NASA calls off attempts to fix Kepler space telescope
reuters.com
reuters.com
However, I do wonder why they build 1 Kepler space telescope and not 2, or 3, and then store the already built ones until they are wanted.
Additionally, you can incorporate lessons learned (from an engineering perspective, not science) into the next instrument. Some things work well and are preserved, and others don't.
Here's a link to some tech awards relevant to future exoplanet instruments: http://exep.jpl.nasa.gov/technology/
On the flipside, I guess the arguments for componentization apply. If off-the-shelf components are improving all the time, integrating them into a full spacecraft should be getting cheaper and easier over time.
What I really wonder about is, given how troublesome and relatively lightweight gyros are, why didn't they have more spares?
Can a robot fix the thing?
and however "powerful" the shuttle was, it couldn't get to something 3 light-minutes away (which was kepler's distance, the last time i looked)
http://en.wikipedia.org/wiki/Armageddon_(1998_film)
"American Components, Russian Components, all the same, all made in Taiwan!"
But yeah, the space shuttles were LEO only. IIRC the highest they ever went was up to Hubble, and I think that was pretty close to the most they could manage.
Though now that I think about it, I am pretty sure they generally jettisoned the external fuel tanks with a little fuel left over, maybe they could delay the jettison to get into slightly higher orbits than usual.
[1]http://en.wikipedia.org/wiki/Kepler_(spacecraft)#Spacecraft_...
Also, it was never economical to fix Hubble with the Shuttle. It was just fortuitous that the Shuttle program was desperate for things to do and ended up subsidizing the launch costs for the Hubble servicing missions. In terms of the ultimate bottom line the US spent nearly $8 billion to fix a $2.5 billion telescope. Granted, I think that even that expenditure was worthwhile but from a strict cost accounting it would have made more sense to simply build and launch additional telescopes.
Kepler, for example, has already used up its budget to build, launch, and operate the telescope for several years. Even if free replacement hardware was available there may not be enough money to launch it. Also, the way most spacecraft are built is as one off prototypes, so building another copy, even if all the spare parts were available, is a very non-trivial endeavor. Even if it didn't take as long as building the original it would still be fairly expensive just to go through all of the necessary testing and integration steps.
The end result of all of this is that it rarely makes sense to spend the time and money to refly a spacecraft and instead it usually makes more sense to simply forge ahead and seek funding for a follow-on mission of a new design.
Here's a few case studies. The inaugural launch of the Ariane 5 rocket failed, which resulted in the loss of 4 "Cluster" spacecraft which were designed to study Earth's magnetosphere (a loss of about $370 million in hardware). However, ESA decided to fund replacements and they built 4 new satellites, making use of flight spares and extra hardware to save on costs, and then launched the new satellites on 2 separate Soyuz-Fregat flights. However, it still cost almost as much money to launch Cluster II as for the originals, so this was a rather heroic effort on ESA's part.
In 1999 the Wide Field Infrared Explorer (WIRE) spacecraft was launched, shortly after launch there was a malfunction that caused a loss of the main science instrument on the orbital telescope. The mission was not replaced. However, the loss of WIRE caused other IR survey mission proposals to receive greater interest, which led to the funding of the "Next Generation Sky Survey" Medium-class Explorer mission. This infrared full-sky survey mission was launched in 2009 (as WISE, the Wide-field Infrared Survey Explorer) and achieved all of the science goals of the original WIRE mission.
In 1993 all contact was lost with the Mars Observer spacecraft right at the moment of planned orbital insertion. This loss led to a re-evaluation of the unmanned Mars exploration program at NASA and eventually led to around 10 new missions to Mars over the following 2 decades, most of them successful.
If you're going to have to launch a new spacecraft anyway, why not take some of those lessons into account when you build the next one?
This strikes me as an excellent thing to do - if they can find a project that can fund the ongoing costs of Kepler, then it makes sense to reuse it.
Kepler looks off the plane of the Galaxy at stars that are typically several kiloparsecs away. In other words, we are never going to visit these worlds. Now that Kepler has revealed that planets are common, it will be much more interesting to find nearby planets that we have a better chance of characterizing, seding probes to, or even visiting one day.
This is what TESS will do: http://en.wikipedia.org/wiki/Transiting_Exoplanet_Survey_Sat...
There are still two more years worth of exoplanets for us.
If you use propellant, you eventually run out and the mission is over. With electric-motor reaction wheels (a minimum of three), you can keep them going with solar energy. So in principle, they're a big improvement over the alternatives.
