This thing is a beast.
When designing the first and second stages of my rockets in KSP, I have been following a rule that I privately think of as the 3/30 rule: cut the first stage at 3km and 150m/s, cut the second stage at 30km and 1000m/s. This is a rule that's served me quite well going all the way back to my earliest "Virgo" launcher. I've read again and again on other KSP websites that there isn't much point to moving faster than 200m/s below 10km, so my step-by-step flight profile from 0 to 30km has generally been:
- Lift-off and acceleration as close to 150m/s velocity as possible.
- If velocity exceeds 150m/s below 3000m, cut throttles to 75% or so to maintain 175m/s.
- Cut the first stage at around 3000m and 150m/s, usually resulting in 110-140m/s up to 10000m.
- Gravity turn at 10000m to 45 degrees pitch at 90 degrees inclination.
- Allow speed to pass 200m/s no sooner than 10000m, then accelerate.
- Decelerate as needed to keep the stress on the vehicle at about 3Gs.
- Aim for 1000m/s speed at 30000m before second stage cut-off.
That last bit -- keeping the stress on the vehicle at 3G -- is something that I picked up from watching real life space shuttle launches. At about 7:20 into their launches, their computer starts dialing back thrust to maintain that load. So I thought it would be fun to mimic this plus who knows, it probably helps things not break off my rockets. ;-)
My first launchers in KSP four months ago would rapidly accelerate off the launch pad. But as I've been building heavier creations I've gotten used to slower lift-offs. Enterprise positively struggled to get into the air and was at the lower limit of the line all the way into orbit.
I say all of this because this new heavy lifter has ridiculous acceleration and speed, particularly for so heavy a vehicle. First stage cut-off and separation was at 5000m and I had to throttle the engines back to 75% at 2500m to keep speed below 200m/s! And that's where the throttle stayed right up to 10000m and the gravity turn. After that, I slowly eased the engines up to 100% throttle for the boost phase and kept them there until G-load passed 3Gs. At that point, I stepped down the throttle in 10% increments to keep the G-load right at 3Gs until second stage cut-off and separation.
That second stage cut-off happens at right around 70km and 1700m/s. That's where I start with a completely full third stage. Wow. Just wow.
Needless to say, this lifter has no trouble at all with much heavier loads. I haven't yet found its practical mass limit to a 400km parking orbit. It's definitely going to be my Jool mission lifter and I wish I'd had it for Duna. So I now know what's getting me to Jool; just have to design the final vehicles. More on that in my next KSP post.
I don't usually share full fittings but this time I'm going to:
[Athena III Heavy Lifter]
Stage 3 (innermost stage), top to bottom:
- 1@ Rockomax Brand Decoupler
- 1@ Advanced S.A.S Module, Large
- 2@ Rockomax Jumbo-64 Fuel Tank
- 1@ Rockomax "Skipper" Liquid Engine
Stage 2 (middle stage), top to bottom:
- 6@ 1@ Protective Rocket Nose Mk7 (optional)
- 6@ 2@ Rockomax Jumbo-64 Fuel Tank
- 6@ 1@ Rockomax "Mainsail" Liquid Engine
- 6@ 1@ TT-70 Radial Decoupler
- 6@ 3@ EAS-4 Strut Connector (space evenly around decoupler)
- 6@ 1@ FTX-2 External Fuel Duct
- 6@ 1@ EAS-4 Strut Connector (heavy S4s, not needed for light S4s)
Stage 1 (outer stage), top to bottom:
- 18@ 1@ Aerodynamic Nose Cone (optional)
- 18@ 1@ Rockomax BACC Solid Fuel Booster
- 18@ 1@ TT-70 Radial Decoupler
- 18@ 1@ EAS-4 Strut Connector (at the top)
- 2@ 1@ EAS-4 Strut Connector each, at top and bottom of SRBs
Add Stage 4 to suit.
As I said, it's a pretty simple design with a minimum of parts. It just turned out the tricky bit was figuring out how to put them together without the vehicle tearing itself to bits. The secret is lots of structural bracing between the stages. Next up, some theoretical vehicle designs for the Jool moon tour.