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back to article Experts stroke beards over LOHAN's vacuity

Last weekend, we finally got LOHAN to breathe fire when we successfully fired a solid rocket motor at an simulated altitude of 76,500ft (23,300m). Click here for a bigger version of the LOHAN graphic Our Rocketry Experimental High Altitude Barosimulator (REHAB) hypobaric test chamber began to pay its way as a Cesaroni P29-1G …

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Bronze badge
Mushroom

Instead of using silicon to seal the motor, why not use wax?

I'm not sure if this will help, it just seemed appropriate.

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Anonymous Coward

wax

Brazilian?

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@phuzz

I tried a wax plug in a KNSU rocket motor to create a waterproof seal to stop the fuel inside the motor going bad from absorbing water from the air (KNSU rocket fuel is very hygroscopic)

I thought with the way the plug sat in the expansion end of the rocket nozzle it would have pushed out really easily with little pressure because the nozzle was made out of rammed bentonite clay.

When I fired the motor, the plug held firm and so did the nozzle, but the plug on the top end of the motor blew out like it was just sitting on there when I know for a fact it had a really great seal on the motor.

Honestly I as much as I think some kind of seal or rupture disk would help here, I am very concerned it won't blow out fast enough to avoid a CATO event.

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(Written by Reg staff) Silver badge

Re: @phuzz

Duly noted. It appears your personal experience means we need to proceed with caution here...

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Silver badge

Re: plugs

Indeed. plugs are dangerous, especially the soft silicone option mentioned in the previous article, since that stuff *will* get rock hard when at the temperature expected at burn time.

Soft silicone is only soft at room temperature, not at minus something-evil....

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Childcatcher

Re: @phuzz

There is a fundamental difference in altitude flight and vaccuum chamber testing. What might be appropriate for altitude ignition could be dangerous if used at low level (after the pressure chamber is popped). If on the other hand you could maintain the vaccuum. But hey howdedodat! Will Farnborough let you put the test motor in the hypobaric chamber?

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If you're worried about the expensive gauge can you use your old one?

Is your previous gauge cheap enough to risk breaking? could you have both gauges, pump down to required pressure on the expensive gauge, take a reading on the cheaper one. Then isolate the expensive gauge and work entirely from your old gauge?

You should know what it read at 20mbar even if it actually read 30mbar you're only interested to see if it changed.

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I would agree with the vacuum tank approach. Draw down the vacuum, isolate the pump and gauge then let the vacuum tank take any residual combustion products whilst buffering the pressure. This can be cleaned out and dried between runs (a pain but not difficult with a decent inspection port) which will remove the need for a cold trap entirely.

AndyG

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Bronze badge

Tanks

I suggested using a vacuum tank a while back. A tank allows the pump-down of the test chamber to be carried out over a few minutes to match the predicted change in pressure during the ascent rather than just relying on how quickly the pump can exhaust the chamber. An air compressor tank good for 10 bar gauge or so working pressure will easily withstand the crush forces from holding a vacuum.

Ruggedised solid-state pressure gauges are pretty cheap and readily available and a lot more useful than a simple dial indicator for logging and recording the pressure changes.

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Silver badge
Boffin

Igniter emissions are raising the pressure?

Easy to check: put one of the igniters in REHAB on its own, set it off, then measure the resulting pressure. Or, as an alternative when you're worried about the vacuum meter, set the igniter off in a sealed vessel with some stuff to measure the resulting increase in volume.

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Silver badge
Paris Hilton

Project planning

Just wondered (once a PM, always a PM...) what the timeline of events was for LOHAN? Is there any form of indication of the launch date yet, or at least a sequence of events to see what problems or stages are next?

If only because you surely want to utter the words "I love it when a plan comes together" at some point?

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Anonymous Coward

It is the "Capacity" that needs to be tested.

Hrm.

There is a minor flaw in the Thermal / Pressure testing for the motors. (I think Cesaroni Technologies Incorporated might be capable of providing insights.)

As some have mentioned, the Propellant grains use a positive pressure method to achive greatest effect.

So a pressure stub plug, or an end cap is a wise choice. So is, too; the choice of the igniter to provide part of the positive pressure upon ignition.. (The Aerotech single use motors use an End Cap for low-temperature firings, but since you are using reloadables, I think this might have been missed as part of your testing parts list for the Aerotech. -- I recognised the same burn pattern on the Aerotech propellant grain.)

The comment about the Rocket nozzle is only pertinent for Large Scale solid-propellant and liquid propellant (Firing times of 20 seconds or more) in this case a simple hole would suffice too, but the nozzle design is for ~ 820 Feet ASL as the average calc height to the nozzle profile.

