamber would be easy you simply need length of tubing of the right diameter a couple of o-ring seals and two flat plates for either end.
I recently had to make a long adaptor to fit long leads in our mass spectrometer/leak detector, we used a length of plastic waste pipe from B&Q with the correct flanges bonded in to either end with epoxy resin. This set-up will go to <6x10-3mbar with no probs (and no leaks).
Remember it is only going to be 14.7lbs/sq/inch which isnt a lot really, I dont know the diameter of the rocket but a length of plastic pipe of suitable thickness will be fine.
Remember it wont "blow-up" if it fails, it will buckle at most, the pressure is pushing in , not out.
A rubber seal will be fine, I have a vacuum vessel in a "Thermotron", a large oven which also chills that we use for testing connectors at altitude and hot and cold, and we regularly test at 45mbar -55deg c. You dont need grease at all, in fact grease would cause the seal to slip and be pushed into the chamber (it happened to me once) a clean dry seal has the benefit of friction holding it in place.
I have one main suggestion: reverse the metals.
Use an inner steel tube because its easier to weld than alloy and welding on the lower end cap would be a good idea. I'd also suggest that you weld a 10 mm wide flat lip on round the top and grind it fairly accurately flat. This way you can get rid of the rubber seal: put a fairly thick layer of low temperature grease on the lip, drop on a flat steel top cap and you should get a good enough seal. Laboratory glass vacuum rigs typically use grease on ground glass mating surfaces and have few sealing problems.
Your biggest problem is likely to be connected with the temperature probe. You may end up having to use flexible epoxy round a length of wire through the inner tube and fit a connector to each end of it. Mounting the sensor on the motor support near the cartridge would be sensible since its probable there will be a big temperature gradient within the tube: hence the internal connector so you can take the innards right out if needed. It will be important not to strain the cable vacuum seal, so gluing the internal connector to the inside of the tube and binding the cable to the outside with Dacron line or linen thread and epoxy would be good.
The rocket motor support can be made of anything, e.g. assembled from alloy disks and steel threaded studding and simply slid into the inner tube. If its a reasonably loose fit there's no need to perforate the plates since vacuum pumps are fairly slow and a 0.5mm gap, or just 3 or 4 3mm semicircles filed in the edge of each plate will be plenty.
Using a tether on the top cap sounds like a very good idea.
Vacuum connection: screw a brass nipple into the inner tube or through one of the end caps and use a rubber vacuum hose to connect it to the pump. Suitable nipples ands hose is readily available from laboratory supply houses. Alternatively the bits and bobs used for vacuum bagging glass/carbon mouldings should be good, and if you're going to build parts of LOHAN from carbon composites you'll need vacuum bagging kit anyway. You could do a lot worse that talk to ASP http://www.acp-composites.com/ or CST http://www.cstsales.com/ about this. Both are good places to deal with and very knowledgeable about their products.
Vacuum pumps: don't immediately rush out and buy one: they can be expensive if you're going to get down to 7.6 mm of mercury (that's 0.01 bar, approximately the air pressure at 100,000 ft) If I was doing it, I'd start by trying a 2nd hand fridge compressor: cheap as chips (often free) and may do the job. They are commonly using them for vac bagging composites at between 0.5 - 0.75 bar and I know they'll go down to 0.1 Bar but may take their time toward the end.
Final hint: you may find that the thermal contact between the dry ice and inner tube isn't all that good and that the dry ice doesn't sublime fast enough to really suck heat out of the system, so make sure that the dry ice compartment is water proof. This will let you use a slurry of dry ice and acetone, which will get you down to -100C, plenty good enough. Acetone is the usual solvent used for a dry ice freezing mixture in the lab because its freezing point is below anything you can reach with the mixture. An ice/salt/water mix won't go below -25C. Of course, using a low boiling organic solvent has implications:
- you can't use styrofoam insulation because acetone dissolves it (but fibreglass loft insulation will do the trick)
- make sure you run the ignition tests outdoors and have a fire extinguisher on hand because, if the rocket ignites it will almost certainly set off the acetone too.
