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back to article LOHAN to suck mighty thruster as it goes off, in a shed

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 at …

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Anonymous Coward

I like the multiple redundancies

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Anonymous Coward

Ah... I almost missed your correction. Just as well you've used a big "O".

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Mushroom

Watch out for low temerature effects on the rubber seal

Thniking back to shuttle disasters, you'd better check on the low temperature characteristics of the rubber seal...

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(Written by Reg staff) Silver badge

Re: Watch out for low temerature effects on the rubber seal

Duly noted.

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Alert

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 :-)

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RICHIE

Rocket Ignition Chamber for Hypobaric Integrity Experimentation

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Transparent lid?

So you can confirm visually that the rocket has indeed fired and is burning evenly.

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Or...

See above???????

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Silver badge

Camera / lights?

I was thinking of the need for a window or internal fast-ish CCD camera and floodlight, bright enough so the camera is not temporarily overcome by LOHAN going off, so you capture the ignition moment. After all, once she blows the camera or window's life is going to be short.

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Angel

Clever Lester's Impressive Test Of Rocket Ignition System ?

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Coat

"Burst Plug to retain internal pressure"

How does it perform in high wind situations...?

Mine is the teenager's hoodie...

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Mushroom

Do it smaller

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 ;)

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Anonymous Coward

Nonsense, always, always, always use the largest rocket you can find.

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Boffin

"...the full size ... engine [isn't] necessary, the smallest ... would test ... just as well..."

Not necessarily...

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.

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Fixed Atmospheric Rocket Testing

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Boffin

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.

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Boffin

Low vacuum

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

http://books.google.co.in/books?id=sdKAPJh5RgQC

has a table of sealing temperatures, it looks like silicone rubber is the best choice.

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Launcher Engine System Test Environment (Reusable)

That way we can finally see LESTER and LOHAN together as nature intended.

You're not planning on setting this off near the combined mission control / donkey sanctuary are you?

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S.L.A.G

Sealed Liquidless Advanced Go-a-matron

Clearly I'm not a technical geek.

However I like the word slag.

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Anonymous Coward

Just a thought?

Why not use a domestic hot water cylinder and pump liquid nitrogen through the hot was circuit to cool it. The benefits are that these cylinders have various openings already installed for you to utilise vacuum ignition etc?

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says:

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.

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Good luck with your...

Cooled

Low pressure

Uniform

igNition

Generation

Experiment

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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!)

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Go

Metal Cylinders

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.

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T.W.A.T

Thrust Widget Altitude Tester

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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.

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Bronze badge
Mushroom

Challenger

Will the rubber ring be a good seal at that temperature - would it not be brittle?

Is there a low temperature grease which would be more appropriate?

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Bronze badge

Pump choice

Use an Edwards - they are the proverbial.

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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.

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Flame

HOT TART

Hypobaric Operational Test Terminal Altitude Release Temperature

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Go

Suggested mods

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.

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ARSEHOLE

Atomspheric Rocket Suitability Experiment Hypobaric Oxidisation LOHAN Evaluation

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Coat

Thin Air

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.

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Boffin

I think the idea behind rocket motors is that they provide their own oxygen...

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(Written by Reg staff) Silver badge

Re:

Yup, they have their own oxidiser. This is not an issue.

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Coat

What's the exhaust gas composition? Is it a greenhouse gas? I think we should be told (then I'll get my coat).

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Coat

LINDA LOVELACE: Lest It Not Describe A LOHAN Orbital Vacuum Explosive-Launch-Accelerator Chamber Experiment.

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Go

A flash of the obvious

LINDSAY - LOHAN Ignition Node System Appliance Yardstick

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2nd valve

Will a second valve not be needed to protect the pressure gauge as the motor is ignited?

Resulting pressure spikes may damage the gauge.

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Meh

Pictures otherwise it never happened

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.

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No no no no no... FFS NO!

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.

http://www.2spi.com/catalog/vac/apiezon/apiezon-waxes-data-sheet.php

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).

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Coat

I have a cunning plan

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....

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Happy

Someone doesn't get irony....

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Bronze badge

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.

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Thumb Up

Keep those crazy titles comming - gave the ladies at work a shock when they glanced at my workstation.

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a title is now optional!

You should either rethink your proportions, and make it longer and thinner, or put some insulation or a bit of dry ice on top - at the moment, too much heat is coming in from the top - besides, doesn't LOHAN deserve something longer?

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(Written by Reg staff) Silver badge

Re: a title is now optional!

Graphic isn't to scale, so the thing will probably end up longer and thinner.

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Anonymous Coward

@Martin Gregorie

Same idea, some form of thether / restaint for lid in case the preasure increase is a tad "vigorous"....

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Bronze badge
Boffin

Are you building a Bomb or a Pressure Vessel?

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.

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Silver badge
Boffin

its a pity

that the united kingdom does'nt have a real space program to keep the rocketery loonies like your good selves off the streets and gainfully employed.

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