To acheive the correct conditions however, you need to have the Volume and Capacity of the chamber to keep the pressure low enough overall for the test to be complete.

I think the chamber itself is a Great Start and proved the initial capability of the test, and the drawbacks of not pressure-sealing the Rocket motor.

The Low Temperature testing is another test that should be done under initial conditions of positive pressure, or even partial pressures (~12000 foot simulated altitude) rather than full-up.

I think that the low temperature testing will reveal another issue due to the reloadable nature of the Rocket Motors themselves. Plastics under extreme low temperatures are my concern, when the pressure/thermal transient occurs. (I have fired Aerotech Single-use Motors in Canadian winter temperatures {-25C} with mixed results. 2 failures and 2 successful firings.)

I think a large 15 - or - 25-gallon vacuum-capable staging tank should fulfull the needs for the large volume to simulate upper atmoshere conditions, and still fit the existing Vacuum pump design you are working with.

-sean

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Anonymous Coward

Massive igniter firmly rammed into LOHAN's exhaust port

So why are the igniters so huge.. do they need to be? surely just some quickmatch packed around the standard motor igniter all packed into a pressure plug on a motor with a smaller apature would be what your after.

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Pressure gauge concern

Your concern over damaging the pressure gauge during ignition and subsequent burn can easily be mitigated: Simple place an isolation valve between the gauge and the manifold; Leave it open (till just before) ignition so you can read the pressure at the time of (or just before) ignition. Close it @ ignition time (or just before) so you can leave the pump running and sucking but protect your gauge. The pump should be fine - in particular if you use the cold trap as suggested.

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(Written by Reg staff) Silver badge

Re: Pressure gauge concern

Failsafe - they're effectively two igniters for connection to two ignition circuits.

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Nozzle and slow ignition.

Can I just point out that I mentioned both of those a while back?

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(Written by Reg staff) Silver badge

Re: Nozzle and slow ignition.

Yes you can.

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Re: Nozzle and slow ignition.

Thanks.

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(Written by Reg staff) Silver badge

Re: Re: Nozzle and slow ignition.

My pleasure ;-)

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Vote for vacuum accumulator

I support the idea of a vacuum accumulator to sustain the low pressure in the firing chamber as long as possible. The tube between the REHAB chamber and the accumulator needs to be large enough to accommodate the sudden change in pressure inside REHAB when the motor fires.

The cost not mentioned is that it will take much longer for the pump to achieve the targeted low pressure.

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Headmaster

Nozzle diameters

If anyone wants an idea about the difference in nozzles for ground level and near vacuum, have a look at the Falcon 9. It uses the same basic engine on both stages, 9 on the first and one on the second, but the nine first stage engine nozzles almost fit within the body diameter whereas the second stage nozzle fills a large part of the interstage.

Ideally you want the hot gas exiting the nozzle to be at the same pressure as whatever atmosphere is surrounding it. If it's higher you're wasting efficiency, if it's lower then you're likely to get flow separation which gives control problems as it tries to kick the engine sideways. If you watch one of the Falcon 9 launch videos you can see the flame of the first stage expanding as it gains height and the pressure drop, and also see the relative size of the second stage nozzle at satge sep.

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Thumb Up

Thirded on the vacuum tank approach - seems like a low effort way to confirm the pressure diagnosis

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Mushroom

Vacuum tank considerations

While a vacuum tank can be made to work in principle, it does require some possibly troublesome-to-meet requirements.

To a first approximation, suppose the gases generated by the igniter occupy a volume of x cc at atmospheric pressure (about 1000 millibar) and 'normal' (ambient) temperature. Boyle's law (PV=constant) allows one to predict that the same gases will occupy a volume of x * 1000/20 cc = 50x cc at 20 millibar. Depending on your best guess of the volume of ignition gasses at 1000 millibar, you will need a 50-times larger vacuum tank for the gasses to expand into, to minimize pressure changes.

This then leads to the second design issue: constructing a vacuum tank of the necessary size that can withstand the vacuum to be used. A weak tank will provide much excitement if / when it collapses*!

Since we know that at least some of the products of igniter combustion are easily condensible (witness the lexan lid getting sooted over), a cold trap is the much easier solution. I don't know if there are any moonshiners in the Spanish hills, but a copper distillation coil (or a segment of a car radiator) stuffed into a dry ice-ethanol slurry in a styrofoam box should work quite well. Protection of the expensive pressure gauge can be had by closing it off at the manifold when ignition occurs.

