"Thread the Needle"

[SiWalder]

I've been thinking about a KSP challenge for a while.  Not sure if it's even possible, but, the challenge would be to get a craft into a very (very very) equitorial orbit with a perilisly low periapsis, almost skimming the surface, on an atmosphereless planet/moon.  Then build a Surface base with a hole in it and potision it carefully so that the obiting craft "threads the needle" each orbit.  better yet, have some kind of short elevated tunnel rather just than a hole.  I think the POV view would be fairly spectacular, especially if you could get the tunnel fairly tight, combined with a high apoapsis for high speeds.

........ or is that impossible.... or just a rubbish idea,.... i'm good at them.

Edited November 20, 2019 by SiWalder

[MiscelanousItem]

Well people have done Mun arch arch stations before...

However I have to remind you the forums rules :  if you want to make this a challenge you have to prove it is possible...

[SiWalder]

oops my mistake. soz.

thinking deeper about it, the orbit would have to be perfectly circular and probably not possible without "set orbit" however that won't let you get low enough for it to work.

I'd delete this, but I'm not sure how.  i'll have a poke around. 

[Vanamonde]

You can't delete your posts, but there's no need to anyway. It's an interesting idea.

[Superfluous J]

Your orbit would have too have the same period as the planet's rotation, and I don't think any moon's SOI is big enough. I think it could work with Dres or Eeloo, though I haven't actually checked.

As to nailing an orbit so perfectly as to hit the same ring every time.... I really can't see that being practical.

Nailing it once though would be really cool, assuming you weren't going so fast that you passed by too quickly to see anything.

Edited November 21, 2019 by 5thHorseman

[Laie]

I recall having seen the occasional video of people orbiting through circular structures, usually on the Minmus flats.

There also was a touch & go challenge where you were supposed to briefly touch the surface with your landing gear (preferably wheels) on as fast a flyby as possible. It didn't get much attention, though.

[vyznev]

3 hours ago, 5thHorseman said:

Your orbit would have too have the same period as the planet's rotation, and I don't think any moon's SOI is big enough. I think it could work with Dres or Eeloo, though I haven't actually checked.

Actually, this should be (theoretically) doable on Gilly. According to the wiki, a synchronous orbit around Gilly needs a semi-major axis (= average of periapsis and apoapsis, measured from the center of the body) of 55.14 km. That's less than half of Gilly's SOI radius of 126.12 km, so a synchronous orbit of any eccentricity (that doesn't intersect the surface) should be possible around Gilly.

Also, a semi-synchronous orbit (i.e. one with a period 1/n times the main body's rotation period, for some integer n > 1) could also work, and would make this possible on most (probably all) moons in KSP. The periapsis of such an orbit would cycle between n different points on the surface, but as long as you made sure to build the "hoop" at the highest one of those points, it should be fine.

Also, probably the easiest way to do this would just be to build the hoop at the highest point on the moon's equator and put the satellite in a perfectly circular equatorial orbit. That should work on any moon, and would have the added advantage of not having to worry about getting the phase of the orbit and the argument of periapsis exactly right. Just zero out the inclination, drop the periapsis to the desired altitude and then do a burn at periapsis to circularize. Fine tune with tiny radial burns if needed.

[Zeiss Ikon]

23 hours ago, 5thHorseman said:

Your orbit would have too have the same period as the planet's rotation, and I don't think any moon's SOI is big enough. I think it could work with Dres or Eeloo, though I haven't actually checked.

As to nailing an orbit so perfectly as to hit the same ring every time.... I really can't see that being practical.

Nailing it once though would be really cool, assuming you weren't going so fast that you passed by too quickly to see anything.

Don't forget that if the orbit has very low eccentricity and is perfectly equatorial, it doesn't matter if it encounters the "hoop" at different local times on each orbit -- it'll be at the same height every time and always directly above the equator, which are the same parameters as a fixed building has.

I've been launching with MechJeb recently, orbit and Lunar contracts in RSS/RO, and I've seen MJ nail an orbit to a few meters of altitude variation on several occasions.  If you launch from the body your hoop is built on, using MJ, the real challenge will be getting the last couple meters ironed out, then build the building around the craft's orbit.

Anyone remember reading "The Holes Around Mars"?

[SiWalder]

Thanks for all the replies, really appreciate your thoughts. 

I think I'm going to try this during some housework procrastination this weekend. 

I'll let you know how I get on. 

[Hotel26]

On 11/20/2019 at 8:15 PM, SiWalder said:

Then build a Surface base with a hole in it and potision it carefully so that the obiting craft "threads the needle" each orbit

There's a deep gully on the Mun - more of a ravine, really -- that qualifies exactly for this.  I regularly cruise over it at 6.5km MSL and it feels so tempting to fly through it, 550m/s (maybe 1km lower).  Crazy thought, coz it's not exactly aligned east-west.  So you'd have to set up your inclination aforehand.  It'd be Star Wars stuff.  So please don't feel crazy alone thinking this kind of stuff.  (There are other crazed people thinking similarly!)

