Ascent trayectory

[poinsot]

Hi, everybody!
I'm looking for a way to calculate the ascent trayectory for a specific rocket. Does somebody know a way to do this?
And where can I find the ascent trayectory of the Ariane V? Or any other launcher.
Thanks.

 

[Urwumpe]

Hi, everybody!
I'm looking for a way to calculate the ascent trayectory for a specific rocket. Does somebody know a way to do this?
And where can I find the ascent trayectory of the Ariane V? Or any other launcher.
Thanks.

Can only be done by iterative simulation.

Important is minimizing the ascent losses of impulse: Gravity losses, drag losses and control losses.

 

[James.Denholm]

I have this problem all the time, when vessels are poorly documented, take-off instruction wise. 'Cause I have no idea how preform initial climbs in planes. Or shuttles.

Or large super big inter-planetary massive-engine ramjet mother ships. Then again, you probably don't need to know how to do accents on those things.

 

[V8Li]

Read the Dynamic Pressure (DNP) and keep a constant ascend speed and forward acceleration. It's as simple as that for DG-style ships and scram ones. If you're using rockets you can have lower DNP values during the ascend (by flying higher) since you want minimum drag and you don't need a support like the atmosphere for your wingless ship. You can't simply jump to 200Km with 1000m/s velocity either since you will not have enough time to accelerate to 7300m/s or so when you get to apoapsis in order to get a stable orbit.

 

[poinsot]

Thanks for the answer!
I was thinking about an iterative simulation. But I have a problem with that. In order to calculate the drag coeficient I need the Mach number and since I need the Mach number I need the speed of the vehicle and the speed of sound. So, my problem is that I can't determine de Cd without the speed.
I try to make an estimate of speed vs altitude, but I find that it isn't too realistic
Thats why I was looking for a table speed vs atltitude and AoA vs altitude for a know launcher, but I can't find it.

 

[Zatnikitelman]

If it helps, Orbiter has a flight data recording feature that exports a text file with a table of data you can put into a spreadsheet. It's under the same dialog box as scenario editor button, under the custom modules button I believe. I've used it a few times, but can't quite get the text data into nice neat columns, they're always all over the place!

 

[Buzz Lovell]

There is also a Flight Recorder MFD which might help with formatting of data, etc.

How do they do it in RL, does anybody know? Compute the desired state vectors at Orbit Insertion point and "fly towards them"?

 

[Urwumpe]

How do they do it in RL, does anybody know? Compute the desired state vectors at Orbit Insertion point and "fly towards them"?

Yes. Also they use simulations for optimizing the open-loop guidance for the first part of flight for each payload and mission.

 

[cjp]

Yes. Also they use simulations for optimizing the open-loop guidance for the first part of flight for each payload and mission.

For each payload and mission??

What things are different for different payloads, except for mass (and CG)? Wouldn't it be sufficient to pre-calculate trajectories for a lot of different payload masses, and simply interpolate the results for a specific mission? Or is this simply a matter of "computers are cheap, so let's use them as much as possible"?

Some time ago I tried to make such an iterative ascend optimizing program, but I had lots of difficulties. One problem was that the derivatives of my result function (in-orbit-mass as function of the parameters of a parametrized ascend trajectory) had discontinuities, which disturbed my optimizing function.

I think it can be solved by using a different way of parameterizing the ascend trajectory. My way of parameterizing was to define the pitch at certain moments (e.g. at 100% fuel, 75% fuel, 50% fuel, 25% fuel and 0% fuel), and interpolate the pitch for all other moments. Maybe writing the pitch as a sum of sines and cosines would be better?

I guess this goes beyond the knowledge of the average Orbiter astronaut, but does anyone know what method the professionals use?

 

[Urwumpe]

What things are different for different payloads, except for mass (and CG)? Wouldn't it be sufficient to pre-calculate trajectories for a lot of different payload masses, and simply interpolate the results for a specific mission?

For example different orbit altitudes, different moment of inertia, etc.

Yes, they do interpolate in the basic mission planning, but when it gets closer to launch, the trajectories for the next launch opportunities get verified in simulation. That is different to the development of a complete new trajectory, but still the possible unknown effects are predicted as good as possible.