Temperature, air pressure, and thin air might all play a part in the engine not firing or continuing the burn. Running this experiment in a decompression chamber that can go to negative atmospheres might be a better way to ensure ignition, unless you’re going to have a pressurise canister providing normal atmosphere and oxygen levels.
Presumably you might want some sort of cheap camera and lighting inside just to check for rocket deformation / other problems. Otherwise you've just got a big lump of metal to look at, and a charred mess afterwards. Mind you, not sure how they'd function at that pressure/temperature, but presumably you need a camera etc to work under these conditions anyway since you'd want to mount them on Lohan for the real launch.
If you ran the exhaust from an RC32/60 into my vacuum pump, I'd beat you to death with a cricket bat. See the rocket motor instructions:
NOTE: Perform RMS-RC motor cleanup as soon as possible after motor firing. Propellant and delay residues become difficult to remove 24 hours after motor firing. These residues can lead to corrosion of the metal parts
Put a safety interlock on that valve; valve open, firing circuit open. Throw the steel outer pipe away as it's unnecessary and heavy. Use a rectangular plywood box for the outer shell with thick expanded foam insulation between it and the vacuum chamber. Since the rocket engine is only 32mm in diameter, use s piece of four inch (100mm) copper pipe for the chamber. Braze the vacuum exhaust line into the side of the pipe.
You're going to need a second line into the chamber for your temperature sensor and the two ignition wires. I would suggest using half inch (1cm) copper pipe for this. Make it a short piece with a female threaded pipe fitting on the end. Use a second short piece with a male fitting leading to an expansion fitting to 3/4 inch pipe. Plug the 3/4 fitting where it narrows with a piece of balsa wood drilled with holes just big enough to take the wires. Fill the rest of the fitting with Apiezon vacuum wax. This will give you a reusable high vacuum sensor port at a not too horrible expense. If you use teflon insulated wires you should be able to heat the fitting up enough to melt the wax with no problems.
Final safety thought - be SURE that it's IMPOSSIBLE for pressure to build up inside the dry ice chamber. Big holes that can't possibly ice up. You might use vacuum grease at the top seal as well as rubber (silicone is better).
Why not attach the rocket motor to some kind of lightweight structure attached to a helium balloon or 3?
A quick pressure switch knock up should be able to attempt to fire the rocket at the right altitude for testing, and if it launches, you know you're good to go for the LOHAN....
I remember, decades ago, my uncle pulling some fancy tricks with Jetex motors. Out of the box, these were a re-loadable end-burn solid fuel rocket. The fuse was held against the base of the fuel pellet.
What melted the casing was carving the pellet into a star cross-section, which greatly increased the burning surface.
End-burn: constant thrust, longest burn-time.
Hole up the middle: Thrust increases with time, reduced burn time
Star-section: High initial thrust, pretty colours when the casing melted, caught fire, and set the model on fire.
But if you want reliable ignition, you may need a hotter igniter, which generates sufficient hot gas to set alight the main charge. And that may need the attention of somebody with the correct licensing to make their own pyrotechnics.
As others have said, I don't see any means for pressure relief. The exhaust gas has to go somewhere and I don't believe that the gasket method will work as there are no mechanical means of fastening the top plate to the tube.
Also the tube needs to be of sufficient strength so it does not collapse under vacuum. Steel pipe or mechanical tubing is your best bet. Plastic or copper are not so good strengthwise, especially at -60 C. Steel may become brittle but not so brittle as the other materials.
Since you are only trying to test ignition at reduced pressure and not contain the exhaust, why don't you look at the following:
1) Pressure Vessel - Find a flanged, welded steel pipe spool in a scrap yard of sufficient size and length. If you could find one that has a "Tee", the branch could be used to hold a polycarbonate window. The inner sleeve to hold the dry ice could be copper as it will not see any pressure differential. Gaskets without flanges will only get sucked into the pressure vessel. The flange will mechanically hold any gasket in place so it will survive pressure or vacuum.
