General Question Optimum ascent profile

[james30]

Hello everyone! I'm trying to make a comparison between 2 Soyuz launches, one from Baikonur and one from Kourou, thus quantifying the improvement in results due to Kourou's better location in terms of initial earth rotation speed. Of course, for this to be realistic, I'm using the same payload and assuming that the desired orbit has an inclination that can be reached from Baikonur (>45º). The only thing I'm missing is a way to calculate optimum ascent profiles for each case. How can I go about doing this? For me the easiest would be to have an autopilot although if not possible, I'll do it manually. Any ideas? How do you launch to a certain orbit while minimizing fuel consumption?

 

[MJR]

In real life you would want to launch eastern because of rotation.

 

[Urwumpe]

How do you launch to a certain orbit while minimizing fuel consumption?

The problem is, that the most effective way is not always the same, even for the same rocket.

The important equation is the ascent formula.

It is pretty much as summary:

Orbital velocity = Total Impulse - gravity Losses - Control losses - aerodynamic losses + Rotation of Earth.

Total impulse is a function of your payload mass and tanking levels, if your rocket stays the same.

Gravity losses are the biggest losses and a function of your trajectory. The sooner you can pitch down, the lower these losses get.

Control Losses are all losses caused by you steering the rocket. This is the most complex factor and can be summarized like that: Control losses are big, if you:
- Fly with high AOA.
- Make fast and abrupt rotations.

You can thus minimize these losses by flying as smooth as possible most of the time, using sharp maneuvers only at the beginning for the initial course.

Aerodynamic losses are caused by drag, the faster and lower you fly, the worse. Usually your structural limits are worse than the drag losses, but I mention them anyway.

Finally, you get a velocity bonus by the rotation of Earth. For eastern launches, this bonus is 469 * cos(latitude of the launch site). If you fly in different directions (for example a 45° orbit from Kourou), this gets a bit of trigonometry (Dot product of the rotation speed vector with the direction vector of the orbit velocity).

Your goal is thus, to optimize this formula, by pitching as soon as possible, fly as smooth as possible and by avoiding as much drag as possible.

Additionally, you can improve the performance by not fueling your rocket completely for lighter payloads - less fuel in the rocket means less fuel to be transported unused into Orbit.

 

[ar81]

Launch from equator, heading 090. That minimizes ascent fuel consumption.

 

[james30]

Thanks a lot for your responses, I really appreciate it. The thing is, I understand these things. The problem lies in the fact that the different losses Urwumpe explains are in conflict with each other (turn as soon as you can to minimize gravity losses, turn as late as possible to minimize drag losses, dont turn to minimize steering losses). OK, good. Now, how do I go about implementing an optimum route to a specific orbit? Does nobody have an addon that will actually do this calculation to obtain a guidance file or at least a manual guidance mfd? How do the rest of you launch into orbit with minimum fuel consumption? Does everyone just start turning when they feel it's right??? Is this really the best we can do?

 

[Quick_Nick]

LaunchMFD: http://orbithangar.com/searchid.php?ID=2802
EDIT: Andy says it's multistage too.

 

[Andy44]

Launch MFD works for multistage vehicles as well as single stage. It's the next best thing to an autopilot, and more fun to fly.

 

[Zatnikitelman]

I'll second that. I use Launch MFD on my Prometheus launcher which is very Multistage and quite an unusual burn profile and it works well.

 

[Urwumpe]

Now, how do I go about implementing an optimum route to a specific orbit?

Additionally to the already said add-ons, it is also possible to optimize a launch profile for by using genetic algorithms. I have done that pretty often for special optimizations, the harder task is just making it work on general purpose launchers (for example by splitting the launch profile into logical stages)