Question Launch autopilot for multi stage LV

[Jarod]

Hi,

I thought about how a launch autopilot could work for multi stage launch vehicles, but I'm way over my head :facepalm:
But if multi stage vehicles had a config file like that :
-stage1 : duration , thrust , mass, deltaMass / deltaT
-stage2 : duration , thrust , mass, deltaMass / deltaT

and then knowing the mass for the payload and final orbital parameters (PeA, ApA, Inc), could it be possible to devise the pitch program necessary to obtain the correct orbit ?
Is there someone on this forum knowledgeable enough in this field to give it some thoughts ?

 

[Urwumpe]

Brief: That is not that easily possible.

First of all, you have to include the difference between physical stages (what you assemble) and logical stages (phases in flight of constant structural mass)

Next, you have to remember that the behavior of such a autopilot depends on much more data, like aerodynamics, acceleration etc. during staging, the rocket has to keep a stable attitude, then you have phases of low AOA and phases where this is no longer of concern, the fairing has to be dropped at the right time, not too soon and not too late...

It is possible to create a rather bad generic autopilot that can tell another autopilot where to point the nose of the rocket, but the final movements of the engines etc is pretty complicated.

The problem is not the final seconds of flight, but the first 80. The complex aerodynamics and high inertia of the rocket make it pretty hard to control the rocket closed loop, only open loop guidance works in real rockets reliable enough for launch.

 

[Jarod]

Sorry I wasn't clear enough, yes I knew about the differences to be made between logical and physical stages.
And I was thinking the pitch program would be fed to an open loop guidance.
I mean, open loop guidance by itself can't work on multistage, it needs to know somehow what kind of stage it will inherit during the ascent.
I used Lua to make some goofy autopilots which were using ApT, ApA and how they were varying to decide how I should change the pitch, but without any knowledge of how the second stage will react, that's impossible to be consistant across different vehicles.