Warming up the thruster that will ram LOHAN to glorious heights
Project team prepares to shove pocket rocket in toaster
Our audacious Low Orbit Helium Assisted Navigator (LOHAN) spaceplane project continues to advance on multiple fronts, and while Neil Barnes and Anthony Stirk put together the Special Project Electronic Altitude Release System (SPEARS) control board, we've been doing some more work on the Vulture 2 rocket motor heater set-up.
To summarise, we know a solid rocket motor will fire at altitude, but the effect of very low temperature on the thruster's ammonium perchlorate composite propellant (APCP) is unknown.
To launch a pre-emptive strike on this potential problem, we decided to heat the motor on the ascent phase of the mission, and got our hands on a space-grade Polyimide Thermofoil flexible heater.
The heater's effective area is 300.257 cm2, and its resistance is 64Ω. At 12V it draws 0.1875A and pumps out 2.24W, which translates to a watt density of 0.0078W/cm2. We hooked it up to a 1300mAh Lithium Polymer (LiPo) battery (11.1V nominal)...
...and the first results were encouraging. Here's how the LOHAN toaster performed when sandwiched between two sheets of styrofoam:
Next page: Two is one, one is none
"lack of atmosphere at altitude may prevent heat radiating away"
Surely, convecting away? You'll get more or less the same amount of heat loss by radiation in a vacuum as you would in sea-level-pressure air, I assume.
Latent heat of freezing?
There's a way to keep it warm using the latent heat of freezing of water. Surrounding the motor with a water-filled coat would keep it above 0 deg C until the water had frozen. The coat could be something like lengths of narrow bore layflat tubing, or even a palisade of straws if some simple and efficient means could be devised to keep the water in them and allow for expansion.
The heater puts out 2.24 W for 60 min, say, which is 3,600 x 2.24 = 8,000 J (approx.) The latent heat of freezing for water is 334 J/g. So it would only require 25 grams of water or so to freeze to provide an equivalent amount of heat to that supplied by the battery. But check my sums first!
Clearly a bit more thought is needed on the actual design, how to add a conducting inner layer and insulation on the outside etc., but it might be a reliable solution.
Re: Warming up my tube.
**SEPTIC ALERT** >>>>>> aluminum tube
Surely SPB only uses aluminium?