Insulate the pressure vessel and precool the engine. A temperature sensor will be very helpful in determining you have reached the proper temperature at the engine. A thermocouple can be read by most decent multimeters and the thin wire can be threaded through a small hole that can be sealed with epoxy.
2) Pressure Relief - Install a burst or rupture disk at the upper end of the pressure vessel. This is a very thin sheet of stainless steel, designed to be mounted and clamped on a flange. It is scored to provide a controlled break in the sheet at a specific pressure range. You can buy them for pressure or vacuum applications. I suggest one for vacuum. When the engine fires, the burst disk will break. If the vacuum is too deep, the burst disk will break. In any case, there will never be any chance of the pipe spool overpressurizing and turning into a bomb and it will not collapse under vacuum
3) Properties of Dry Ice or Solid CO2 - Dry Ice will sublimate, the solid turns directly into a gas. This will tend to decrease the vacuum you are trying to pull. The more vacuum you pull, the more the dry ice will sublimate. You may want to allow everything to cool down to below the desired temperature and the majority of the dry ice to "evaporate" BEFORE you pull the vacuum.
Also, CO2 is used in fire extinguishers for a reason as it displaces oxygen and puts out fires. This "may" cause an issue with your ignition.
4) Vacuum Source - While a vacuum pump is great and can pull very deep vacuum, it does not move a lot of volume so keep the pressure vessel as small as possible and see how much volume it contains by filling it with water and measuring the liquid volume in a large beaker or graduated cylinder. However, the volume you are trying to pull is not all that deep and you could use an "Eductor" which is a venturi nozzle that uses water flow to pull a vacuum. These are commonly found on residential water pumps as part of the priming mechanism and can be purchased easily. They can handle a significant volume and are not as expensive or finicky as a vacuum pump. Note that you are only trying to reach about 60 mmHg or 0.1 kg/cm squared at 60,000 feet above sea level. Once most of the volume is evacuated with the eductor, you can switch to the vacuum pump if needed. A proper bidirectional pressure gauge is absolutely required which can easily be threaded into the steel pipe.
...what's the plan if the motor doesn't fire? If it works, the thing blows itself apart, job done. If it doesn't, someone who's possibly feeling a little crestfallen has to take some delicate decisions.
Sorry to be a pessimist, but I don't want to hear of anyone getting their face melted off in this venture. Not even A Orlowski.
I'd like to see a risk assessment and hazard mitigation plan for disassembly on failure.
Just a few comments on what people have or haven't said:
- Don't put the dry ice in the vacuum chamber - you'll have to sublime all of it off before you can reach a nice vacuum. And then it won't be cold any more.
- As far as the dry ice goes - I'd ice the rocket motor to get it down to the proper temperature, then place it in the chamber, start your pump, and when that starts to struggle, ice the outside of the chamber. Cooling the chamber will drop your vacuum faster while your pump is having trouble, and cooling the rocket motor indirectly, when it's inside a nice vacuum insulator, will take forever. Then again, I don't know your balloon ascent rate, maybe the rocket won't have time to cool on the actual trip.
- See if you can measure the temperature of your igniter. I don't know how large they are for a rocket motor like the one you've got, but I imagine that you could get a thermocouple and a tiny dab of thermally conductive epoxy pretty close.
- If at all possible, run your vacuum lines through flat surfaces, like your end caps. Welding or threading connections into curved surfaces is just not that fun.
- Aluminum for the vacuum chamber and steel to contain some dry ice pellets? Aluminum might be fine for your pressure cylinder (if it's thick enough - there are some relatively simple equations which I can't be arsed to look up, but I'm sure you can,) though it does complicate welding, and it's not exactly cheap. Someone mentioned a copper pipe, which would also not be cheap, for that size, and it's harder to find material properties of plumbing materials, but at least you could braze it together quite easily. As for the outer ice containment, large plastic drain pipe or plywood will work fine, just remember to insulate well so you don't waste dry ice cooling your shed (also don't fire it in the shed, please.)
