Low thrust, heavy asteroid. Power track to LKO?

[Laie]

Not a problem I'm currently having, but might get myself into. If I know a solution.

Last time I wen't asteroid-hunting, I eventually left it in a sublunar very eccentric orbit. Lowering it to LKO would have taken a lot of time. Not merely burn time, I can live with that (and better timewarp helped a bundle). But it would have required many burns, and that was ultimately too much.

Detailed description of problem: I was in a high orbit, used whatever combination of thrust and munar gravity assists to lower PE, then found that I'd rush through PE very quickly. On every round, I could only effect <10m/s worth of decelaration to lower the AP. However, with attitudes shifting on every round, it was also necessary to re-orient the rock+pusher which often took more time than the burn that I could perform. I simply wasn't willing to repeat this as often as would have been necessary.

I'd be interested in other solutions... especially, I expect that it should be possible to just burn retrograde for a long time, and thus spiral inward to a low orbit. The question is, how do I set this up so that it will mostly work with only a comparatively small amount of intervention and corrections? Also, how lossy would it be in terms of dV?

Also, is there another possible approach I didn't think of?

Please do not suggest either MOAR boosters or aerobraking. I've thought of both, but here I'm asking about alternatives.

[Espatie]

How low are you trying to take it, and (more importantly) why?

 

Sinking stuff deep into a Gravity Well is rarely a good idea.

[Laie]

Because missions to elsewhere typically start in LKO. If I leave it high up, I might as well refuel on the Mun, or Minmus. Leaving it in an eccentric orbit is even worse in that regard.

[GoatRider]

When I captured an E asteroid, I was able to do long burns by making sure I was balanced exactly through the CoM of the asteroid, and also had a lot of verniors because even when you're exactly centered, you really aren't. Also, to re-center, I found it's just easier to un-grab, and re-grab. Swivelling the claw never seemed to work for me.

I like how you have extra solar panels on the other side of the asteroid. I was always having to spin around to expose the panels on my ship when refueling.

By the way, I left mine in about a 400km orbit of Mun in case I ever decide to use it for refuelling an interplanetary expedition.

Edited February 16, 2019 by GoatRider

[linuxgurugamer]

6 minutes ago, Laie said:

Because missions to elsewhere typically start in LKO. If I leave it high up, I might as well refuel on the Mun, or Minmus. Leaving it in an eccentric orbit is even worse in that regard.

Actually, that might be ideal.  If you launch and rendezvous with the asteroid while in LKO, and then leave the asteroid when at the AP, you would need very little dV to get to escape speed.  Yes, you need to do the rendezvous, and will need a bit more dV to do that, but that will leave your ship  with a lot of useful dV once it leaves the asteroid

[MaverickSawyer]

LGG's got a point. If you make most of the ejection burn just to rendezvous with your fuel depot, then leave it topped off, you only need a small burn to complete the ejection. You'd be saving 150-250 m/s, compared to orbiting Minmus, as now you don't have to make the burn to eject from Minmus orbit.

[Laie]

2 minutes ago, linuxgurugamer said:

Actually, that might be ideal.  If you launch and rendezvous with the asteroid while in LKO, and then leave the asteroid when at the AP, you would need very little dV to get to escape speed. 

Only if the Asteroid PE happens to be in the right place. I could position it so that it's fine for one a particular launch window, but for all other's I'd have to fix the argument of PE... more often than not that requires me to circularize high first, then drop the PE again. Worse than just starting from Minmus.

[linuxgurugamer]

1 minute ago, Laie said:

Only if the Asteroid PE happens to be in the right place. I could position it so that it's fine for one a particular launch window, but for all other's I'd have to fix the argument of PE... more often than not that requires me to circularize high first, then drop the PE again. Worse than just starting from Minmus.

No.  It's a bit harder, but you can time it right.

For example:

1. Get your ship into a circular orbit at approximately the Pe altitude
2. Wait until your ship is at the Pe, just ahead of the asteroid (ie:  less than one orbit away)
3. Burn into the asteroid's orbit, just a bit slower than the asteroid.
4. Wait for it to catch up to you.
5. Match velocities and Rendezvous with it

[Laie]

1 minute ago, linuxgurugamer said:

No.  It's a bit harder, but you can time it right.

Did you just describe how to RV with the rock? That's the easy part

I'm thinking about how I get from the asteroids' eccentric orbit to my destination. That's only cheap if the argument of PE about matches my needs for whatever transfer I want to do next.

[linuxgurugamer]

6 minutes ago, Laie said:

Did you just describe how to RV with the rock? That's the easy part

I'm thinking about how I get from the asteroids' eccentric orbit to my destination. That's only cheap if the argument of PE about matches my needs for whatever transfer I want to do next.

Leave the rock at it's Ap, circularize the orbit.  That's cheap.  Then leave orbit at an appropriate time

[Laie]

25 minutes ago, linuxgurugamer said:

Leave the rock at it's Ap, circularize the orbit.  That's cheap.  Then leave orbit at an appropriate time

Haven't we long since hashed out that the cheapest way from Mun to anywhere goes through a low Kerbin PE?

