We turned up the heat a little this week at the Low Orbit Helium Assisted Navigator (LOHAN) headquarters, with the first test of our space-grade Polyimide Thermofoil flexible heater, intended to keep the Vulture 2 spaceplane's rocket motor nice and toasty as it it ascends into the stratosphere. Click here for a bigger version …
LiPo at altitude
may I respectfully suggest that you load-test LiPo in REHAB?
LiPo usually is kept together using the coffee-pack principle, meaning that the active ingredients are inside an air-tight bag at (close to) vacuum. The outside air pressure then presses the cathode, the conductive polymer and the anode together, do that beautiful chemistry may occur. Remove said air pressure, and chances are that the spark will be missing, with the sad result that LOHAN will fail to see an ejection while suspended under her massive orbs. Should LiPo fail in REHAB, then you may want to consider using potted Li-Ion. The hard enclosure means that energy/weight is somewhat reduced (chemistry can be the same), but these batteries can work successfully in space (check specs for individual cell types).
Also note that you will get more energy out of the pack if some energy is used to keep the pack itself at temperature.
..mine is the one with a 12.5 kWh / 289V space grade Li-Ion pack in the pocket...
Re: LiPo at altitude
Not just that.
LiPo capacity drops nearly exponentially with temperature.
I suspect it needs to keep itself warm as much as it needs to keep warm the motor. That will add some more weight and logistics around insulation.
Re: LiPo at altitude
Fair enough, we'll give the battery a blast in REHAB
Re: LiPo at altitude
"LiPo capacity drops nearly exponentially with temperature.
I suspect it needs to keep itself warm as much as it needs to keep warm the motor. That will add some more weight and logistics around insulation."
First: Use low temperature capable cells.
Second: Insulate them well (keep them inside truss payload compartment
Third: Do something like this: http://lange-aviation.com/htm/english/products/antares_20e/img_battery_module_xl.html
The red stuff is the battery heating wrapped around 3 individual cells.
May I suggest
LiPosuction for the code name of that experiment?
<coat, many sizes too large afterwards...>
Re: LiPo at altitude
If the battery chosen for LOHAN falls to pieces in REHAB, maybe duct tape or heat shrink might help keep things together? Add two balsa boards to make a battery sandwich and tape it up tight.
At altitude, heat will still radiate away, as radiative heat transfer still works in a vacuum. It is convective transfer which will be much less. However, the very low temperatures may offset that (larger heat gradient).
Heat loss by radiation, at that differential (less than 50K), will be very low.
My only point is that it is unaffected by the low density of the air. It is convective transfer (along with a bit of diffusion) which takes a dive. If the object is in direct sunlight, radiative transfer may cause it to warm up.
Just a thought... If the battery pack is in the truss and the heater is on the rocket, how do you seperate the 2 at launch?
We're working on that. All suggestions welcome...
>> Just a thought... If the battery pack is in the truss and the heater is on the rocket, how do you seperate the 2 at launch?
The playmonaut will carry a small pair of scissors, or possibly a hacksaw...
First thought involves two metal strips in some sort of conductive grease, inside a close fitting (but not tight) plastic sleeve. Shouldn't have any problems with the conductors freezing together, easy enough to assemble and disassemble, shouldn't impose any undue force when it comes to separation time.
Use the thinnest suitable copper wire for the connection and route the wires across the rocket motor nozzle. Simple, effective, zero weight.
I would recommend simply using exposed mated pins (ie. DBx pins with no housing). You can keep the wires leading to the pins at different lengths to ensure you don't get any shorting. I'm not sure how a bit of dangling wire off of LOHAN will affect flight characteristics but it is very simple and the pins don't bind tightly allowing them to be pulled apart once you have ignition. The only consideration is if low temperature may cause them to bind but then again the surface area where they connect is very low so any freezing shouldn't have too much of an impact given the force generated by the motor.
Regardless, based on you battery usage testing it appears that you can use a less than ideally conductive interface since there appears to enough battery power remaining to deal with high resistances. I have in past had success using iron filings lacquered to tissue paper (burns up very quickly).
