Flight Question Incorrect orbital Insertion

[Wrangler]

Hi all,

My first thread. I recently discovered Orbiter (6 months or so). I have learned alot from you folks, and I hope that you will bear with me if I ask a very elementary question.

I have been trying to perfect my shuttle launches so that I can insert them at the exact inclination angle, without the need for a plane alignment later in the mission. I have done the azimuth calculations by hand. Even used the Space calculator for the Hanger. But when I choose an Azimuth for the shuttle autopilot ( I have tried both the stock Atlantis in orbiter100830 and one of the Shuttles from David413's fleet), I always end up with ~2 degree RInc between the shuttle orbit and the target orbit (usually the ISS). Now I have been going through the old footage of shuttle launches, and they always say that the guidance 'converges' after the SRB Separation. Is there some sort of course correction that is supposed to come after the SRBSep? Am I missing a step?

Also if this has been answered elsewhere, I would appreciate the link. I have not been able to locate it.

Regards,

Wrangler.

 

[Zatnikitelman]

Sounds a little like a timing issue. You need to wait until your target orbit track is real close to your launch site. How close? I've only ever been able to work it with trial and error and launching when it "feels right." Also, as you're launching, you might could bring up the Align Planes MFD and nudge the vehicle left or right, whatever reduces the RInc as your ascending. It sounds like you're very close though, 2 degrees RInc is pretty good.

 

[csanders]

LaunchMFD is good for timing when to launch. I've found launching at about 500seconds to interception is a good place to start.

 

[Wrangler]

Sounds a little like a timing issue. You need to wait until your target orbit track is real close to your launch site. How close? I've only ever been able to work it with trial and error and launching when it "feels right." Also, as you're launching, you might could bring up the Align Planes MFD and nudge the vehicle left or right, whatever reduces the RInc as your ascending. It sounds like you're very close though, 2 degrees RInc is pretty good.

Well, 2 degrees is practically nothing if you are in a DG. However, you have to burn a lot of fuel in a shuttle. Anyway, I will try your advise on the target orbital track. However, if that is what I am doing wrong, it should only effect my LAN, right? My inclination should be spot on. But it is not. Anyway, Thanks.

---------- Post added at 05:41 PM ---------- Previous post was at 05:37 PM ----------

LaunchMFD is good for timing when to launch. I've found launching at about 500seconds to interception is a good place to start.

Yes, I have seen that. and you are correct. However, with historic scenarios (like the ISS construction missions I am doing these days), Launch windows are usually rather fixed. But again, something else to revisit. Thank you.

 

[Urwumpe]

Does the Shuttle Fleet still fly at constant azimuth or was this minor bug corrected already? if not, you need to apply a little bias to the launch azimuth, since you are not flying on a great circle during ascent.

But this bug is so old, it likely already sits in his retirement home in Florida.

 

[Wrangler]

Ok, I just did another launch and I rechecked the parameters through the LaunchMFD as advised by csanders. The time to intercept was a bit larger, but I was running the STS-88 scenario from thorton's ISS package (The launch timer is active when the scenario starts). Anyway, please refer to the attachment. I estimate that I will have to spend nearly a third of the fuel to aling the orbits ( though that might also include the OMS1 burn at apoapsis). Is that really the best I can hope for?

Also, @Urwumpe, yes the shuttle does seem to maintain a steady heading during ascent until the heads-up maneuver before MECO. Could that be the issue?

Regards,

Wrangler

 

[Urwumpe]

lso, @Urwumpe, yes the shuttle does seem to maintain a steady heading during ascent until the heads-up maneuver before MECO. Could that be the issue?

Yes, because Launch MFD and most publications have the launch azimuth as great circle heading. For Shuttle Fleet, you have to use essentially the azimuth at MECO as launch azimuth... complex stuff to calculate, easier to just trail-and-error.

 

[C3PO]

Yes, because Launch MFD and most publications have the launch azimuth as great circle heading. For Shuttle Fleet, you have to use essentially the azimuth at MECO as launch azimuth... complex stuff to calculate, easier to just trail-and-error.

You can do a test launch using the usual launch azimuth, and make a note of the latitude at MECO. If you use that latitude to calculate the "azimuth" for the STS fleet, you should be a bit closer.

 

[Urwumpe]

You can do a test launch using the usual launch azimuth, and make a note of the latitude at MECO. If you use that latitude to calculate the "azimuth" for the STS fleet, you should be a bit closer.

