OpenRocket and RASAero adapted to orbital rocket sims?

The most difficult part of getting a trajectory sim for a rocket to orbit is estimating the air drag on the rocket. The losses that arise from gravity drag and steering drag are easy to calculate.

Then it is notable that free rocket trajectory simulators such as OpenRocket and RASAero can do sims even up to high altitude and at high Mach values while including the effects of supersonic air drag.

Then can these sims be adapted to do sims of rockets to orbital space? Most rockets to orbit follow a flight to orbit making use of a "gravity turn." What this is is that the rocket makes a small angle pitch over maneuver, commonly only in the range of 3 to 5 degrees, and then we let gravity gradually turn the rocket horizontally so that the rocket is eventually thrusting horizontally, to achieve the needed tangential velocity for orbit.

The OpenRocket and RASAero sims though don't seem to have included this capability of changing the pitch angle, i.e., angle of attack, during the flight. So this would need to be a modification added to the software.

At least with OpenRocket you can somewhat emulate it by doing two separate sim runs. The first is done so that it has the first booster stage run to completion and get to some altitude. Then you run a separate sim with the remaining stages starting at that ending altitude provided by the first stage and at your desired starting pitch angle.

Problem is though while OpenRocket allows you to input starting altitude and starting pitch angle when you run a simulation it does not allow you to enter a starting velocity, so this emulation would be of the case where the upper stages start from 0 velocity. This though is not the most efficient trajectory for an orbital flight.

So still OpenRocket would need an adaptation where a stage could pitch over some time during the flight. You would also want the adaptation to also allow the complete flight without having to break it up into separate sims.

Another adaptation that would be required is to allow multiple parameters to be varied to find the optimal combination to get the most payload to orbit and/or the highest orbital altitude. For instance, in addition to finding the optimal pitch angle and altitude to perform the pitch over, you also need to know what's the best combination of coast phases. Users of the sims are aware of the fact that changing the length of the coast time between stage burns can significantly change the altitude. This is also the case for what is the maximal velocity that can be achieved, and therefore maximal payload deliverable to orbit.

This capability of varying multiple parameters would be computationally intensive though. For instance, my laptop is a ca. 2010 Windows 7 computer, and often I get a "Out of Memory" error just with fixed pitch and altitude, when I tried to do the OpenRocket emulation of an orbital flight, as broken into two sims.

Bob Clark

I doubt it. OpenRocket isn't designed for that kind of trajectory- it's not even designed for supersonic flight.

May I recommend Kerbal Space Program?

Doesn’t RockSim PRO do what you’re asking?

timbucktoo said:

Doesn’t RockSim PRO do what you’re asking?

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Rocksim Pro is also suborbital. But, it's a 6-degree-of-freedom simulation. Last I heard, it has major problems that make it unusable for serious applications.

You can run an orbital rocket on RASAero II, but there are a few work-arounds required.

RASAero II does not currently have full rotating oblate Earth Equations Of Motion. But, if you fly to a Polar Orbit, the errors from what is missing from the Equations of Motion go to close to zero, so it will work.

So first, Polar Orbit only.

Second, remember that the thrust curve data in the rasp.eng file read by RASAero II is assumed to be Sea Level thrust. So if on your orbital rocket, you have a vacuum thrust curve, you have to convert it to a Sea Level thrust curve.

You'll have to simulate the vertical rise off the launch pad, and then the pitch-over to the gravity turn, by mounting the rocket on a rail and adjusting the launch angle until you intercept the gravity turn trajectory. Make sure the wind input is zero, you don't want to weathercock, you want a pure gravity turn.

So suppose you have a Vanguard three stage rocket. You run the first two stages in RASAero II, and then coast the third stage, until the coasting third stage reaches apogee at the orbital altitude. The only error will be that instead of a vertical rise, and then a pitch-over to a gravity turn trajectory, you start with the rocket "leaned over" on the launch rail, and the gravity turn starts immediately. But in the end you'll be adjusting the launch angle anyway until you go through apogee at the orbital altitude, so the errors here will be small.

Once you have reached apogee with the coasting third stage, and it is time to light the third stage (a solid in the case of the Vanguard rocket), at that point your RASAero II sim is done. You'll have to do hand calculations using the Ideal Rocket Equation to fire the third stage at the orbital altitude, to get the final Delta-V. And you'll have to check that you have the right inertial velocity to be in orbit at that altitude.

On a RASAero II trajectory, there has to be an apogee and a descent. If you try to light the third stage, and you exceed orbital velocity, there never is a descent and the program runs infinitely or locks up, and a Ctrl-Alt-Del will follow.

Also note that liquid fuel orbital rockets with low initial thrust-to-weight ratios (1.2 to 1 in many cases) will "fall over" and impact the ground when the launch angle from vertical gets too large, you'll find that a small launch angle from zero will produce the appropriate gravity turn which will intercept the targeted orbital altitude.

Again, many orbital rocket have Stage-1 burn, Stage-2 burn, Stage-3 coast, Stage-3 burn, or Stage-1 burn, Stage-2 first burn, Stage-2 coast, Stage-2 second burn. RASAero II will work for a Polar Orbit for the first two burns and the coast. Once you are at the orbital altitude for the final burn, you'll have to do hand calculations.

So for orbital rockets on RASAero II, Polar Orbit only, and hand calculations are required for the final orbit insertion stage.


Charles E. (Chuck) Rogers
Rogers Aeroscience

LithosphereRocketry said:

I doubt it. OpenRocket isn't designed for that kind of trajectory- it's not even designed for supersonic flight.

May I recommend Kerbal Space Program?

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I've heard of Kerbal being used by people to model their own designs, but can it model existing orbital rockets and has it been proven to be accurate?

Bob Clark

jsdemar said:

Rocksim Pro is also suborbital. But, it's a 6-degree-of-freedom simulation. Last I heard, it has major problems that make it unusable for serious applications.

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Also quite expensive.

Bob Clark

RGClark said:

I've heard of Kerbal being used by people to model their own designs, but can it model existing orbital rockets and has it been proven to be accurate?

Bob Clark

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Yes; no.

It can model just about anything, especially with help from the modding community, but since you have to hand-fly the trajectory you want it's hard to get anywhere close to the actual mission specs of real vehicles.