Lowest Stable Orbit for Large Space Station

[Plusck]

2 hours ago, Tatonf said:

...in order to check the influence of the Oberth effect we should use non-circular orbit, with a fixed apoapsis and a variable periapsis.

So, I replaced the swivel engine by the vector engine (so I could get a shorter burn time), and I used a constant 5 000 km apoapsis so I can have a great change despite my small dV (this is what we call a "big response") :

71 km : Speed at Pe is 3066.0 m/s ; Ap raised to 32 787 857m

85 km : Speed at Pe is 3031.1 m/s ; Ap raised to 31 556 426m

99 km : Speed at Pe is 2997.2 m/s ; Ap raised to 30 443 993m

101 km : Speed at Pe is 2992.5 m/s ; Ap raised to 30 299 403m

110 km : Speed at Pe is 2971.3 m/s ; Ap raised to 29 648 300m

As we expect, the faster I go, the more my apoapsis is raised. According to my results, the Oberth effect is functioning totally fine. Or maybe the use of Hyperedit fixed it ?

Good work. :up:

However, to really test Claw's statement about there being a physics break at 100km, wouldn't it be best to test at 95km and 105km?

For example:

95x95km orbit: theoretical orbital velocity = 2254.2 m/s

95x105km orbit: theoretical orbital velocity = 2262.2 m/s at Pe, 2230.2 m/s at Ap

105x105km orbit: theoretical orbital velocity = 2238.2 m/s

If you take a hyperedited ship with, say, 100m/s dv and a decent TWR, then the final orbital velocity at Pe should be each of those figures +100m/s:

95km Pe @ 2254.2+100 = 2354.2 m/s => Ap at 233.6km

95km Pe @ 2262.2+100 = 2362.2 m/s => Ap at 246.2km

105km Pe @ 2238.2+100 = 2338.2 m/s => Ap at 246.8km.

These are theoretical values derived from the standard orbital equations. I hope they are correct. If they are out at all, then they should at least be consistently out... and therefore doing a 100 m/s burn from 105km circular should take you approximately to a 600m higher Ap than doing the same burn at Pe from a 95x105km orbit, and approximately 13.2km higher than a 95x95km orbit.

If the figures tally, then the Oberth effect is working as it should for small burns.

 

However, the other possibility is that you need a very large burn to test the hypothesis. From my calculations for gate orbits, you basically need a burn of 2250 m/s or more for it to be more efficient to burn from a 95x95km orbit than from a 105x105km orbit.

So to test this, you need a high TWR ship with 2250m/s dv starting from a 95km then a 105km orbit, burning prograde. Then either be very careful to time it precisely to get maximum Ap in sun orbit (should be just outside Eeloo's orbit) or warp to the SOI edge and check velocity on leaving the SOI (V_inf, should be about 3180 m/s).

Again, my figures may be slightly out (especially V_inf, since my calculations don't take the SOI edge into account), but the basic principle should be correct, and better performance for a 2250m/s burn at 95km should demonstrate that Oberth is doing fine...

[Plusck]

OK, well I tried that last idea with a ship with 2248m/s then another with 2261m/s dv, put into orbit using HyperEdit in KSP1.1.2, no gimbal, prograde hold, measuring Vinf at 4m30 from SOI edge, and... there was no noticeable difference.

95km orbit: velocity 2254.2m/s:
+ 2238m/s => Vinf 3191.2 m/s
+ 2261m/s => test 1: Vinf 3208.8 m/s
+ 2261m/s => test 2: Vinf 3211.3 m/s

105km orbit: velocity 2238.2 m/s
+ 2238 m/s => Vinf 3191.4 m/s
+ 2261 m/s => test 1: Vinf 3209.1 m/s
+ 2261 m/s => test 2: Vinf 3210.7 m/s

The main problem was that prograde hold wasn't actually holding much at all, hence the two different attempts with the larger rocket. I let the rocket drift off prograde in exactly the same manner in test 1, but manually tried to keep it on track in test 2.

So there is certainly some error in the figures due to the failure to maintain the heading. However, I can't see any clear signs of a 100km "edge" to the Oberth effect...

If anyone wants to try repeating this to get more accurate results, the ship was:

2238 m/s: 1k battery, size 1 reaction wheel, size 1 advanced probe core, size 1 to 2 tank, skipper.
2261 m/s: oscar tank, 200 round battery, size 1 reaction wheel, size 1 advanced probe core, size 1 to 2 tank, skipper.