And so, about some of the points made above: you'd need a nav point at which you'd perform an initial alignment for the approach.  You'd target to hit the nav marker and do a precise alignment there...  (waves hands knowingly/confidently)  It'd make a wonderful Youtube clip...

Edited November 23, 2019 by Hotel26

[Klapaucius]

I think this is a really cool idea.  I had thought about a challenge ages ago just looking for lowest combined periapsis and apoapisis.  All you do is go the the Mun or Minmus and set the lowest orbit you can get away with. The lowest number after adding the apoapsis and periapsis wins.

 

But I think your idea is way cooler.

 

I don't know if it is in the spirit of the challenge, but I imagine you could at least prototype this reasonably easily with the Making History Mission Builder.

Edited November 25, 2019 by Klapaucius

[SiWalder]

Finally had a crack at the lowest moon orbit.

If you're bang on the equator of the Mun, you can get away with a 5900m x 5900m (give or take 50m) orbit which includes a fairly frightening skim of the southern peak of the canyon.  There's also a short time in which you are pretty much IN the canyon.  Might try and squeeze it in a little.  It looks like a structure about 70 meters tall would do the trick.

 At the moment though, I'm just gonna pump the graphics settings up and kick back with the interior view out the prograde window.  It's quite a treat.

[swjr-swis]

39 minutes ago, SiWalder said:

If you're bang on the equator of the Mun, you can get away with a 5900m x 5900m (give or take 50m) orbit which includes a fairly frightening skim of the southern peak of the canyon.  There's also a short time in which you are pretty much IN the canyon.

 

I've done 5900m and that's still quite safe. You do fly through the higher part of the canyon, but the limiting obstacle is not the canyon, it's a crater edge a bit further away.

A long while ago, back in KSP 1.0.5:

Spoiler

"Jeb... those are canyon walls. To our sides. At eye level. And rising. Jeb? JEEEB!"

Full album: https://imgur.com/gallery/968gz

Some say Bob came back from that trip without his suit trousers. Some say those trousers are still stinking up the Mun canyon to this day. All we know is... Bob is still touchy on the subject.

[swjr-swis]

On 11/20/2019 at 1:15 PM, SiWalder said:

I've been thinking about a KSP challenge for a while.  Not sure if it's even possible, but, the challenge would be to get a craft into a very (very very) equitorial orbit with a perilisly low periapsis, almost skimming the surface, on an atmosphereless planet/moon.  Then build a Surface base with a hole in it and potision it carefully so that the obiting craft "threads the needle" each orbit.  better yet, have some kind of short elevated tunnel rather just than a hole.  I think the POV view would be fairly spectacular, especially if you could get the tunnel fairly tight, combined with a high apoapsis for high speeds.

........ or is that impossible.... or just a rubbish idea,.... i'm good at them.

I've done this with an Mk3 'tunnel station' (just a couple of open-ended Mk3 cargo bays and a probe core) in orbit around Gilly, and a tiny 0.625m probe with some lights shot in the exact opposite direction. The only place this was even remotely possible to achieve was Gilly, as orbital speed is low enough that a prograde/retrograde flyby is slow enough to be able to record it (as opposed to just one or two frames anyplace else).

Took a whole bunch of attempts (getting something in that exact a retrograde orbit is... exasperating), but finally managed to make one actually pass through the cargo bays, which was plain to see as the lights on the probe illuminated the inside of the bays in passing. Unfortunately it's one more of those videos I can't seem to find anymore; I must've archived a bunch of them to make some room on my SDD, but heck if I know on which USB disk I saved them.

Not exactly the same thing you mention, since you want a structure on the ground - but I have to concur with @vyznev that that's the most plausible place to try this.

[SiWalder]

Right, I've placed a 50m tower with a command pod at the top.  the orbiter passes within 150m of it each orbit, I've passed 10 orbits and visibly at least, it's hitting the same spot each time.  there's a slim chance this is doable.

The tower about 100m too far south but I'll leave that there as a marker and build the loop tower into a rover for fine adjustments.

[SiWalder]

Each orbit is the same pretty much

[swjr-swis]

A quick setup with a cheated orbit (I don't have the time or inclination right now to try it otherwise), just to show the potential futility of trying to record such a thing on the Mun:

This fragment is in low orbit, with the cargo bays and the probe in almost identical orbits, except one is prograde and the other retrograde. This GIF is slowed down 10x and shows just 7 frames, of which only 3 are in clear vicinity of the bay. In real time you basically just see a quick flash and it's gone. Relative velocity is about 1100m/s, which is twice(*) what it would be when you do it with a structure on ground level... so you get a couple frames more, but it's still going to be over in half a blink.

Seriously, before you spend too much time trying this on Mun just to find out you can't really get a good recording... head out to Gilly. Orbital speed there is in the order of 15-20m/s in the lowest orbits... much more likely to get some sweet footage of your feat.

(*: less than twice, because lowering orbit to ground level will increase orbital speed.)

Edited January 12, 2020 by swjr-swis
PS

[swjr-swis]

Just for contrast, the same thing in orbit around Gilly:

This GIF is rendered in real time. Relative velocity is about 31.4m/s, which gives plenty of time to actually see the fly-through... when you succeed.

I highly recommend it.