> All suggestions welcome...
What about induction?
There would be efficiency losses, but you could also get 2.24 watts of power in AC form over a couple of millimetres, and you wouldn't even need a physical link.
Could even push data through such a link if you want to go silly.
"...area is 46.54 in2* (300.257 cm2)"
46.54 is an odd size to make anything, but 300 is nice 'n round. I suspect that someone else may have already converted this the other way and not been too precise about it.
This baby is growing up
Have you recalculated the expected altitude you hope to reach before launch, given all the extras that have been added to the payload over recent weeks? Might be simpler to just launch the damn rocket off a tall building.
Re: This baby is growing up
Pah. Big balloon, more helium.
Low Temperature Batteries?
Are you sure the battery will still work once it starts to lose temperature and pressure? You might need to put the battery somewhere next to the heater..
can I just be the first to point out that...
"Sexy Flexible Motor Toaster" would be an excellent name for a rock band
Look, everyone knows what happens if you keep switching units: you'll crash into Mars
Re: SI Units
Sounds good, if they can get the video back to Earth...
Bog standard linguine units!
re: temperature issues
Temperature should not be a problem for the battery used, since afaik the current plan still involves incorporating man+dog into the flying truss in a properly insulated box with a couple of handwarmers tossed in for good luck. So while testing performance in REHAB is a good idea, temperature should not influence performance significantly.
A sliding connection would be needed to hook things up anyway, given the way LOHAN is suspended on the rail.
...simply keep the whole heating module as a wrap around pot (mostly styrofoam) around LOHAN's backside, which then can simply blast out of it---a much simpler setup.
Re: Why not...
To be close enough to heat effectively it would pose a risk of freezing together.
I wonder if it is also worth looking into LiFe batteries? They are safer, but slightly heavier. Is it possible to run the heater on a lower voltage / higher amps to compensate since you seem not to have issues capacity wise?
You need to test any battery at the expected temp and pressure regardless - Temperatures are a noticable factor when I run my RC, and since Lipos are in soft bags, pressure could certainly affect them.
re: Is it possible to run the heater on a lower voltage / higher amps
Ohm's law would suggest not.
at ambient a 30 degree celsius delta is possible with some insulation included? What's the outside and inside temperature at 35000ft like a plane thingy for example?
Does the battery generate heat itself while discharging? - apart from the exploding variety!
If so, that will help as you won't be trying to start the system at altitude, just running it while you get there.
Also, you could preheat everything while still on the ground - take the motor near to it's maximum safe temperature before launch.
For over-temperature protection you need something like:
Airpax sanata Part No. 67L070
These are very small (TO220) and operate as a simple switch in series with the heater.
It might be worth considering LiMnO2 primary cells to power the heater. They have a higher energy density than LiPo and some are specified to operate down to -40C. See http://data.energizer.com/PDFs/la522.pdf and http://www.houseofbatteries.com/documents/U9VL-J.pdf.
LiPos don't generate heat during discharge, which is a good thing. The battery will be inside the insulated electronics enclosure, so we don't see the external temperature being an issue, for the battery at least. As regards the heater, insulation is obviously the key.
A nice lump of plutonium would keep the whole thing warm indefinitely and you'd easily find it afterwards. You should ask BNFL if they could lend you a chunk.
Seriously, what's the worst that could happen?
probably the worst would be that it could fall on your head from 100000 ft. It's a little bit more dense than gold. 1/2mv^2 and all that.
You will be wanting a GPU ...
That's Ground Power Unit, not the usual computer stuff.
Keep any heaters etc powered by a GPU until ready for launch, then plug in the battery pack for the ascent. That way you can pre-heat motor (and anything else you decide to heat) without using up (wasting) energy from your precious payload. Get the thermal mass of the rocket fully warmed through to your target temperature, and you'll need a lot less energy to keep it warm till ignition (smaller batteries, less payload weight).
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