 

Edit: tried again with Jeb driving (to hold prograde better) and with a second craft with 2323m/s dv. TWR was somewhat lower for both craft, particularly the one with lower dv:

95km orbit: velocity 2254.2m/s:
TWR 4->8: + 2238m/s => Vinf 3191.1 m/s
TWR 6->13: + 2323m/s => Vinf 3298.3 m/s
+ 2261m/s => test 2: Vinf 3211.3 m/s

105km orbit: velocity 2238.2 m/s
TWR 4->8: + 2238 m/s => Vinf 3191.2 m/s
TWR 6->13: + 2323 m/s => Vinf 3298.0 m/s

So with more careful piloting, everything seems to be doing what it's supposed to in 1.1.2. Unfortunately I haven't got 1.1.3 properly installed yet.

 

Therefore, the only explanation I can see for Claw's comments about a switch at 100km is that it might make a difference at extremely high velocities, i.e. at Pe for a highly elliptic orbit. Since in that case you only have a very short time for a burn at Pe anyway, and even less time if you take it down to around the 70km mark, is this really a problem with the physics calculations or rather simply a question of burning inefficiently off prograde since you have no time to make an efficient prograde burn?

Edited July 9, 2016 by Plusck

[Claw]

Well, I didn't mean for this thread to get hijacked by the Oberth Monster, but I suppose that's my fault.

My comment about the 100km discontinuity was based on testing I did back in November. I went back and tested again, and either: 1) My original test was incorrect; or 2) Changes to the orbit code in 1.0 has fixed the issue.

So I will concede (and retract my earlier statement) that Oberth is not effected by any discontinuities across the 100km boundary. However, there are other effects that still happen below 100km to watch out for (such as apparent maneuver node drift, and occasional issues with "cannot warp while under acceleration").

What's really awesome is that you guys actually did some testing, and didn't just jump back to "no, you're wrong [insert bad name here]." That's the thing I really like about the players/community/game here.   I will say that in testing Oberth, one has to be careful about comparing answers. It's not as simple as "the AP was higher here than here so it must be better." Orbital mechanics do not occur as isolated pieces, and Oberth effects total orbital energy. Starting out in a 150x150km orbit already has higher Orbital Energy than starting out in a 70x70km orbit. And testing in a 70x150km orbit will also yield different results than either of the first two. Leaving Kerbin's SOI with a craft from a 70x70km orbit will have less Vexcess than a craft leaving from 150x150km (assuming same fuel load). This trend reverses itself between 1Mkm and 2Mkm.

Also, someone asked earlier if there's much of a difference putting a station up at Munar altitude (I never said Munar orbit) and diving down to LKO, vs. just leaving straight from Munar altitude. The difference there is huge, if you're trying to maximize Oberth...Around 40% increase in excess velocity, depending on PE dive altitude.

 

But, back to the original question:

On 7/7/2016 at 5:51 PM, castille7 said:

What's the Lowest Stable Orbit around Kerbin or your best guess for a large Space Station?

It really, really, depends on what you're going to use it for. If you're asking for the straight up lowest stable orbit, then the original answers still apply, no less than 70km. There's still a 100km discontinuity that caused instability in low orbits, but I think (for the most part) that has been fixed in 1.1.3. Though it looks like you've already had your question answered.

[Castille7]

56 minutes ago, Claw said:

Well, I didn't mean for this thread to get hijacked by the Oberth Monster, but I suppose that's my fault.

My comment about the 100km discontinuity was based on testing I did back in November. I went back and tested again, and either: 1) My original test was incorrect; or 2) Changes to the orbit code in 1.0 has fixed the issue.

So I will concede (and retract my earlier statement) that Oberth is not effected by any discontinuities across the 100km boundary. However, there are other effects that still happen below 100km to watch out for (such as apparent maneuver node drift, and occasional issues with "cannot warp while under acceleration").

What's really awesome is that you guys actually did some testing, and didn't just jump back to "no, you're wrong [insert bad name here]." That's the thing I really like about the players/community/game here.   I will say that in testing Oberth, one has to be careful about comparing answers. It's not as simple as "the AP was higher here than here so it must be better." Orbital mechanics do not occur as isolated pieces, and Oberth effects total orbital energy. Starting out in a 150x150km orbit already has higher Orbital Energy than starting out in a 70x70km orbit. And testing in a 70x150km orbit will also yield different results than either of the first two. Leaving Kerbin's SOI with a craft from a 70x70km orbit will have less Vexcess than a craft leaving from 150x150km (assuming same fuel load). This trend reverses itself between 1Mkm and 2Mkm.