The problem is that they're constantly in motion -- constantly. If you turn a spacecraft and bring it to a new stable position, that new position must be maintained by constant reaction wheel motion. So they're essentially running all the time, there's constant friction, and their parts and bearings eventually wear out.
> Seems like either a more durable item or some sort of cartridge system where you can have like 20 spares ready to go, eject one and slot in a new one or something.
You're overlooking something. To turn a spacecraft like Hubble or Kepler in a reasonable time, you must either have a massive reaction wheel, or you need to make it spin fast (it's all about momentum). Typical reaction wheels represent a compromise between mass and rotational speed. If you want to reduce frictional wear, you make them bigger, more massive, which increases the mission cost. If you need to reduce the size and mass of the spacecraft, you make the wheels spin faster, and they wear out sooner.
So solving the problem by adding more reaction wheels is a non-starter -- they're too physically large and massive to just add a lot of spares.
Correct me if I am wrong, but don't most systems use six? It is a whole lot easier to use three pairs of wheels. Pairs can be spun up from a dead stop and in the event of reduced power can be used to turn in either direction by braking action alone.
Or does "a reaction wheel" imply two counter-rotating flywheels by definition?
For an alternative, I've heard some good things about laser ablative thrusters. Still fairly young, but they are powered by electricity and have no moving parts. They do need reaction mass, but unlike traditional engines the mass is a solid block and can't boil off, corrode, leak, freeze or whatever problems affect liquid systems.
Kepler was equipped with four -- three in use, one spare. The idea is that you need one reaction wheel per dimension.
> It is a whole lot easier to use three pairs of wheels.
All that six wheels would provide is three spare wheels, one per dimension.
> Pairs can be spun up from a dead stop and in the event of reduced power can be used to turn in either direction by braking action alone.
That's not how reaction wheels are normally used. Two counter-rotating wheels cancel each other out, producing zero net rotational force on the spacecraft.
> Or does "a reaction wheel" imply two counter-rotating flywheels by definition?
No, just one. If you have two, and if they counter-rotate, they cancel each other out.
> For an alternative, I've heard some good things about laser ablative thrusters. Still fairly young, but they are powered by electricity and have no moving parts.
Not nearly enough momentum to rotate a large spacecraft.
are you sure? i thought it was four (in a tetrahedral arrangement), but they were all constantly in use, and one failure could be tolerated.
Source: http://kepler.nasa.gov/mission/faq/
Quote: "Only 3 reaction wheels are needed to control the 3 degrees of freedom of rotation of spacecraft. But Kepler was provided with 4 reaction wheels, one extra for redundancy in case a wheel fails."
kepler also had thrusters, for desaturing the reaction wheels. took at least half an hour or something every few days, i think.
thrusters spew stuff into the local environment, cloudying it up (relatively speaking). so it might not have been a solution for kepler, even if you could pack on the necessary reaction mass.
However... What little I can puzzle out about systems like this suggests that six active wheels (plus spares) would allow for desaturation of spin (cutting back when one flywheel approaches its maximum RPM) without either additional spacecraft rotation or thrusters.
But in the absence of thrusters or another method, to stop one reaction wheel from spinning, you have to start another one spinning -- in other words you have to transfer the conserved momentum to another wheel. So wheels acting by themselves cannot cancel their own momentum, they can only transfer it from one wheel to another.
Another approach is to stop a wheel spinning with a brake pad, which would convert the momentum into heat. But this has obvious drawbacks, among which the energy is irretrievable.
Why not use wheels that use magnets instead of bearings so that there is no friction?
In any case, for various reasons, frictionless bearings aren't ready for spacecraft use.
Magnetic bearings have been used in fans, and I'm familiar with a bearing-analogous application in Inductrack maglev trains as well.
Has this type of tech penetrated into aerospace?
i worked with a spacecraft launched in 2003 that was definitely not using reactions wheels like that. (and saw a failure.)
Such a correction might be needed simply to change the pointing of a spacecraft telescope from one target to another -- let's say, to depart one stationary angle, rotate 30 degrees, and come to a new stationary pointing angle. Such a change of pointing angle would in most cases require all three reaction wheels to change their speed of rotation, and all three wheels would have to continue spinning when the new position was reached.
Remember that rotational momentum is conserved, there is essentially no friction on orbit, therefore the initial rotational momentum must be present in the spacecraft or its reaction wheels. And trace gases, micrometeorite hits, and even light pressure, over time contribute rotational momentum to the spacecraft.
They are doing the same thing with the Webb.
Great job!