- Someone suggested acetone in the dry ice - sounds good to me, though if you want something a bit more tame, I've heard that ~90% isopropyl alcohol works well too. It should also be easier to put out burning isopropyl than acetone - be careful with that fire extinguisher not to let the pressure spray flaming liquids too far.
- As someone else mentioned - close the vacuum pump valve (and probably turn off the pump) before ignition, unless you really don't like your vacuum pump.
To properly test this rocket motor you need to cool it and subject it to an air pressure drop over the same sort of timescale that you expect the balloon launch to altitude to take rather than doing it over an arbitrary curve.
Cooling it down from ground temp (10 to 20C) to firing altitude air temp (-60C) could probably be done following a specific temperature vs. time profile by using a sealed jacket around the motor with a few mm separation between it and the inner wall of the jacket. Blow dried (to prevent ice formation from humid air) compressed air through a coil in an insulated box filled with dry ice pellets and into the jacket, adjusting the cooling flow rate to match the simulated launch to altitude temperature profile by monitoring a temperature gauge on or in the rocket body.
For the vacuum part of the test use a vacuum pump to exhaust a remote tank, don't connect the pump directly to the test chamber. A simple hand-valve connected between the tank and the test chamber and an eye on a pressure gauge monitoring the motor test chamber should allow the experimenter to control the decreasing pressure over a time curve which will be close to expected conditions. Right now the setup as specified will freeze the motor quickly in the presence of moist air at 1 bar before the air pressure drops and the moisture disappears for all intents and purposes.
A regular air compressor tank of 50 to 100 litres capacity should easily be enough to act as a vacuum tank. They're built for 6-8 bar operation and tested to beyond 10 bar so they will cope with 1 bar of "crush" without a problem. Do remember that the vacuum hose has to be quite thick-walled otherwise it will crush flat under vacuum but there is flexible plastic hose on the market that will do the job. Keeping the hoses short also helps as they also have to be pumped out as the system runs.
Using a tank also means you don't need a hefty vacuum pump as you can pump the tank down overnight, say, before you carry out the experiment. As someone else said a hard shutoff valve in the line between the test chamber and the tank is a good idea. A couple of poppet valves in the chamber lid is also recommended just in case the pop-off lid doesn't, err, pop off.
What if you sealed the business end of the rocket inside a condom or balloon at sea level, thus ensuring the rocket was in an atmosphere of the correct pressure at ignition time? The rocket exhaust should very quickly burn through the balloon once it fires. The balloon would swell as it ascended into thinner air and I suppose explosive burstage might be an issue if not designed for, but those sound like surmountable challenges.
If your pressure test fails then maybe you could consider moving the mountain to Mohammad, as it were.
I have no prior knowledge or insight whatsoever so quite frankly haven't got a clue but thought it might be worth asking as you intend to use aluminium as the inner tube. What is the melting point of aluminium, what temperature will the rocket ignition be, and how close together will these two temperatures be.
I think this all looks terrific (taking into account the suggestions to reverse the thrust arrow on the diagram, include an internal temperature sensor, bring it to "altitude" slowly, and tether the lid to the rig).
Remember folks it isn't the burn profile your testing, but the ability to ignite at all. And ignition is probably controlled by pressure at the reaction face, not the "air" in the rocket or the interstices of the propellant, as some people seem to think. The only reason it's rocket propellant in the first place is because it has oxidizer built right in (like the intel ad...oxidizer-inside).
There were a couple of commentators who worried that the rig wouldn't test ignition correctly because it's so small and the vacuum would be lost so quickly; they were worried that there would be a slow start with the burn coming up to regular pressure more slowly than at sea-level and that this wouldn't be detected but the test. I think that might not be so bad a result. If you look at the thrust graphs provided in an earlier post, they all start with a dramatic spike. Anything that levels that spike out will be kinder to the air frame and other components and give the control system a bit of time to get stabilized as the craft accelerates. Just sayin'...
Do you need to worry about ice forming on the rocket as it goes up on the balloon? Any way you can test this?