Also, someone asked earlier if there's much of a difference putting a station up at Munar altitude (I never said Munar orbit) and diving down to LKO, vs. just leaving straight from Munar altitude. The difference there is huge, if you're trying to maximize Oberth...Around 40% increase in excess velocity, depending on PE dive altitude.

 

But, back to the original question:

It really, really, depends on what you're going to use it for. If you're asking for the straight up lowest stable orbit, then the original answers still apply, no less than 70km. There's still a 100km discontinuity that caused instability in low orbits, but I think (for the most part) that has been fixed in 1.1.3. Though it looks like you've already had your question answered.

Yes sir I agree!

[kBob]

On ‎7‎/‎8‎/‎2016 at 2:56 PM, OhioBob said:

The ideal altitude for ejection from orbit is called the gate orbit. 

The wiki page seemed a bit um...sparse, but the PDF linked on it was great.  Thanks for the link.

[Castille7]

8 minutes ago, kBob said:

The wiki page seemed a bit um...sparse, but the PDF linked on it was great.  Thanks for the link.

Lots of great information being shared here, thank you @kBob!

[OhioBob]

13 minutes ago, kBob said:

The wiki page seemed a bit um...sparse, but the PDF linked on it was great.  Thanks for the link.

We had a big discussion about this last year, in which the gate orbit concept was independently discovery and derived.  It wasn't until some time after the conclusion of the discussion that somebody found the Wiki page.  It was at that time that we realized what we had discovery was called a "gate orbit".

 

[linuxgurugamer]

I created a ship which is available here:

https://kerbalx.com/crafts/15338/edit

Mods used for this test were:

• HyperEdit
• MechJeb
• Mechjeb for All
• KW Rocketry Redux
• Kerbal Alarm Clock

 

Picture below.  The main stage has two tanks.  The FT-400 is disabled, is used merely to add mass to the vessel

The ship is two stages.  I did the following steps for each of the tests below:

1. Use HyperEdit to get orbit to 1Mm altitude
2. Use MechJeb SmartASS to point Retrograde
3. F5 to save

The following steps are repeated for each test:

1. Use MechJeb Maneuver Planner to drop orbit to test Pe
2. Once maneuver is complete, decouple (decouple force is set to 0 to avoid any impact to the Pe)
3. Switch to tug section, use RCS to back away from main test stage
4. Use KAC to warp to 1 minute before Pe
5. Use MechJeb SmartASS to point Prograde
6. Pause at Pe -5, record speed
7. At Pe, engage engine
8. When tank is empty, record Ap
9. F9 to return to test start, ready for next test

Very interesting

And here are the results:

Orbital start:	1,000,000m
Tug used to drop Pe, which then disengages with 0 force
Total Dv:	323
Pe (km)	Speed at Pe  (m/sec)	Final Ap (Mm)
70.095	2725.7	4.38876
94.985	2661.7	4.33023
104.999	2637	4.30865
125.003	2589.2	4.26857
175.033	2477.7	4.18364
249.998	2329.5	4.08814
349.995	2159.9	4.00367
500.003	1950.7	3.93731

 

 

[Warzouz]

On 8/7/2016 at 0:33 PM, Tatonf said:

HOLY SMOKES WHAAAAAAAAT ?

I've been playing KSP for 3 years, always putting my spacecraft at the absolute minimum altitude (~71 km, 1 km margin in case of), and you're telling me it's not the most efficient way to do things ?

I refuse that.

Remember that LKO stations are usualy designed to dock with. When docking you usually have to timewarp a lot. Having a high timewarp factor is a very important factor for station. You save a lot of time.

 

Further more, Docking with a 71km station is hard to get a precise rendez-vous. If you miss, you don't have any choice than wait on a high orbit that the station gain on you. You loose fuel to go higher then go lower. When your target is higher, you can wait lower thus saving fuel. Further more, you always have the option to wait higher and save time (but you waste fuel)

Finally, orbits may not be always stable.Your 71km station could end at 65km... But I think this has been fixed now. Even though, safer is better !

[Montag]

On 7/9/2016 at 5:44 PM, Claw said:

However, there are other effects that still happen below 100km to watch out for (such as apparent maneuver node drift, and occasional issues with "cannot warp while under acceleration").

 

So on these other effects on different planetary bodies... If we get to the 100x warp factor altitude does that clear these effects?

[Gojira1000]

Given possible orbital fluctuations and timewarp, go >100km

Edited July 11, 2016 by Gojira1000
spellin'

[Claw]

9 hours ago, Montag said:

So on these other effects on different planetary bodies... If we get to the 100x warp factor altitude does that clear these effects?

I'd have to look to be sure, but in general that should keep you above the rotational reference frame cutoff.