Original URL: http://www.theregister.co.uk/2012/07/11/rehab_result/

LOHAN fizzles forlornly in REHAB

Rocket motors feel the lack of pressure

By Lester Haines

Posted in SPB, 11th July 2012 12:04 GMT

It's been a lively couple of days down at the Special Projects Bureau, with the first of our Rocketry Experimental High Altitude Barosimulator (REHAB) experiments, designed to determine whether solid propellant rocket motors will fire under simulated high altitude conditions.

Click here for a bigger version of the LOHAN graphicAs we previously explained, we decided to do the low pressure test first without using the dry ice required to bring the motor temperature down to a chilly -60°C.

Our reasoning was pretty simple. If the motors wouldn't fire at low pressure in an out-of-the-box state, then we'd need to address that issue first.

We promised you an impressive location for the first foray into REHAB, and we think we delivered. Here's me and SPB volunteers Federico Buenadicha (our CAD man) and Rui Luz posing in the Plaza de Toros in Béjar - a town in the neighbouring province of Salamanca. Note the official SPB T-shirt and some retro Reg apparel:

The REHAB team: Lester Haines, Federico Buenadicha and Rui Luz

Béjar's bullring is reckoned to be the oldest in Spain, with some of the original stone structure dating back to 1711. We're obliged to the mayor of Béjar, Alejo Riñones Rico, for allowing us the use of this magnificent structure, and to Juan González Castellano – the mayor of the nearby village of La Garganta, in Extremadura – for acting as intermediary in the negotiations.

Here's another snap of Federico and Rui, as we prepared the REHAB rig for a first blast:

Federico Buenadicha and Rui Luz in the bullring in Bejar

So, our two test units were: an AeroTech RC 32/60-100NS – seen here (clockwise from left) with its G-class reload, motor case, sealing ring, end cap and nozzle...

The AeroTech rocket motor components

...and a Cesaroni P29-1G casing, with a 57F59-12A "White Thunder" reload:

The components of the Cesaroni P29-1G motor

First up to the plate was the AeroTech motor. The Copperhead ignitor is, as the rocketeers among you will know, a delicate flower which required careful handling by me and Rui:

Lester and Rui prepare to connect the motor ignitor

Once we'd got the igniter cable connected to the Copperhead with croc clips, we wrapped the whole thing in insulating tape, to prevent a short against the REHAB chamber's metal casing...

Connecting the AeroTech Copperhead ignitor

...and then I carefully lowered the motor into place...

Lester lowers the motor into the REHAB chamber

...and seated the perspex lid on the silcone seal:

The motor in the chamber and the perspex lid in place

The silver thing you can see passing under the blue and brown igniter cable cores is the thermocouple cable, coated in high-temperature-resistant silicone and metal foil, as protection against the rocket exhaust.

In this case, the thermocouple wasn't connected to the motor, since temperature wasn't a consideration. For the record, though, the interior of the chamber was at a balmy 18°C.

We then just had to rig the mirror and monitoring camera, and we were ready to rock'n'roll:

The chamber with the monitoring video camera mounted on its tripod

Here's a closer view of the whole REHAB rig. In the foreground is a vintage piece of SPB kit: the Kodak Zx1 video camera which recorded the famous flight of PARIS's Vulture 1 aircraft:

The complete REHAB rig, ready to roll

Having drawn the short straw, Rui positioned himself to shut off the vacuum gauge isolation valve, as soon as the rocket motor fired:

Rui prepares to shut off if the vacuum pump isolation valve

This precaution was to prevent damage to the gauge caused by a sudden rise in chamber internal pressure, although in the event, there was little risk of that.

With the vacuum pump up and running, and the system at 20mbar, a dramatic countdown culminating in a slightly nervous press of the ignition button resulted in... absolutely nothing.

Or rather, an electrical continuity test showed that the Copperhead igniter had fired, but had failed to set off the motor propellant.

This didn't concern us unduly, since Copperheads have a reputation for being temperamental. Unfortunately, though, the Cesaroni unit behaved in a similar fashion, with the igniter blown and no rocket burn.

Here's what should have happened, as seen in our first run of the AeroTech motor last year:

The question is, why did the igniters fire but the powder fail to burn?

Both motor reloads have their own oxidiser, so the lack of oxygen shouldn't be an issue. Nonetheless, AeroTech president Gary Rosenfield told us last year he wasn't sure "if the motor will ignite reliably at that altitude without a burst plug or other means to retain internal pressure".

This could be the explanation, or more specifically, oxygen might be required to facilitate initial motor ignition.

Suffice it to say, we're going to get straight down to another test, and our first idea to close the end of the motor with a sacrificial silicone seal, which should act as Gary's "burst plug".

As ever, we invite reader comments/suggestions, and let's see if between us we can't crack this hypobaric problem...®

Further LOHAN resources:

LOHAN - A Special Projects Bureau production in association with...

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  • University of Southampton logo
  • Applied Vacuum Engineering logo