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Alexandicity

"Body" longitude vs "orbital" longitude

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Hi!

Some friends and I are trying to fly to Minmus manual mode IVA without map view. We're therefore needing to plan out all our manoeuvres ahead of time. One that is giving us difficulty is the plane correction. We launch equatorial, but need to correct to a 6deg inclination to match Minmus. The burn itself is easy enough, but getting the timing right in order to be co-planar is problematic.

We are using telemachus to give us telemetry (aka API access!) and so can see our various orbital parameters. We know that we need to burn at the ascending node - and we have the longitude of this relative to an arbitrary reference line. However, we're not sure how to see our vessel's own longtitude as measured from this same reference point. The longitude offered by the api/telemetry appears to be relative to the orbited body's prime meridian, which of course is rotating in inertial space. 

Looking at wikipedia, it seems that the way to do it is to take our argument of periapsis, and when our true anomaly is the negative of this, we make the plane change. We'd do this twice: once to correct launch errors (using the vessel LAN, INC->0), a second time to match minmus (using the minmus LAN, INC->6). Does that sound right?

Is there any way to time a launch such that we can insert directly into a minmus plane (applying our inclination burn during launch)? Without an orbit, we'd not have a true anomaly or AoP, so we'd not know when we were at the LAN/when we were coplanar with minmus....

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>Is there any way to time a launch such that we can insert directly into a minmus plane (applying our inclination burn during launch)?

Yes. You can time the launch visually by waiting in map mode until the plane of Minmus's orbit passes over your craft (at the KSC). I realise you want to fly the mission in IVA view, but perhaps you would allow map mode prior to launch? It's quite awkward to do so, but you can try to fly at an azimuth of 83.5°/96.5° by hand. That's the initial compass heading you need to take at launch to end up with an inclination of 6° in LKO.

Alternatively you can use maths and mods (for the maths to be doable you probably need an information mod at least). Myself, I use kOS to calculate the launch time and control the launch. Yes, being able to convert between geographic longitude and universal longitude via the universal reference vector is important, but you need to know where that reference is. It's not exposed in stock. It would be useful to work out what longitudes Telemachus is giving you.

To answer your earlier questions: You can do the plane change to match Minmus's orbit in one go, rather than zeroing your inclination first, but that's harder to calculate. OhioBob's website is pretty handy for this kind of thing: http://www.braeunig.us/space/orbmech.htm

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If you're flying by eye, then trying to match planes is probably a mistake in the first place, unless your "telemetry" gives you an ascending/descending node readout.

What you need to do is treat it as a rendezvous. Set Minmus as your target. When Minmus is directly prograde of you, burn prograde until your Ap is about 40Mm. Go into target mode on your navball. After you get to 20mM, every so often, burn to pull your prograde marker on top of the target marker. Keep it there, and you will hit Minmus.

Edited by bewing

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Ah yes - to be clear, we're not flying by eye all all: it's all in the maths. The pilot is ham in a can who points the s/c where we in mission control tells him to :)

"Docking" with Minimus is indeed a possibility, but we'll not have target relative velocity information until we're in the SoI. It's also a pretty fuel- and time-inefficient. Minmus SoI is quite a small target so we may find that no matter how good our TmI burn is, we may struggle to hit it. In this case, we'll need to do corrections and the only way we (currently) have for that is the way you describe.

Out of interest, ElWanderer, why that extra half degree in your heading targets? 

For launch, we've managed to get <1 degree heading error by simply reading out heading corrections to the pilot while launching. Seems to work fairly well! It's just that timing we're having trouble with. I guess we could go map mode just for launch, but we're hoping to get a specific time if we can. The longitude we're getting is surface longitude. I think what we need to construct is a local time -> orbital longitude conversion. 

Fortunately, I just found this by user Dib, which does just that :) I'll have to give it a whirl but it seems sensible to me.

Thanks for your thoughts both!

 

 

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6 minutes ago, Alexandicity said:

but we'll not have target relative velocity information until we're in the SoI.

Yes you do, starting in ver 1.2.0. Try it. :)

But yes, flying by eye is certainly inefficient with fuel.

Edited by bewing

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ElWanderer: aah, thanks for the clarification. We fly with the navball set from the start to orbital, so hopefully we don't need to correct for this and can go direct to 6deg...

 

27 minutes ago, bewing said:

Yes you do, starting in ver 1.2.0. Try it. :)

But yes, flying by eye is certainly inefficient with fuel.

We're trying to be uber-strict with the information we allow ourselves. One could argue that target velocity/distance calculations could easily be done by some nameless computer in mission control (as, presumably would things like Pe/Ap/alt etc), so I guess I wouldn't be unhappy to have this information - but then there's a practical issue of not being able to actually to select a target :P I guess we could install RPM and do it through that, or via the console somehow...

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