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posted by LaminatorX on Thursday September 03 2015, @05:05PM   Printer-friendly
from the freight-train-hopping-IN-SPAAAAAACE! dept.

Traveling around space can be hard and require a lot of fuel, which is part of the reason NASA has a spacecraft concept that would hitch a free ride on one of the many comets and asteroids speeding around our solar system at 22,000 miles per hour (on the slow end). Comet Hitchhiker, developed at NASA's Jet Propulsion Laboratory, would feature a reusable tether system to replace the need for propellant for entering orbit and landing on objects.

The spacecraft would first cast an extendable tether toward the object and attach itself using a harpoon attached to the tether. Next, it would reel out the tether while applying a brake that harvests energy while the spacecraft accelerates. This allows Comet Hitchhiker to accelerate and slowly match the speed of its ride, and keeping that slight tension on the line harvests energy that is stored on-board for later use, reeling itself down to the surface of the comet or asteroid. A comet hitchhiker spacecraft can obtain up to ~10 km/s of delta-V by using a carbon nanotube (CNT) tether, reaching the current orbital distance of Pluto (32.6 AU) in just 5.6 years.

Unfortunately rocket scientists apparently don't read SN, or they'd know from discussions last year that it simply won't work. It seems that the idea defies "basic orbital mechanics" and "makes no sense".


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  • (Score: 4, Informative) by zocalo on Thursday September 03 2015, @05:44PM

    by zocalo (302) on Thursday September 03 2015, @05:44PM (#231868)
    I don't think they are planning on using propellent to match speed with the comet; that's the job of the tether system. The basic idea seem to be that you put the hitchhiker probe on a lower energy orbit that *intersects* the orbit of a higher energy comet, then fire a tethered harpoon at the comet at the appropriate point. The probe then plays out the tether to soften the sudden change in delta V that would result when the tether goes taut and uses the friction of that to harvest energy to be used later when reeling itself back in. The probe can then detach from the tether at a convenient point and use remaining on-board propellent to put itself onto a more interesting orbit - most likely to visit the ice giants and/or kuiper belt objects.
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  • (Score: 4, Informative) by deimtee on Thursday September 03 2015, @10:24PM

    by deimtee (3272) on Thursday September 03 2015, @10:24PM (#231998) Journal

    Given some parameters, it is actually pretty easy to work out the maximum delta V you could get from this.
    You need the length of the line (S) and the maximum acceleration the craft can stand (A).
    Assuming the line is strong enough not to break, the anchor holds, and you hit zero relative velocity at the full extent of the cable, then :

    V = sqrt(2AS)
    where
    V = velocity change in metres/sec
    A = Acceleration in m/s2
    S = line length in metres.

    Given the (I consider extreme) suggested values in another comment of 50 G and 100 km, you could get 10 km/s.
    At 5G and 10km you get 1km/s.

    If you stored the energy released by spooling out the cable, and used it to wind it in again and slingshot past you could theoretically double that.

    --
    No problem is insoluble, but at Ksp = 2.943×10−25 Mercury Sulphide comes close.
    • (Score: 2) by zocalo on Friday September 04 2015, @06:30AM

      by zocalo (302) on Friday September 04 2015, @06:30AM (#232147)
      I was also wondering whether it might be possible to have multiple tether release points at different tensions, so instead of getting a single sudden wrench on the tether as it went taut you could have multiple smaller ones, or to try and have a system where the tension on the extending tether, and thus the amount of acceleration stress on the system, increases in proportion to the length of the extended tether so the acceleration takes place over time instead of at one point. That would probably mean more mass though, so we're back to the trade off of whether this system would save enough to make it worth doing vs. just boosting more propellent to orbit.
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      • (Score: 1, Informative) by Anonymous Coward on Friday September 04 2015, @07:02AM

        by Anonymous Coward on Friday September 04 2015, @07:02AM (#232152)

        That was the point of having a brake. Rather than the acceleration taking place at one point, you let the tether spool out, but apply enough brake force to the tether that the vehicle is also pulled along.

        For optimum results, the brake force needs to be matched so that when you reach the end of the tether, the velocity difference between the vehicle and tether reached zero at the end of the tether.

    • (Score: 2) by gnuman on Friday September 04 2015, @06:23PM

      by gnuman (5013) on Friday September 04 2015, @06:23PM (#232366)

      If you stored the energy released by spooling out the cable, and used it to wind it in again and slingshot past you could theoretically double that.

      With what? A magic spring for the magic string?

      This idea is completely infeasible.

      http://www.nasa.gov/sites/default/files/capabilities.jpg [nasa.gov]

      This entire thing is based on ifs and buts of "future technologies that don't exist". Considering that we have enough problems slowly unfurling wires in space so they don't tangle, or failing to anchor on a comet while at 0 delta-V, well, well.... we'll sooner get reactionless drives working then this.

      https://en.wikipedia.org/wiki/RF_resonant_cavity_thruster [wikipedia.org]

      • (Score: 2) by deimtee on Saturday September 05 2015, @04:03AM

        by deimtee (3272) on Saturday September 05 2015, @04:03AM (#232506) Journal

        Did you see the word "theoretically" in there? I do not expect any system we could build soon to use the slingshot doubling, I only put in that line for completeness.

        I thought the idea was ridiculous when I first saw it, but achieving a delta V of close to a km/s might be engineeringly possible, and on a large vessel could well be worthwhile.
        This is a technology that would actually scale well, and something to keep in mind for a manned mars mission. A large ship pulling five gees for 10 or 20 km would be able to save a lot of fuel or reduce travel time significantly.
        Given that you were going to do it, it might be better to have two equal mass ships with a cable strung between them and a spool on each. You could line them each side of the comet path and not have to worry about the anchor holding.
        The hard part would be waiting for a comet or asteroid that is going the right way at the right speed.

        Also, you don't have to match the comet speed as long as you can let go at the end of the tether, but efficiency goes down as the residual speed difference goes up.

        --
        No problem is insoluble, but at Ksp = 2.943×10−25 Mercury Sulphide comes close.
  • (Score: 3, Insightful) by frojack on Friday September 04 2015, @12:25AM

    by frojack (1554) Subscriber Badge on Friday September 04 2015, @12:25AM (#232051) Journal

    I doubt the harpoon concept will work.
    They should have a backup plan to use a big net, like a purse seine. [noaa.gov].

    These things tend to be big rubble piles, and a harpoon might not obtain nay purchase.

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    • (Score: 3, Informative) by AnonymousCowardNoMore on Friday September 04 2015, @02:39PM

      by AnonymousCowardNoMore (5416) on Friday September 04 2015, @02:39PM (#232257)

      You've hit on one of the central problems with this approach. The reason for catching a comet with this harpoon "propulsion" system is to get more delta-v with the same mass than some other propulsion system. But if the harpoon misses, or doesn't stick, or sticks but then breaks loose during acceleration, you've missed your absolute one and only transfer window and the probe is a multimillion dollar monument to Things That Look Good on Paper.

      I don't think I like the net idea. With that kind of mass budget I'd just get a solar sail or ion engine (depending on destination, timeline and electric power budget).