Exactly. Iterative improvements... But it does sadly not converge unless you keep the rate of change in your calculations.

 

[C3PO]

Exactly. Iterative improvements... But it does sadly not converge unless you keep the rate of change in your calculations.

That's true, but i always modify my heading according to AlignMFD during the later part of the ascent anyway.

 

[Wrangler]

That's true, but i always modify my heading according to AlignMFD during the later part of the ascent anyway.

So I take it you are not using the autopilot? cause if you are, how do you override the autopilot?

 

[DaveS]

So I take it you are not using the autopilot? cause if you are, how do you override the autopilot?

The Shuttle Fleet Ascent AP allows manual yaw inputs during second stage to correct the orbital plane prior to MECO.

 

[Wrangler]

Thanks!!!!!!!!!!!

The Shuttle Fleet Ascent AP allows manual yaw inputs during second stage to correct the orbital plane prior to MECO.

Thank you DaveS and C3PO. I know what to do now. attached is the latest launch. I was able to get very close. :thumbup:

 

[DaveS]

Thank you DaveS and C3PO. I know what to do now. attached is the latest launch. I was able to get very close. :thumbup:

You should aim to get it under 0.1°. 0.8°s is too much.

 

[C3PO]

The thing to watch out for is when the nodes on the AlignMFD start to move faster. Once they are 90° away from you, you can't reduce RInc any more. In a perfect launch this happens just as you hit your target orbit, but that never happens without full closed-loop guidance.

I use the HUD in orbit mode to minimize the error. Once the nodes on AlignMFD are 90° away from my position, I yaw the "nose" so it's aligned with the vertical zero-line on the Orbit HUD. Once you get used to the method, you can eyeball the "slip" angle to get a lower RInc.

 

[jthill]

Some things I do to get the Atlantis to orbit manually, all lined up:

Launch anywhere from 250-300 seconds Tn on Align Planes. For the ISS, 45 or 135 degrees is close enough for an initial azimuth.

Don't roll or start pitch-over until the heading's correct. Use the yaw and pitch RCS lightly, with a bit of killrot toggling to keep things under control, keeping velocity within a few degrees of vertical. Then roll inverted while still keeping velocity near vertical..

Watch DNP on the surface MFD and when it starts dropping (or a few seconds before, anywhere in there's fine) start the pitch-over. Lead the velocity vector by something like 15deg all the way until velocity is something like 20 deg up.

From there, use pitch to manage ApA and ApT on the Orbit MFD and yaw to manage Rate, Tn and RInc on Align Planes. Pitch controls the speed of the ApT clock, yaw controls Rate and soon begins to control the Tn rate. I generally keep a very rough running product of Rate*Tn in my head and keep that close to RInc. When Tn rate gets very sensitive to yaw (later, likewise with ApT and pitch) things get almost too easy -- missing MECO while chasing tiny errors is my usual mistake.

I jettison the main fuel tank just when PeA goes positive, and use translate mode on the RCS to get out of its way.

 

[Wrangler]

Some things I do to get the Atlantis to orbit manually, all lined up:

Launch anywhere from 250-300 seconds Tn on Align Planes. For the ISS, 45 or 135 degrees is close enough for an initial azimuth.

Don't roll or start pitch-over until the heading's correct. Use the yaw and pitch RCS lightly, with a bit of killrot toggling to keep things under control, keeping velocity within a few degrees of vertical. Then roll inverted while still keeping velocity near vertical..

Watch DNP on the surface MFD and when it starts dropping (or a few seconds before, anywhere in there's fine) start the pitch-over. Lead the velocity vector by something like 15deg all the way until velocity is something like 20 deg up.

From there, use pitch to manage ApA and ApT on the Orbit MFD and yaw to manage Rate, Tn and RInc on Align Planes. Pitch controls the speed of the ApT clock, yaw controls Rate and soon begins to control the Tn rate. I generally keep a very rough running product of Rate*Tn in my head and keep that close to RInc. When Tn rate gets very sensitive to yaw (later, likewise with ApT and pitch) things get almost too easy -- missing MECO while chasing tiny errors is my usual mistake.

I jettison the main fuel tank just when PeA goes positive, and use translate mode on the RCS to get out of its way.

I can never get my pitch rate right during manual launches. I find the auto pilot script to be rather handy. Still I will give it a whirl. Thanks...