<icon: inverse = implosion>

* http://www.youtube.com/watch?v=n-3cu_Q119s

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Re: Vacuum tank considerations

@VeganVegan > " I don't know if there are any moonshiners in the Spanish hills, but a copper distillation coil (or a segment of a car radiator) stuffed into a dry ice-ethanol slurry in a styrofoam box should work quite well."

The condensation coil will also have to withstand sea level atmospheric pressure and maintain the vacuum, so I doubt a segment of car radiator will be sufficiently robust. The other option would be to enclose the condensation coil within a cylinder made in the same way as the rehab iteself, chilled and atmospherically sealed.

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Bronze badge
Holmes

What were the engines DESIGNED for?

As AC 13:22 stated, perhaps Cesaroni Technologies could be helpful in determining what pressure differentials are allowable or desireable across the specific nozzle orifice you are using. If there is enough backpressure inside the nozzle, it should have sustained combustion once ignited. If the backpressure is too low (nozzle orifice too large) it could easily blow itself out (which may have happened to the Aerotech).

Since the propellant is solid and contains both fuel and oxidizer, the flameout issue would not be related to available oxygen but how much pressure differential exists between the chamber and inside the nozzle at the point of ignition. Nozzles designed for Atmospheric use are not the same as those for high altitude use which is another possible issue to consider.

See link for much better description and math. http://en.wikipedia.org/wiki/Rocket_engine_nozzle

Aslo see http://en.wikipedia.org/wiki/Solid-fuel_rocket#Nozzle

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Silver badge

Bah!

Why not stop faffing about and actually attempt to fire the motor at altitude with a balloon-lofted test rig?

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(Written by Reg staff) Silver badge

Re: Bah!

In a word: money. I also subscribe to the "suck it and see send a motor aloft and see if it fires" approach, but the balloon and helium are very expensive. We need to do our homework before sending anything upwards.

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Go

Got to be done...

LOHAN *must* feature a burst diaphragm, if only for comedic reasons.

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Gold badge
Happy

Re: Got to be done...

Seconded.

I'd like to offer some help here by providing a precedent for your reasoning.

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Happy

"we'd still worried about the possible terminal destruction of our pressure gauge".

Have you considered an absolute pressure transducer? The transducer will be more robust and you just need to calibrate before and after the test. You can do it yourself as you already have a precision reference gauge.

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Meh

At this point..

I'm thinking liquid fuel.

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Bronze badge

Re: At this point..

Liquid fuels?

http://www.tor.com/stories/2012/07/a-tall-tail

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Silver badge
Pint

Re: At this point..

There you go....

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Silver badge

Re: At this point..

Liquid fuel would need an oxygen supply, which isn't available at the launch altitude. Unless you can think of a simple way of storing and carrying LOX with the payload?

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Ru
Flame

Re: At this point..

"Liquid fuel would need an oxygen supply"

It would need an oxidiser; LOX is not the only game in town. The traditional approach taken when people wanted liquid fuels without the need for any sort of cryogenics is UDMA and IRFNA, a couple of chemicals which make a hydrogen-filled balloon seem positively benign. Their hypergolic nature makes ignition a lot simpler too, which is part of what makes handling them so much fun.

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Some thoughts

A bit of searching around brings a few facts up.

PIC uses Lead Dioxide and Silicon, and yields Lead and Silicon Dioxide as reaction products, neither of which are gasses at low temperatures. That and any vaporised plastic. The electric match uses Antimony TriSulphide and Potasium Chlorate, which yields only some Sulphur Dioxide as a gaseous product, the Potasium Chloride and Antimony Oxide not mattering. Thus the igniter may well not be reducing the pressure in REHAB all that much. Clearly the easy test is as suggested above - just ignite one inside the test chamber and see what happens to the pressure. That alone should set to rest any issue of whether any additional hardware is needed. There are two places where the pressure matters. In the test vessel (REHAB) where you want it to remain low, and in the motor chamber, where you simply need it to be accurate. In the motor chamber the temperatures may be much higher, and the reaction products may stay gaseous for a tny bit longer, but this is what they will do in the actual launch, so this remains accurate. Outside the motor chamber everything will be cold, and I suspect that little of the reaction products will remain gaseous for any meaningful time except the the Sulphur Dioxide.

Next, the equation of burn rate of the motor is Rate = constant x pressure^n. Where n depends upon the propellent composition, and seems to vary between 0.2 and 0.5 Basically the burn rate only depends upon the chamber pressure. Whilst this was known in the abstract, just how critical it is is perhaps a surprise.