Also, don't forget that dry ice is CO2, so unless you're careful you'll end up with a chamber full of CO2 not air, which'll somewhat invalidate the test. (Plus all the usual things about making sure you don't have the dry ice in a confined space - I'd suggest the entire test be done out in the open.
While the drawing in the article is a graphic not an engineering drawing, it suggests that the base of the inner tube is mounted to the base of the outer. As this is likely to allow a significant path for heat transfer, it would be better to mount the inner tube on some form of stand-off. This would allow the insertion of either a layer of insulation in the gap between the bases, or the entry of some of the dry ice pellets to assist cooliing.
Good luck with the test !
The rocket motor will produce a known amount of hot gas equal in mass to that of the consumed motor. It will produce this gas over the motor burn duration. If you know the exhaust temperature and the desired external pressure, the total volume of generated gas may be approximated via the Ideal Gas Law (PV = nRT see http://en.wikipedia.org/wiki/Ideal_gas_law).
The motor exhaust gas is both highly corrosive and abrasive. You won't want it to get anywhere near a conventional vacuum pump.
To use a conventional pump,. it must be off while the motor is burning, so you will need the vacuum system to contain a truly immense vacuum reservoir that will not only be able to be pumped down to the desired initial vacuum pressure, but also to take up all the rocket motor exhaust gas yet have a final pressure that is low enough to still adequately mimic the space environment.
Taken all together, I doubt you will have access to anything remotely close to what will be needed. Instead, you will need to dynamically create the vacuum while the test motor is burning, something no conventional vacuum pulp can do for the reasons noted above (filters make the problem worse).
The best way to do this, as noted above, may well be to use a venturi. But this venturi will need to be powered by a separate rocket motor, and the venturi will likely need to be fabricated as part of that motor's exhaust nozzle. To ensure the vacuum is maintained while the test motor is burning, the motor used to generate the venturi will need to be many times larger than the test motor (SWAG = 10x-20x, depending on the desired vacuum). And remember, the venturi will need to cope with the corrosive output from both motors, so material selection will be important if more than a single test run is needed (or use disposable venturis).
That's not all the bad news: The back pressure on the venturi exhaust will also have to be minimized. You may need a second, vastly larger, secondary 'gas mover' behind the primary one to keep the outlet pressure near ambient atmospheric. Fortunately, this need may be met by large electric fans (a meter in diameter, or so).
Please turn away. I hate seeing a grown man cry.
Why would they need any of that?
They only need to test ignition. Once ignited, the motor will burn regardless of outside temperature and pressure (just make sure the casing is strong enough for it not to burst open). Therefore, if the ignition test is successful they don't need to maintain vacuum anymore - just let the fuel burn out...
I just re-read my prior post. Fuggeddaboudit.
Send the motor aloft on a minimal balloon that can exceed the desired ignition altitude. Trigger the motor igniter when the balloon pops. Wait for the spent motor and shredded balloon to descend (no parachute needed). Inspect the motor casing and nozzle to see if even and complete burning occurred.
I doubt you really need much more test information than that.
I think the main problem is ensuring you get to the real burn before the lid pops and 1 ATM is restored.
pointing the rocket at the lid doesn't help that.
Also, you really want an area that can accept the thust/extra gas without affecting the vacuum majorly.
What I'd suggest, is a reservoir tank piped in at the bottom of the tube. A water heating tank would be ideal.
This, plus reverse the thust so pointing away from lid.
thus, when the rocket fires, the lid will not pop(ATM will not be exceeded) until the extension reservoir tank pressure has been restored.
i.e. effectively this gives you a large hyperbaric chamber rather than a wee tube.
This should give sufficient time to reach real burn with the oxidiser, or not - then you'll know you have a problem.
This is all getting a bit out of hand, you dont need no tig welding, burst disks etc
all you need is a length of plastic pipe and a couple of end caps.
Let me explain.
As I said in an earlier post we used a length of plastic waste pipe from B&Q to make
a test piece for our leak detector. The pipe says on the side "1.25/36mm BS EN 1455 B
ABS1.8mm" it has a diameter of 36mm and a wall thickness of approx 1.5mm. We bonded
some quick release flanges into the bore of the pipe using epoxy resin and this takes
a port pressure on our leak detector of 2x10-3mbar no problem, no leaks nothing,
remember the max external pressure it'll ever see is 14.7lbs/sq/inch.
