Flight Question On gravity turns and other ascent / descent profiles

[Delta_Glider]

Hi Orbinauts,

I've just completed a lenghty study of the book Orbital Mechanics for Engineering Students (Curtis). It gives great explanations for delta-V computations of various orbital maneuvers we are all familiar with. However, on the question of ascent profiles, there is only a small section in Chapter 11 that briefly mentions a gravity turn.

I am planning a flight from Brighton Beach to the orbiting station Luna OB-1. I suppose the simplest way to launch would be to fire the engines in from horizontal hover, pitching the nose just above the horizon to clear any mountains, and then clean up the resulting orbit at apoapsis. But I would like to know if the community here has resources for understanding how to do other launch profiles, for example a gravity turn.

Kind regards

 

[Thunder Chicken]

A gravity turn on ascent is really necessary for a vertically launched rocket going through an atmosphere. Thrusting against gravity doesn't put you into orbit, but it is a necessary evil to get the vehicle up out of the dense atmosphere before aerodynamic drag loads get too high. Determining an optimal ascent profile that minimizes the gravity and aerodynamic losses really has to be done by numerical methods and there aren't any analytical solutions for such a profile that I am aware of, which is why that text spends so little time on the subject.

For ascent off a body with no atmosphere, how you are doing it is probably pretty efficient. You just need to have enough thrust vector downward to keep from falling back to the surface and to clear terrain, but otherwise you want to be burning tangentially to the surface as much as possible to get your speed up to orbital or escape velocity. Just remember that your periapsis will be pretty much at the surface launching this way, so you certainly would need to either escape the body completely or lift your periapsis with a second burn once on orbit.

 

[Delta_Glider]

Thanks
I think I am going to try to script a simulation of a "launch" wherein the pilot maintains a fixed pitch relative to the horizon, say 20 degrees, for a given time, and then examine the resulting postion, velocity and mass at burnout.

 

[Thunder Chicken]

If you are ascending from a body without an atmosphere there is an analytical solution for your pitch program. Assuming full thrust, you set the pitch such that the downward component of thrust matches the weight of the vessel. The horizontal vector of thrust will accelerate you downrange into orbit.

 

[Urwumpe]

A gravity turn has also some small advantages in minimizing the fuel you consume for controlling your attitude. But for something with no atmosphere and low gravity, that is really requiring a radical initial pitch over and you can't stop it as smoothly as on Earth.

Its really about gaining only as much altitude as needed for clearing the geology, without wasting any fuel in the vertical. Same in the opposite direction: For landing on such a body, only stay as high as really necessary, don't fight gravity more than needed. Concentrate on losing horizontal speed in the shorting possible time and distance. Just look at how high the Apollo missions were at PDI, merely about 50 kft or 16 km above the surface.