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Old 06-17-2017, 07:56 PM   #1
TeamJackSwigert
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Default Increasing eccentric or apoapsis problem

Hey Everyone

First of all I am new to the forum so I say hey to you all! Therefor I am also quite new to orbiter. I have been playing it for 1 month now, but I can't seem to to stabilize my height once in orbit. I do everything as the guides tell me to do. Once I have reached orbital velocity, got my 0,0000 eccentric and engine shut-off, my apa and thus my ecc start to increase at a rate 0,0001 per 10 seconds or so. What am I doing wrong or is this normal? Thanks a lot!

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Old 06-17-2017, 10:54 PM   #2
Zatnikitelman
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Is nonspherical gravity enabled? What's your altitude? You are likely seeing small perturbations caused by some kind of external force. Nonspherical gravity and the atmosphere are the two most likely sources.
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Old 06-18-2017, 05:39 AM   #3
Thorsten
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My bet is non-spherical gravity - apoapsis is unlikely to be increased by atmosphere...

If so, the drift should be symmetric around a full orbit as energy is still conserved, i.e. after some period of increase, you should see a second period of decreasing apoapsis.

At the level you're reporting, drifts are perfectly normal - even cooling evaporator exhausts etc. can cause minute drifts. At some level, gravity perturbations due to the Moon are felt. At some other level, Orbiter's numerics will drift (no calculation is ever perfect).

There's never perfect 2-body dynamics in nature or realistic simulation.
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Old 06-18-2017, 08:36 AM   #4
TeamJackSwigert
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The drift is indeed symmetric. An increase followed by a decrease and thus making it quite difficult to allign with the ISS properly for my R.Inc start increasing as a result of the apa drift . I am speaking of altitudes of + 200km where the atmosphere forces should be minimal.

Gravitational power of the moon is excluded at this height I suspect.

@Zatnikitelman my nonspherical gravity sources is indeed enabled. Should I disable it ?

Thanks for the quick reply guys!
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Old 06-18-2017, 06:33 PM   #5
Linguofreak
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Originally Posted by TeamJackSwigert View Post
 
@Zatnikitelman my nonspherical gravity sources is indeed enabled. Should I disable it ?
That's a matter of personal taste. On better reflects the realities of actual spaceflight, off presents a simpler environment for learning to fly.
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Old 06-18-2017, 07:06 PM   #6
TeamJackSwigert
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Quote:
Originally Posted by Linguofreak View Post
 That's a matter of personal taste. On better reflects the realities of actual spaceflight, off presents a simpler environment for learning to fly.
Hmmm I was wondering that too if it would affect the realism. Ofcourse I'd like to have my simulator as realistic as possible. I think I mastered getting into orbit by now at a pretty high level and it still occurs... How could you perfect it or let's say how would real astronauts/pilots cope with it then?
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Old 06-18-2017, 07:38 PM   #7
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Originally Posted by TeamJackSwigert View Post
 Hmmm I was wondering that too if it would affect the realism. Ofcourse I'd like to have my simulator as realistic as possible. I think I mastered getting into orbit by now at a pretty high level and it still occurs... How could you perfect it or let's say how would real astronauts/pilots cope with it then?
The Apollo Guidance Computer can deal with the non-spherical gravity in its trajectory calculations and uses the same gravity model as used in Orbiter, at least for the Earth. So the answer, even already for the Apollo program is: the computer deals with it.

So in NASSP you always need to have non-spherical gravity enabled, or else the AGC gets confused about where you are.
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Old 06-18-2017, 07:45 PM   #8
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Quote:
Originally Posted by indy91 View Post
 The Apollo Guidance Computer can deal with the non-spherical gravity in its trajectory calculations and uses the same gravity model as used in Orbiter, at least for the Earth. So the answer, even already for the Apollo program is: the computer deals with it.

So in NASSP you always need to have non-spherical gravity enabled, or else the AGC gets confused about where you are.
Ow thanks a lot, I will remember that for just in case! NASSP is next on the list. It it very tempting to only use the cockpit view I must say whilst playing... but very hectic for beginners...
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Old 06-18-2017, 11:57 PM   #9
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Protip: Unlike Orbit MFD which oscillates, the IMFD Map will display your true orbit including non-spherical gravity.
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Old 06-19-2017, 07:01 AM   #10
Thorsten
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Quote:
How could you perfect it or let's say how would real astronauts/pilots cope with it then?
It's a question of Delta v - or as they say, navigation is for people who don't have enough thrust - if you have enough thrust, you can just point and burn away

In real life, there's near-infinite computing power on the ground to pre-compute every maneuver a dozen times over, spacecraft are remote-tracked by radar or probes on interplanetary journeys use star tracking cameras to get precise attitude fixes and then use angles to planets and lots of trigonometry for improved position fixes and astronauts do as they're told by mission control, they never really have to solve the task of navigating anywhere by themselves.

In fictional spacecraft, the error you can allow depends on Delta-v reserves you have. For a remdezvous, you just need to get 'near' your target - if you have ISS in sight, the Delta glider can just take you there even if it's 100 km away.

So by and large you can live with small errors - you can fly stock Atlantis to ISS using only default instruments, you might have to point and thrust the last 30 km, but it's doable. It's not like a 0.1 % eccentricity error is going to take you massively off-course in a normal orbital situation.
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Old 06-19-2017, 10:08 AM   #11
TeamJackSwigert
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So in the end it's mostly mission control thanks a lot guys!
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Old 06-19-2017, 08:24 PM   #12
Buck Rogers
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For practical purposes in Orbiter I would suggest using Rendezvous MFD 050621, works in Orbiter2016!
https://www.orbithangar.com/searchid.php?ID=1199
It has it's quirks and comes with no documentation but is relatively simple to use after a little practice.
I usually setup an intercept course with InterplanetaryMFD and then course correct every half of orbital distance for a suitable low Dv approach with Rendezvous MFD, Velocity match again with IMFD, and dock with PursuitMFD.
Hope this helps.
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Old 06-22-2017, 08:17 PM   #13
TeamJackSwigert
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Does anyone know why the NASSP Project Apollo site advises to enable the nonspherical gravity sources? Just curious... or is it mainly for the realistic-factor?
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Old 06-22-2017, 08:37 PM   #14
indy91
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Originally Posted by TeamJackSwigert View Post
 Does anyone know why the NASSP Project Apollo site advises to enable the nonspherical gravity sources? Just curious... or is it mainly for the realistic-factor?
NASSP uses the Virtual AGC emulator, running the actually flown AGC software for LM and CSM. And many of the internal calculations of the AGC assume that the Earth has a nonspherical gravity field. One of the major features of the AGC is to keep track of the state vector of the spacecraft at all times. And without using nonspherical gravity in Orbiter, the AGC would quickly not have a precise state vector anymore.
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