Gravity Turns, Clockwork Arrows and Power Shaping

[Hotel26]

The perfect gravity turn is elusive!

Spoiler

(If I could hand-fly one perfectly every time, I would a) give up KSP, b) turn Pro Golfer, c) start dating again and d) retire.)

                         

One method, employed during exhaustive testing, is the Clockwork Arrow.  Use a "clockwork" procedure, over and over, tuning it for best result.  Publish the 'clockwork' in instructional form.

A simple form of recipe is: to launch SAS Sfc Radial Out; pitch downrange 5 degrees reaching speed X m/s, raise the AP to Y km, then coast to circularization.

Sometimes, it is cute to pre-tilt the vehicle on the launchpad by 5 degrees and ignite with SAS Hold, allowing some build-up of horizontal velocity to achieve the precise initial trajectory before transiting to SAS Sfc Prograde.

With very high TWR (modular) boosters, one can stipulate launch with a retarded TWR (e.g. 1/3 throttle) to get a more accurate transition to SAS Sfc Prograde at the precisely-desired speed.

                         

Super-precision is fine, most of the time, but what about when you just want to get to space in one piece?  Or when you are in the midst of all those test launches and you know the current one is tending to the pear shaped...?  Or you are launching a complete unknown, designed by someone else.

This is where I use a technique I call "Power Shaping".

If your launch is not turning fast enough, you can retard the throttle.  (Since drag is the square of speed, this is not a terrible option in the low atmosphere.)

And you may want (or have) to take a retardation or pause in thrust to avoid 'sploding in a heat-induced fireball by imposing speed limits in the lower 20 km...

And if you think things are going well, you can add power.  Or switch to SAS ORB Prograde (as long as you are confident of having the attitude control authority to take the nose 'off prograde').

The metric to watch is the Time to Apoapsis.  I have this displayed in a KER HUD.  The moment it stops increasing, one needs a) to increase power and/or b) (most likely) to go to SAS Hold on the attitude.  Be advised that this will induce more drag and heat, but it will likely be necessary then to save the launch.

In the worst case, you may have to pull the nose up, applying full power, but this will indicate that you are experiencing a terrible launch, never to be repeated.  Think instead about ESC-Return-To Launch to save your own valuable time.

                         

Other ideas?  Techniques?  Please feel free to share/discuss.

[Bej Kerman]

On 12/1/2023 at 3:16 PM, Hotel26 said:

If your launch is not turning fast enough, you can retard the throttle.

On 12/1/2023 at 3:16 PM, Hotel26 said:

And you may want (or have) to take a retardation or...

Heads up, I think that word has the potential to confuse future readers

[TheFlyingKerman]

On 12/1/2023 at 11:16 PM, Hotel26 said:

The perfect gravity turn is elusive!

I end up using the same lifter again and again after perfecting the gravity turn.

[Hotel26]

This post by @Dakitess is worth reading!

https://forum.kerbalspaceprogram.com/topic/222559-what-does-your-ascent-profile-look-like/?do=findComment&comment=4355775

Dakitess correctly identifies TWR as the prime variable that leads to the correct choice of my 'X m/s', the speed at which to commence the gravity turn.

This had only been an unidentified intuition with me; Dakitess formulates the relationship very clearly.

                                                       -                       

On 12/10/2023 at 12:00 PM, Bej Kerman said:

Heads up, I think that word has the potential to confuse future readers

Thanks, Bej.  You might find this helpful, then.  https://en.wiktionary.org/wiki/retardation  Please see meaning #1, specifically.  But do note 'wiktionary.org', in general, as an excellent online resource!

I am a firm believer that people should continue learning/increasing their knowledge of their working language for the whole extent of their lives.  (Lest they spend their whole lives dazed and confused.) The internet makes it easy to continue learning and provides an indispensable aid to good communication!

                                                                      -        

On 1/5/2024 at 11:24 PM, TheFlyingKerman said:

I end up using the same lifter again and again after perfecting the gravity turn.

Brilliant!  That's it.  A range of modular lifters is an essential tool.  Makes the investment to find out how to perfect (and document) its ascent an easy choice -- and as reliable as 'Clockwork' to run.

Edited January 7 by Hotel26
Edited at user's request.

[Dakitess]

Thanks haha