Welcome to Jester's Trek.
I'm your host, Jester. I've been an EVE Online player for about six years. One of my four mains is Ripard Teg, pictured at left. Sadly, I've succumbed to "bittervet" disease, but I'm wandering the New Eden landscape (and from time to time, the MMO landscape) in search of a cure.
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Friday, March 28, 2014

Athena III

Remember how I said I was trying to design a heavy lifter based around KSP's Mainsail engine? And how I was having lots of trouble with it? I finally succeeded, and what a success! It turned out in very Zen fashion that all I needed to do to succeed was to go back to basics, keep thing simple, and focus on fundamentals.

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:
  1. Lift-off and acceleration as close to 150m/s velocity as possible.
  2. If velocity exceeds 150m/s below 3000m, cut throttles to 75% or so to maintain 175m/s.
  3. Cut the first stage at around 3000m and 150m/s, usually resulting in 110-140m/s up to 10000m.
  4. Gravity turn at 10000m to 45 degrees pitch at 90 degrees inclination.
  5. Allow speed to pass 200m/s no sooner than 10000m, then accelerate.
  6. Decelerate as needed to keep the stress on the vehicle at about 3Gs.
  7. Aim for 1000m/s speed at 30000m before second stage cut-off.
I find if I meet all of these goals, the resulting vehicle almost never has any issues making orbit. Hell, I followed a variation of this plan with my space planes: use the air-breathing engines to get to 30km and 1000m/s, then boost from there. It worked fine.

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
Link to Stage 3 with:
  • 6@ 1@ TT-70 Radial Decoupler
  • 6@ 3@ EAS-4 Strut Connector (space evenly around decoupler)
  • 6@ 1@ FTX-2 External Fuel Duct
Link to Stage 4 with:
  • 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
Link to Stage 2 with:
  • 18@ 1@ TT-70 Radial Decoupler
  • 18@ 1@ EAS-4 Strut Connector (at the top)
Link each SRB left to right with:
  • 2@ 1@ EAS-4 Strut Connector each, at top and bottom of SRBs
Stabilize with 6-12 TT18-A Launch Stability Enhancer to suit

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.


  1. Wow dude, didn't know you played KSP.

    What are the weirdest designs you've ever made? And have you tried out the new NASA pack?

    1. Yeah, I've been playing and blogging about KSP since December.


    2. Oh, and no, I haven't done anything one-off yet. All stock so far until I exhaust the possibilities of that.

    3. NASA pack is not released yet, only few youtubers got it to test it.

    4. I'm a rather recent reader, please forgive me for not knowing what all you blog about Ripard.

      I didn't know that the NASA pack wasn't out yet.

      Have you ever watched any of Scott Manley's videos on KSP, I know they've helped me a lot when it came to my rocket and space plane designs.

    5. Yep, I've watched a lot of Scott's videos, they're quite good. The one he did on orbital rendezvous and docking in particular really helped me learn that topic.

    6. Yeah, I've been meaning to get a station into orbit, that I've tentatively dubbed the Hyperion Project. Any tips on construction you could share?

      Unfortunately I can only seem to get small "comms" satellites into any sort of orbit. Since we don't really need the communications parts for anything but transmitting science at the moment.

      I do seem to have a talent for making planes and ballistic "missiles" though.

      It was Scott's videos and those by Macey Dean that really got me into KSP and inspired me to get the game off Steam.

      I think I need to work on my launchers for getting the station parts into orbit. And watch those videos on rendezvous and docking.

    7. Seriously, go back through the older KSP posts I've done by clicking on the Kerbal Space Program link at the bottom of this post. That said, I've built two space stations, both with the same basic design. Here's some notes on the first:

      And on the second:

      That said, you might find it most useful to go straight to my pictures directory here:

      From there, look for files starting "freedom-*" to see my first station built step-by-step, and "liberty-*" for my second.

    8. I've already got your other blog posts open, and thanks for the help, I'll be sure to put it to use.

  2. Your general design seems good. Do you use fuel lines to asparagus stage your boosters together? If not, you might want to experiment with that. It's generally more efficient as you get to run all of your engines and shed mass as tanks run dry.

    The simplest thing is to run some fuel lines from your Stage 2 boosters to your Stage 3 tanks. (You only need one per booster, not one per tank part). That way you can keep Stage 3 running all the time but it will still have full fuel because it will be draining from the outer tanks. The next step is to pick a pair of boosters (on opposite sides) and run fuel lines to their adjacent two boosters to it. Now those two boosters will, initially, be fueling your entire craft. They'll run dry relatively quickly and you jettison them. That saves you the tank weight, the strut weight, and the engine weight. Now you're a few km up and all of your remaining tanks are full. With six boosters you could wire them to eject in three different pairs, but I usually find that too much work. Oh, and you can give Stage 3 a Mainsail too, and run it the entire time, because it's getting fueled by your outer boosters and will still be full when all of Stage 2 is gone. And you got thrust from it the entire ascent.

    1. I've tried various asparagus launchers and just not had good luck with them, except with Enterprise (which was a modified asparagus). I find the loss of engines is not worth the loss of accompanying mass. That said, I guess a lot of asparagus designs rely on multiple engines per fuel tank cluster.

    2. Oh, and yes, I run fuel lines from S2 to S3. They're up there in the fitting.

  3. If you're consistently throttling back to keep your speed down, why not remove/downgrade some engines to reduce your vehicle weight and increase dV?

    1. I thought about it, but I really am using the full thrust of the Mainsails after 10km and I'm convinced their excellent thrust-to-weight ratio is what's pushing these vehicles to these high speeds at the top of the boost phase.

  4. Now install FAR and get rid of these ugly pancake rockets. :D

  5. Just thought you might like to know, the shuttle would cut its throttle after its roll maneuver because there was still significant air resistance and that would reduce aerodynamic stress on the launch vehicle. Once air pressure dropped low enough it could throttle back up.

  6. max Q it was called I think.
    have you tested payload weights yet, would be interested to see.

    Im using a mainsail heavy lifter that can do 60T to 100km

  7. an interesting discussion about rocket acceleration and drag: https://what-if.xkcd.com/24/ this made me think of it. I really should fire up KSP... i bought it a few months ago, and never played yet


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