Digging for that old thread, I instead found a table of gateway orbits suggesting that a fuel depot at about Munar altitude is well-placed as long as the destination is Eve or Duna. Hmmm.

[OHara]

Your original thinking, @Laie, made sense to me.

You want an LKO staging base for the usual reasons. You asteroid has more energy than it will in LKO -- specifically the extra energy-per-kg is 0.5(3200m/s)²  - 0.5(2400m/s)².  Each chunk of dV that you can manage lowers the specific energy by dV×Vorbital for the orbital speed Vorbital at the place you make it.   Conservation of energy requires a particular sum of dV×Vorbital over all your (pieces of) burns to get where you want.

So, if you lengthen your burns, continuing them through the arc where you orbital speed is at least half its value at PE, you loose some efficiency but it the extra cost in dV, and burn time, is less than a doubling.  When I did this, I burned through 120° of arc each low orbit.

'Gate orbits' is a strange name (given by just their original proposer but not really explained) to the altitude that divides two cases of circular orbits.  For a given interplanetary destination, is it better to (1) lower PE and burn from there, or (2) leave directly?  Circular orbits above the gate-orbit altitude benefit from the step or lowering PE.  There seems to be no reason to actually enter an orbit at the gate-orbit altitude. It is not a 'gate' in the sense of a doorway; it is a dividing line.

Edit: Now I see. If you want a refueling center in circular orbit, for a given interplanetary destination, and you care mostly about minimizing dV needed after refueling, and you don't want to bother with the step of dropping PE, then the gate-orbit is a little better than other altitudes.  

Edited February 16, 2019 by OHara
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[Espatie]

58 minutes ago, linuxgurugamer said:

Leave the rock at it's Ap, circularize the orbit.  That's cheap.  Then leave orbit at an appropriate time

I think he means that the orbit is inclined, rather than eccentric.

[bewing]

2 hours ago, Laie said:

Also, how lossy would it be in terms of dV?

IIRC, it's a factor of root 2? So the worst you would get by braking down in a spiral from Kerbin's SOI boundary is about an extra 410 m/s?

[Kryxal]

I'd probably consider raising the PE to maybe 600 km, and bringing the AP down to match ... you'll be able to do a nice burn from there, and get longer burns to circularize anyway.

And then there was the time I grabbed an asteroid and took it to Moho without bothering to circularize anywhere...

[Laie]

12 hours ago, OHara said:

Now I see. If you want a refueling center in circular orbit, for a given interplanetary destination, and you care mostly about minimizing dV needed after refueling, and you don't want to bother with the step of dropping PE, then the gate-orbit is a little better than other altitudes.

Basically, yes. I'd still prefer parking it in a low orbit, but that would be a lot of effort and so far noone has brought up a good idea of how to avoid it, while it turned out that a higher orbit isn't all bad for a fuel depot.

12 hours ago, OHara said:

When I did this, I burned through 120° of arc each low orbit.

I did something similar, but the tug in question really had ridiculously low thrust: checking the screenshot, I see  20mm/s². I will definitely bring more boosters the next time I do this, but still not enough so that I could circularize in LKO just like that.

[Laie]

Oh boy, it's been long enough to carry a child to term... but I've got a few ideas abot my original problem.

First off: if I want to decelerate at full thrust for a long time and eventually end up in one particular circular orbit, I also have to start from one particular, well-defined orbit, need to start the burn at just the right time, and have absolutely no steering errors along the way. I dare say that this is not practical.

In order to actually do this, I need some parameters I can watch and control: as in, "if X gets too high, throttle down".

This is a retrograde maneuver on a mildly eccentric orbit, just for illustrative purposes.

• decelerating before PE will move the PE away from me. There's a combination of thrust and distance from PE  that will allow me to constantly burn retrograde while maintaining the same position relative to PE, somewhat similar to "maintain 1 minute to apoapsis" on ascent.
  If I have more thrust, I can maintain a position closer to PE.
• The above also requires some eccentricity. Burning closer to PE will reduce eccentricity, up to the point where it flips and eccentricity increases again. PE starts running away quickly, soon I will find myself near AP.

Taken together, there's one combination of eccentricity and distance from PE (probably best expressed as an angle, Mean Anomaly), where I can keep decelerating at full throttle while both remain constant.

Proto-Algorithm: throttle down whenever distance to PE or eccentricity gets too high. As those two are linked, it may turn out that only one of them really matters. I don't have the slightest idea how much will be "too high" for either, but guess that I can find out applicable values with some experimenting.

*looks outside*  Weather is overcast, drizzly, near-freezing. I don't think I have anything better to do today.

[Laie]

This ran overnight on kRPC autopilot. Basic algorithm was to point retrograde and throttle up the closer I get to PE.