This seems the right size pipe for you (your motor is 30mm dia?), place the motor in
the pipe bring the temp probe lead and ignition leads out the bottom through a hole
drilled in and aluminium plate. Seal this with epoxy resin. You will also need a hole
for the vac pipe fitting, attach this using epoxy resin. Attach to this a length of hose
from a acetylene welder or equivalent (this is designed to resist internal pressure but
will do the same for external pressure) attach this to a valve, then to a gauge then to
Bond this plate to the bottom of the tube with epoxy resin, bond on the top cap too.
Lay this in your dry ice and monitor the temp, once it reaches temp you'll have to
let it "soak" for an hour or two to makes sure the core of the motor is at the correct
temp, remove and fit in a retort stand (or just leave it in the dry ice), vac it down,
this will only take a few seconds due to the small volume of the chamber. turn off the
pump close the valve and ignite.
The motor will probably burn thru the tube almost immediately but all you want to know
is if it will ignite at cold and altitude.
If you are worried about the assembly going bang, do not bond on the top cap, use a
rubber sealing ring instead to allow the top cap to pop off under the pressure.
This is a low cost disposable set-up but will do the job.
To reliably reproduce the conditions expected in LOHAN, you might need some enlarged vacuous volume. Take a beer keg, drink it empty, tap it and connect it with a tape-sealed conical tap / tube to your test chamber. That should allow the rocket to burn some second before the assembly blows.
Are you aware of the difference between burning an explosive (as commonly done in a rocket engine) and exploding an explosive (as usually done in a sealed container similar to the one you've drawn)? You better make sure your lid comes off before the fuel had time to find it's thermodynamic equilibrium, else you'll get all the propulsion power of the rocket in a single undirected bang.
I dont think the low temp will be a problem for the plastic pipe, I guess it will make it more brittle but as long as you dont hit it with anything too hard I think you'll find it is surprisingly tough.
You could always just leave it laying in the dry ice, there is no need for it to be vertical, just make sure it is not pointing at anything flammable for when the end cap pops off!
I'd suggest running with rg20's idea of using plastic pipe. One variant on using household piping would be to use the yellow gas pipe you see used for gas mains. If you ask nicely, most crews will let you take any scrap.
I picked up some 180mm diameter pipe yesterday, which has a 10mm thick wall (so it'll be plenty strong enough). It's also super-easy to work with. I was able to cut the tube to length with a bandsaw in minutes and drilling/tapping it for vacuum feeds would be easy.
...as the point is to test only ignition I was assuming that after the evacuation the pump would be isolated by a valve and even disconnected. That way, as you are not intending to maintain a vacuum the top will be blown off (for the purposes of insurance risk assessment call this 'released') by the exhaust gasses.
Put the whole mechanism in a couple of yards of sand with only the top exposed. Blast and fallout will be limited to the top which if you were really risk averse you could chain to a big lump of concrete.
In the event of a failure to fire open the pressure valve and let the chamber return to normal temperature and pressure, fire the engine just for fun. If it doesn't go then you have to make a decision about how long to leave it before you call it really a dud. 24hrs?
I'm not a materials guru, but what happens to aluminium at -60 when its raised to whatever the rocket engine exhaust temp is in a short space of time, and simultaneously taken from external atmospheric pressure vs vacuum to internal pressure vs atmospheric?
I guess it smashes to bits and redecorates the inside of the shed?
"what happens to aluminium at -60 when its raised to whatever the rocket engine exhaust temp is in a short space of time, and simultaneously taken from external atmospheric pressure vs vacuum to internal pressure vs atmospheric?"
Nothing much, really.
I used to test samples of aviation alloys (including Al) to destruction, also at low temperatures (-90). Yes, everything gets more brittle at low temps but not to an extent which may affect the LOHAN test rig.
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