Comment by mturmon | Hacker News Reader

Comment by mturmon | Hacker News Reader

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"I don't know why Hershel used liquid helium instead of a cryocooler...it's a risk thing"

The cryocooler technology (say, to a few kelvins) has proved hard to get ready for space. For example, out of the 10 or so technologies that were judged most risky for JWST, the 6 K cryocooler for the MIRI instrument was the last to be judged ready for space ("at TRL 6" in the jargon) (http://www.stsci.edu/jwst/news/2007/jwst-passes-tnar).

Despite being judged ready, the JWST cryocooler has proved very challenging to build. The effort now has frequent reviews with the director of JPL (and a high-level counterpart at NGST), and tens of engineers are now working on the system.

Part of the problem, as I understand it, is that the heat has to be taken away and radiated at a site distant from the IR detector. This requires a large structure, and a deployable radiator. This large structure can't leak much heat back into the spacecraft bus or instruments, and must not be disturbed by the vibrations of launch. Additionally, vibrations of the cryocooler must not affect the telescope optics (2 micron resolution).

You can tell that these requirements are fundamentally opposed to each other ("be large, don't vibrate, don't touch anything else").

Precisely what I was thinking. Expelling liquid helium doesn't introduce any heat. Any active cooling needs to somehow expel the heat generated while compressing the gas back into a liquid. In the vacuum of space, irradiating heat is the only way to remove it from the system, and that isn't going to be very efficient. Given the choices and the temperatures they needed to reach, liquid helium seems like a rational solution.

There's also a lifetime issue. Say you decide, "forget the whole vibration thing, we'll run the cryocooler half the time and take data half the time while the cooler is powered off"

Then you either get half the mission data, or extend the mission by a factor of two, which means twice the operational labor cost, plus every other part of the craft has to be double lifetime rated, which could get expensive. Hubble used to burn thru gyros on a regular basis, so now you need them to last twice as long or launch with twice as many spares. Or maneuvering propellant if any, now you need twice as much for station keeping.

You can easily get painted into a corner where the cheapest way to run a mission twice as long is to launch two of them. At that point you're better off saying "you know the cryocool half the time and take data half the time idea? Yeah scrap that idea"

This is before we started on energy issues. A heavy helium tank doesn't use much energy. But cryocoolers on earth take quite a bit indeed, well, at least compared to a couple watt transmitter and all that. Whats heavier, a tank big enough to last mission lifetime, or a cooler and a stunning array of solar panels to run the cooler? Or since it only runs half the time as per above to prevent vibration issues, you could put a battery in which is heavy and becomes another exciting point of failure. This makes the existing power system more complicated and less reliable possibly shortening the theoretical craft lifetime to less than you'd get if you just launched a big simple tank.

How about just making a detachable LHe tank and send some replacement helium every few years? Shouldn't be too costly even given the day-to-day cost of running a space observatory.

Given the weight of the LHe tank, I think they'd rather send up a new telescope along with the tank.

Even worse the helium was the limiting consumable in this particular case, but for the next scope the last gyro might burn out in 5 years. So now you need a supply of gyros, more likely an entire nav unit bolted permanently on. And next time the battery croaks so you need a battery and a way to install it and isolate the old one such that you can never end up with neither installed. And next time the transmitter final power amp goes thru too many thermal cycles and cracks so you need a complete telemetry unit.

Yes I know there are good reasons why they put it at a L point, but unscheduled unplanned maint is exactly what the ISS could have been perfect for. Position this dude in the same orbit but at a phase 10 miles away so its not too close, and send a live astronaut with a new battery or whatever anytime "something" fails. To say the ISS project is not managed this way would be an understatement. But someday, an unscheduled repair shop in space will exist, and probably be quite profitable.

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