Launching and Trajectories

Basic Rocket Launch Principles

This page is by user Zorg from the KSP forums. If you have questions please ping me on the BDB forum thread.

The process of getting a payload into orbit goes something like this:

1. Launch rocket into space.
2. Boost the payload into a "parking orbit" - typically by circularizing at Apoapsis to enter into a low orbit.
3. Boost the payload into an elliptical "transfer orbit" by raising Ap to the final desired altitude (or escape velocity if going interplanetary)
4. Kick the payload into final orbit by raising Pe to the final desired altitude.

Typically the first stage will launch the rocket into the upper atmosphere and the second stage will circularize. "Apogee kick motors" are small, cheap and reliable solid rocket motors such as the BDB Staara (Star), which are used for boosting payloads the final two steps when they reach the apoapsis (apogee). Some upper stage designs can deliver a payload direct to it's final orbit. This requires the engine to be able to restart multiple times, which requires more complex engineering and thus more prone to going wrong (though this is not an issue in KSP unless you're running a parts failure mod).

Calculating the exact burn time needed for solid rocket motors is challenging. Thankfully BDB SRMs intended for upper stage use can be manually shut down once the desired altitude has been reached (using Part Action Window, an Action Hotkey or a mod like Smartparts). It is recommended to include an Orbital Adjustment Module (such as the HAPS/PIBS or Athena OAMS) to tweak your orbit & trajectory before releasing the payload. Alternatively the payloads own thrusters can be used to trim the orbit.

Mechjeb Prime Vector Guidance and single burn to orbit

Mechjebs launch settings have an additional option called PVG, which was derived and built upon further on top of real world Atlas Centaur guidance algorithms. PVG was developed for playing at real scale such as Real Solar Solar system but it is usually quite effective at 2.5x/2.7x stock scale for which BDB is optimised.

Compared to the standard mechjeb ascent mode, which aims to follow a fixed trajectory, PVG dynamically adjusts the trajectory based on the TWR (thrust to weight ratio) of the stages. However it is not an AI and still needs some set up.

1. Set desired altitude and inclination

• Somewhat counter intuitively, the first thing you need to set up is the Target Periapsis. Normally you think of the altitude you are aiming for as an Apoapsis but this field is the periapsis of your final orbit. So for instance if you want to enter into a 90km orbit, set 90 in the Target Periapsis field. This will send you into a 90km circular orbit
• For the Ap leave it at 0 if you want to enter a circular orbit. As long as the Ap field is smaller than the Per field, PVG will aim for a circular orbit equal to the Per field.
• If you want to launch directly into a high orbit, the Target Apoapsis field becomes quite useful. Rather than setting the Target Periapsis field high, you leave it at a low figure such as 100km and then enter your desired Target Apoapsis such as 300km. PVG will then initially raise your altitude to 100km and then when circularising at 100km will contiue to burn until your apoapsis is raised to 300km. So your final orbit at the end of launch guidance will be 100km x 300km. You can then use a relatively small burn to circularise at 300km.
• inclination is pretty self explanatory. Set to desired inclination. 0 for equitorial launch.

This method can be used to launch directly into GTO. Set 100km for Per and GEO altitude as Ap and you will launch into an elliptical transfer orbit.

2. Pitch settings

• Booster pitch start sets the velocity at which the gravity turn begins (when the rocket begins pitching over). The default value of 50 m/s works well for most craft but you can increase to 100 m/s to be safer if you have trouble with rockets burning up before reaching orbit.

Increasing is safer for lower TWR craft as it will start the turn later and decreasing is more efficient for rockets with very high TWR (over 2) at lift off.

• Booster pitch rate controls the maximum rate at which the turn is executed. The default value of 0.5 degrees per second generally works well. Special cases are discussed below.

3. Coast

• If you leave Omit Coast unchecked, PVG will usually burn until the Per is established and then circularise on a second burn. If you check Omit Coast it will do the ascent in a single continuous burn. This is closer to most real world rocket launches. However Omit Coast can be very inefficient if your upper stage TWR is too high. If you see the upper stage burning radially downwards towards the planet by a large angle at the end of the ascent, then either uncheck omit coast or use a lower TWR upper stage.

4. Other settings

• The rest of the settings are the same as normal mechjeb ascent. Usually there is no need to limit Q (basically makes MJ throttle down to limit Q or dynamic pressure to the set limit). If you have a very high TWR craft and you feel the need to limit Q, make sure to set keep limited throttle over to something reasonable like 70% or higher. I find it unnecessary usually although players using FAR might find more use for it.

Lofted trajectories

Some rockets can be quite challenging to launch either manually or with default mechjeb PVG as suggested above. In particular

1. Rockets that have low TWR throughout most of the ascent for instance the Atlas CELV

2. Rockets that have a low TWR second stage responsible for a large proportion of the ascent such as Saturn IB trying to launch an Apollo CSM into low orbit or an Atlas Centaur (or any generation) attempting to maximise payload to low orbit.

For such craft an approach called a lofted trajectory is required. Indeed many real world rockets such as Saturn V, Saturn IB and most Atlas Centaurs use this approach.

On most rocket launches the aim is to go as horizontal as possible as quickly as possible. However with a lofted trajectory you are aiming to "throw" the upper stage higher than you normally would. The second stage then has more time to circularise and often ends up in a lower orbit than its initial apoapsis (this doesnt really happen with PVG it will end up at the target).

A few examples

Saturn IB

The Saturn IB is a craft BDB players often have trouble with. Especially with the more conservative thrust scaling BDB now uses for upper stages. However with the right approach you will see it can make orbit comfortably with an Apollo CSM.

Make sure to right click and use the part action window to select Orbital configuration for the Apollo service module. The Saturn I is not capable of launching a full lunar configuration of the Apollo CSM, setting to orbital will cut down the fuel to similar levels used on Saturn I apollo missions and Skylab missions.

Conditions

• This example is with the JNSQ planet pack which is 2.7x the size of the stock system. The atmosphere ends at 85km
• Service module set to Orbital configuration
• The Saturn IB first stage is configured with 4xH1C and 4xH1D in their original configuration rather than the 1973 upgrade.
• The SIVB second stage is configured with a single J2, original model rather than an upgrade. Upgrades will of course improve performance but I want to demonstrate it can be done without.

Mechjeb PVG Ascent

• Target Periapsis set to 130km (120 might be enough too but the important here is that are aiming higher than the usual 90-100km in order to have more time to circularise)
• Target Apoapsis set to 0 for circular but we are aiming for 100x300km in this example so I set to 300
• Booster pitch start set to 100
• Booster pitch rate set to 0.3 Make sure to jettison the escape tower as soon as the second stage ignites

Manual Ascent

If you have trouble, studying a mechjeb PVG ascent with the above settings might be helpful. However the below principles should be enough:

• Start pitching over gently once the rocket has reached 100 m/s. Normally most players set the rocket to hold prograde at this point but it may be better to control manually. Alternatively you can hold prograde for a minute or so and then take control.
• Use the readout of your choice (kerbal engineer, mechjeb etc) to see how far your apoapsis is. The idea is to control your pitch and try to get your apoapsis to around 90s away before first stage is depleted. You will most likely be maintaining a positive AoA (angle of attack). ie your rocket will be aiming higher than your prograde marker.
• Stage and ditch the escape tower.
• Once that is achieved start gradually pitching down. You should be pointing towards prograde by the time you reach your apoapsis. You should circularise some time after reaching apoapsis. You may need to pitch up a bit after passing the Apoapsis to keep the Ap from not shooting too far behind you before the periapsis rises above the atmosphere.