The point of the PIC is to slam heavy hot particiels into the rocket grain, obviating the need for heat transfer by conduction though hot gas.. But the grain won't stay burning unless it is subject to sufficient pressure. If it doesn't start burning fast enough to build chamber pressure it seems it fissles out. Probably as soon as the PIC is depleted. So it may be argued that a problem with the PIC is that it may not be producing enough hot gas. Given the above this is perhaps not a surprise. A reliable ignition might be achieved by a modification that simply adds something that produces hot gas as well as the hot sparks in the motor chamber. This could be much more reliable and less subject to catastrophe than a burst plug. However it may simply be that the difference in composition between the different manufacturer's motor grains may be enough to bridge the gap. The burn time of the motors might provide some clue as to this since the motors are much the same weight.

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On the other hand

Following up from above, the problem with the test chamber pressure probably remains. Even if the igniter doesn't create much gas, the motor grain is designed to create much gas, so a sputtering grain that is doomed to fail in a real launch might manage to create enough pressure in the REHAB chamber to cause itself to light up. This presents a difficult problem. It suggests that a completely valid test does need a large enough vacuum chamber to cope with a significant amount of gas production.

One is reminded that some tests of real rocket motors use basically a very large water jet ejector that can sustain a vacuum even after the motor starts running. I wonder if something designed to fit on a fire hose would work? (Only partly in jest here.) An industrial size water jet ejector would probably work if you could find one.

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Bronze badge

Re: On the other hand

Getting enough vacuum from a water jet ejector is probably going to require a heck of a lot more pressure and flow than a fire hose can supply.

I agree with my fellow commentards that the easiest way to determine the effect of an igniter on the REHAB vacuum is to simply ignite JUST the igniter inside and see what happens to the vacuum. (If it's enough to pop the lid we can be very certain this might be a problem)

Possibly adding a tiny bit of low nitrated gun-cotton inside the nozzle could help. The PIC gives of the heat, the gun-cotton raises the chamber pressure (and I suggest low-nitrated as this goes more fwoosh instead of bang. High-nitrated can give a bit too much bang when confined)

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Windows

I am DEFINITELY no rocket specialist/enthusiast...

But I've often wondered why LOHAN has only one motor stuffed up its jaksi, instead of, maybe 3? Cost, I guess, but redundancy trade-off? (however, if all DO fire up, it'll sure make its eyes water....)

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Re: I am DEFINITELY no rocket specialist/enthusiast...

Like all comedy, the answer is in the timing. Much better to use one that is three times more powerful than mess about with worrying that the three don't ignite exactly at the same time (or worse, one fails to ignite) and then have to cope with asymmetric thrust just as it tries to launch. Failure of one SRB to ignite on the Shuttle was one of the unsurvivable accidents (and possibly most spectacular) possible.

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igniter gas

Have you considered testing the pressure in the chamber after just using an igniter wihtout a motor? That should determine the effect of the ignition gasses, and may be cheaper than including either a large additional chamber or a condensing coil.

(Apologies if this suggestion has already been made, not had time to read all of the posts).

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Bronze badge

Look back in anguish

Aside from going back to my suggestion of just using gun wadding soaked in an oxidizer (which will form a pretty good pressure seal at low temperatures), could I suggest looking at the principles behind the old pulse-jets. Create a combustion chamber at the exit that allows a build up of pressure inside the rocket motor while also providing a tuned exit pipe that allows the pressure to be released in a controlled pulse of thrust. With the right dimension you might even be able to prolong the firing into a series of jet pulses. Just a thought or am I basically saying the same as all the others.

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Alert

vacuum gauge protection

I agree with the various comments w/r/t cold traps and, possibly, large vacuum reservoirs. Note, however, that the latter represent considerable stored energy--not only will they require proportionally longer time to pump down, but there's a risk of implosion (or at least collapse) with a mighty bang and possible shrapnel.

As far as protecting the costly vacuum gauge is concerned, you might consider simply placing it at the end of a much longer line, and possibly installing very small orifices at both ends of the line. In addition, you could stuff the line to the gauge with something like steel wool or fiberglass insulation, which would be permeable to air (and its absence) but which would tend to trap particulates, while the orifices would slow gauge response to sudden pressure changes including those which occur when the motor lights off and the REHAB lid is blown away.

Finally, while off-topic:

A Higgs Boson goes into a catholic church. The priest asks it, "what are you doing here?" The Higgs Boson replies, "you can't have Mass without me."

(rimshot)

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