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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".
Original Submission
(Score: 3, Insightful) by gman003 on Thursday September 03 2015, @05:28PM
Doing this is not physically impossible. The core problem is that matching velocities with an orbiting object requires as much fuel as going out to the furthest point the object reaches. You save no fuel by softly docking or landing on it.
So you have to basically crash into it, and have your craft have far less inertia so the end orbit is dominated by the comet's energy, not your craft. Problem is that orbital velocities are immense - you'll be crashing at thousands of meters per second. You're getting all the energy you need to match orbits for free, but you're getting it all at once, and your craft probably can't survive that much energy.
This method does spread that energy out over a longer period, but it's still pretty hard to engineer. Your harpoon needs to survive a 10,000kms impact, and then you'll need a massive spool of cable, which will weigh quite a lot. They seem to think they can get the weight down to something reasonable by using carbon nanotubes, which tells you about how much unobtainium this method requires.
(Score: 4, Informative) by zocalo on Thursday September 03 2015, @05:44PM
UNIX? They're not even circumcised! Savages!
(Score: 4, Informative) by deimtee on Thursday September 03 2015, @10:24PM
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.
If you cough while drinking cheap red wine it really cleans out your sinuses.
(Score: 2) by zocalo on Friday September 04 2015, @06:30AM
UNIX? They're not even circumcised! Savages!
(Score: 1, Informative) by Anonymous Coward on Friday September 04 2015, @07:02AM
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
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
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.
If you cough while drinking cheap red wine it really cleans out your sinuses.
(Score: 3, Insightful) by frojack on Friday September 04 2015, @12:25AM
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.
No, you are mistaken. I've always had this sig.
(Score: 3, Informative) by AnonymousCowardNoMore on Friday September 04 2015, @02:39PM
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).
(Score: 2) by bzipitidoo on Thursday September 03 2015, @05:55PM
Yeah, my first thought is that there is too much delta-v for a grappling hook and cable to be practical. It'd be worse than trying to accelerate your skateboard by hooking on to race cars during a race. Hooking on to passing cars happened in Snow Crash, but that after all is fiction, no matter how scientific.
(Score: 2) by Zz9zZ on Thursday September 03 2015, @06:46PM
All depends on the material science that goes into the harpoon and how long of a tether they can use. A skateboard could safely tether to a race car if it had a mile long tether. With diamond nanothreads exceeding 500x the strength of steel, and being much lighter, I don't see why this is impractical.
~Tilting at windmills~
(Score: 2) by tangomargarine on Thursday September 03 2015, @10:03PM
With diamond nanothreads exceeding 500x the strength of steel, and being much lighter, I don't see why this is impractical.
Or better yet, power the craft with magical fairies and unicorn farts!
"Is that really true?" "I just spent the last hour telling you to think for yourself! Didn't you hear anything I said?"
(Score: 3, Insightful) by Zz9zZ on Thursday September 03 2015, @10:29PM
No joke! Alone neither of those would work be enough, but the fairies are able to increase the power AND efficiency to levels we can not even measure.
~Tilting at windmills~
(Score: 2) by draconx on Thursday September 03 2015, @06:50PM
Well, what matters are the magnitude of the forces involved (acceleration), the tensile strength of the cable, and the mass of the whole assembly (compared to a more conventional thruster). As well secondary issues, like how reliable the system is. The idea in the article is that you fly by the comet, fire an anchor into it, and the cable unwinds as the spacecraft flies by the comet. This powers the spacecraft via a generator, and simultaneously slows the craft (relative to the comet).
For the concept to be useful, I imagine that the cable would be unwinding for the entire usable life of the spacecraft. Which means it needs to be unimaginably long, low mass, and have high tensile strength. The article suggests carbon nanotubes, which of course make all sorts of cool things possible except that, well, nobody has ever actually managed to build a cable out of the stuff.
For reference, 10 km/s delta-v is appox. the same as the delta-v budget of Dawn (after separation from its launch vehicle), which carried 425kg of Xenon propellant.
(Score: 2) by Snow on Thursday September 03 2015, @07:22PM
https://en.wikipedia.org/wiki/Skitching [wikipedia.org]
There was a game for Sega Genesis called Skitchin' that you had to get around town by hanging onto cars. It was actually a pretty fun game.
My armchair analysis seems to think that the deta-v might be too great for a tether. Just some napkin calculations suggest that even if your tether is 100km long, then the craft would be subjected to 50Gs of acceleration.
(Score: 2) by soylentsandor on Saturday September 05 2015, @07:24AM
the craft would be subjected to 50Gs of acceleration
No worries there: the cable would simply snap :)
But apart from that, they claim it could be up to 620 miles long, that's 1000km in the rest of the world. Which would reduce the acceleration to 5G. Still quite a bit but feasible. As for the rest of the ingredients, I'm not so sure those are all that feasible. But one can dream. And research. And that might even result in something valuable.
(Score: 0) by Anonymous Coward on Thursday September 03 2015, @08:49PM
This is possible: https://en.wikipedia.org/wiki/Fulton_surface-to-air_recovery_system [wikipedia.org]
But trying to attach a cable to a comet that's traveling many times faster... You'd need a lot of cable that can be released at high speeds and won't tangle...
And you definitely have to be very very accurate in timing and positioning if you're hoping to get much of a boost. Otherwise if you're having to adjust and match, you've probably spent almost as much energy by then.
(Score: 3, Informative) by Absolutely.Geek on Thursday September 03 2015, @10:57PM
It is a cool idea; but I have a few issues with it.
Lets say that you want to grapple with something that is going at 10km/s; you get on an intersecting trajectory going 2km/s so your delta is 8km/s. Now we will assume that the latching onto the thing is a solved problem.
If you somehow managed to make a 160km line you would pay that out in 20s; thus your acceleration is 400m/s^2 for 20s. Your craft has to handle approx 40g; your line and grapple point has to maintain its grip under 40 x the weight of your craft to get up to speed. This all has to happen without damage to the components that will wind the craft back into the object.
Don't trust the police or the government - Shihad: My mind's sedate.
(Score: 1) by Absolutely.Geek on Friday September 04 2015, @12:22AM
Replying to myself to clarify; the 40g that I stated above is the MINIMUM acceleration achievable. It is quite likely that the actual acceleration would be much higher then 40g; if there was a failure of the unspooling mechanism half way along the craft would need to gain 4km/s is a very short amount of time....most likely outcome would be the unspooling mechanism tearing away from the craft or the teather snapping at a weak point somewhere along its length.
Don't trust the police or the government - Shihad: My mind's sedate.
(Score: 2) by mhajicek on Friday September 04 2015, @02:32AM
Yeah, Mythbusters tried it with a car. It didn't work very well.
The spacelike surfaces of time foliations can have a cusp at the surface of discontinuity. - P. Hajicek
(Score: 2) by Adamsjas on Friday September 04 2015, @04:19AM
Stretchy tether?
Not necessarily a bungie cord, just a weave that could stretch slowly, (no rebound, its stays lengthened).
You could soak up a lot of Gs in the cable itself.