Date: Fri, 23 Jun 2000 09:07:38 +1000 From: Peter Jeremy <peter.jeremy@alcatel.com.au> To: Narvi <narvi@haldjas.folklore.ee>, handy@lambic.physics.montana.edu Cc: freebsd-hardware@FreeBSD.ORG Subject: Re: Hardware in space? Message-ID: <00Jun23.090747est.115316@border.alcanet.com.au> In-Reply-To: <Pine.BSF.3.96.1000622113850.2206U-100000@haldjas.folklore.ee>; from narvi@haldjas.folklore.ee on Thu, Jun 22, 2000 at 11:44:01AM %2B0200 References: <4.1.20000622075529.00979b80@mail.rz.fh-wilhelmshaven.de> <Pine.BSF.3.96.1000622113850.2206U-100000@haldjas.folklore.ee>
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On 2000-Jun-22 11:44:01 +0200, Narvi <narvi@haldjas.folklore.ee> wrote: >On Thu, 22 Jun 2000, Olaf Hoyer wrote: >> Basically, you need to transport the heat=energy away from the chip. >> On earth, you may take air as transport and dissolver. >> But why no liquid cooling? >Crazy! Liquid weights a lot - and besides, how do you cool the liquid? If the system have to run for an extended period (ie, reach steady state equilibrium) then your only option is radiation. Since the proposed flight has a limited endurance, you could just store the unwanted heat or rely on consumables to dispose of it: - Using specific heat: You use a massive heatsink and rely on the heatsink's specific heat to limit the temperature rise. Aluminium has a specific heat of ~0.95J/g per degree C. 10W for 15mins is 9000J so you'd be looking at ~200g for a 50 degree temperature rise. Good alternatives include lithium (3.3J/g/degree) and water (4.2 J/g/degree), but both have a much lower density (meaning more volume to store the heat) and are more difficult to handle (lithium is highly reactive and water is a liquid under room conditions). - Using latent heat of fusion: ie melting a solid. The advantage is that latent heats of fusion are 1-2 orders of magnitude higher than specific heats (with a commensurate reduction in heatsink mass). The temperature also remains constant (since the heat is being absorbed via a phase transition). Picking an appropriate substance is more difficult but I believe there are a range of eutectic salts intended for thermal storage - you might be able to find a suitable one. Alternatively, Rose's Metal (from memory) might do. - Using latent heat of vaporisation: ie evaporating a solid or liquid. The advantage is about another order of magnitude reduction in mass. The disadvantage is that you'll need some way to vent the vapour, without losing the liquid/solid. Water is probably a reasonable choice if you're venting into a vacuum. In general, I wouldn't recommend Peltier cells because of their poor efficiency (you'll probably triple your power consumption), but could be justified if you find a good phase-change material that is just a few degrees too hot. BTW, there are low power (CMOS) variants of both the 386 and 486 - these will further reduce your power consumption - especially if you reduce the CPU clock speed. As for the problem of extended pre-launch holds, the best solution would seem to be implementing a low power hibernation mode (or even total power off, if this is practical) that is broken by the launch g-forces. Peter To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-hardware" in the body of the message
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