Smart Parts to Orbit

The question to answer was, is it possible to automate a launch vehicle using Smart Parts?

Craft: Automatic Launch

0.8 ton relay satellite delivered into a circular orbit

As you might expect, it is possible to automate the launch. We will attempt to cover the detail of the considerations for the design of the launch vehicle and the kind of launch profile needed to get into orbit.

The end result of this is that from the launch pad, all that is required to get into orbit was:

• Enable SAS
• Stage
• Set SAS to Prograde

The rest of the sequence is entirely automatic and will deploy the payload into a fairly tunable orbit.

However there are some caveats. The launch vehicle will require extensive testing to ensure the timing is correct, and in addition I found in testing that getting two completely consistent launches was not possible. There appears to be minor variations in each launch, perhaps due to the physics processing or some other subtlety.

Requirements

• Stock Kerbal Space Program
• Mod: Smart Parts

Launch Profile

The launch profile is a gravity turn which aims to be reasonably efficient. It is made up of three separate burns:

Initial: Full power burn at a slight angle to start the turn.

Middle: Moderate power long duration burn continues gravity turn to orbital altitude

Circularise: Full power burn adds enough horizontal velocity to circularise orbit

Using a gravity turn like this allows us to automate the launch with no input from the user.

Launch Vehicle Requirements

The launch vehicle must be aerodynamically stable. With fins at the rear and a simple balanced design ensures that it will remain stable throughout the flight. The first two burns could actually be performed with no stability control at all. Only the last one requires that we use Prograde SAS.

The payload mass in the demonstrated craft is small, to lift a larger load would require a larger launch vehicle but would follow the same principles. In this case the payload is close to the maximum this launch vehicle could lift. Switching to Liquid Fuel engines would improve efficiency.

Initial Burn

The launch vehicle should be set at a slight incline. This is achieved in the VAB by selecting the craft, and holding Shift + D to tilt the craft in small increments. Two increments is optimal. Use the support towers to hold the craft at this angle for launch.

Launch Vehicle at slight incline, ready to launch - Full Size

Attach a timer to the last stage of the craft which is in the first staging group. This timer will trigger staging of the circularise burn later. About 4 minutes, tune to launch vechicle

The launch burn I found worked best with Solid Rocket Boosters (SRB), 4 x Hammer at 100% thrust worked well for a small payload. The SRB's should jettison once spent with separators.

Trigger staging with the fuel sensor

Use timer trigger set for about 4 minutes to trigger staging

Set SAS on, and to Prograde. Stage to launch.

4 SRBs take us up to about 5km - Full Size

Middle Burn

Middle Staging triggered - Full Size

The next burn immediately follows the initial and is a longer duration, lower thrust burn. This has to do the rest of the work of lifting the mass out of the atmosphere. It needs to be long enough for the gravity turn to slowly curve the orbit into a useful ballistic orbit with an apoapsis of 70km or more. Too low thrust and the vehicle will not gain enough height and burn up. Too much and there will not be enough horizontal speed for the final burn.

Middle stage sub-orbital burn to 70km+ - Full Size

The burn can finish before the craft reaches apoapsis however it is recommended to jettison the spent stage before the final burn so that the final SRB is clear to fire.

Trigger staging with fuel trigger

Middle stage complete and separation - Full Size

Circularise Burn

Coasting to final burn - Full Size

Now the craft is coasting towards apoapsis waiting for the Timer part to trigger. The important detail here is that we have used SAS set to Prograde to keep the craft aligned with the prograde vector.

Timer triggered Circularising burn - Full Size

When the timer elapses, the final SRB will fire (15g of force, not recommended for crew) and propel the payload to orbital velocity. The fuel amount of this SRB will need to be tuned to the mass being propelled. The SAS control here is crucial (I could find not other way to achieve this without SAS).

When the SRB expires it needs to trigger an action group which will trigger the parts of the payload that need to be activated. Separator, solar panels, antennas etc are all ideal to trigger in this last group. In the example I used the "Lights" action group but any other would work as well.

Fuel Sensor to trigger final action group

Circularising burn complete, jettison stage and deploy payload - Full Size

We now have deployed the payload to orbit.

A roughly circular orbit - Full Size

Testing and Tuning

The most fiddly part of this processes is the testing and tuning and requires a number of test launches to perfect.

In the example the payload weighs 0.8 tons.

Initial: 4 x Hammer SRB @ 100% thrust

Middle: 1 x Thumper SRB @ 62% thrust

Circularise: 1 x Hammer SRB @ 100% thrust (275 out of 375 fuel)

Timer Trigger: 3 minutes 45 seconds

The variable parts are:

• Middle thrust: Scaled to lift TWR for gravity turn
• Circularise fuel: Scaled to payload mass lifted
• Timer: Based on all of above

Appendix

One aspiration I had for this approach was that I would be able to create repeatable launches for the purpose of launching satellites to make up a network for RemoteTech. However I found that even with automated launches, there was a surprising amount of variability in the final altitude of the probe.