Launch profile for 3.7 scale Kerbin

[NFunky]

Hey guys,

I0've been playing with the Kerbol 3.7 mod since I got to 1.0 and I really can't see going back any time soon. I really enjoy the extra challenge and slightly more realistic velocities involved. I've gotten pretty decent with it, except that I still haven't built up good "rules of thumb" for ascent.

My main problem is, while the atmo in 3.7 is still 70 km, the planet is WAY bigger, so horizontal velocity has to be around 4500. This means I need a shallower ascent path than I used to use. The trouble really seems to be when using a particularly low TWR final stage. I launch, let speed rise to 100, then tip a little. Solids jettison just about as vehicle goes supersonic, and TWR on Mainsail core is around 1.1, so I maintain a very high AoA during the first minute or so of this stage. I let time to AP rise to between 35 and 40 seconds. Lower stage jettisons, and upper stage instantly fires, but has a TWR of only 1.05, so I bring the nose back up quite a lot to compensate. I try to maintain 40 seconds to AP until it reaches 80 km. And here's where I'm not sure what I should be doing.

So, I at this point in flight, I still need at least two minutes of burn to reach orbital velocity, but time to AP is only 40. So, no engine cut possible, but I don't quite know how to orient the ship for a continuous burn to orbit. I find that if I aim for the horizon, I end up passing AP well before the half way point of this final burn phase. If I aim to high, I end up pushing my AP up to 90 or more km before I can get close to speed. Is there a happy medium aim point? An angle at which my AP will remain the same while I increase horizontal velocity?

Is this even possible? Oh, and the final 30 seconds or so of burn is with the final, LV-N powered probe, so TWR drops again to 0.5 for that last little bit. Is there a better ascent path in general I could be taking? Am I just expecting too much performance with too little TWR?

Thanks

Edited July 18, 2015 by NFunky

[Jouni]

I haven't played in bigger solar systems for a while, but here are a few thoughts:

• If the angle of attack is higher than a few degrees, something is probably wrong. Good rocket ascents follow prograde.
  
• If you use low-TWR upper stages, a steeper ascent profile could help. See what happens, if you start the gravity turn at 120 m/s or even at 150 m/s.
  
• A 200 km orbit may be easier to reach than a 100 km orbit.
  
• The first couple of minutes are the key to a successful launch. If you can't reach orbit, try using a bigger first stage and/or more/bigger boosters. Increase the initial TWR and/or the burn time before the first staging event.
  

[mrclucks]

Try less AoA with your twr being 1.05. Unless your time to apoapsis starts falling fast, you'll be increasing your orbital speed more and your apoapsis height will increase at a lower rate, but then your orbital insertion burn won't be as great.

If everything is in the perfect balance you'll be able to keep prograde after the initial kick untill just about apoapsis where you hold your heading to circulize.

I hope this isn't confusing and makes sense

[Streetwind]

Real rockets actually sometimes encounter the same problem going into low Earth orbit. At 7500 to 8000 m/s horizontal speed, with an atmosphere that ceases to be significant after maybe 120 to 130 km, that's a lot of very flat trajectory flying to do. Pretty much every real rocket burns for 9 to 11 minutes to LEO, and if the target orbit for the satellite doesn't mandate pushing the initial apoapsis fairly high, they need to optmize their trajectory a whole lot to avoid falling back down.

You can especially see this in some of the launch vehicles with longer burn times, like the Ariane 5: Towards the end of the burn, it will actually already have passed apoapsis, and is losing altitude before circularizing.

But without computer-aided trajecotry planning, a much more practical approach would be this: after you have pushed your apoapsis to where you want it, allow your rocket to reach it. And then, sit on that apoapsis, and take it with you. If you raise your nose, you will pull it forward, even if it is already behind you; if you lower your nose, it will slip backwards again. With careful steering, you can make it so that it always remains more or less directly on your vessel's location. This may or may not require you to keep a fairly steep AoA with a low TWR stage, so it's not the most efficient of maneuvers. It really depends on how much you managed to accelerate by the time you get to that point, too. But it's practically your only alternative to simply packing more TWR.

One thing to keep in mind is that in KSP, engine weights and tank dry masses are artificially high, to avoid completely trivializing the task to reach orbit on stock size planets. You may think Isp is the main factor that governs how easy it is to reach orbit, but that's not the case. If you play Kerbol 3.7 with stock parts alone, you will frequently encounter a situation where you struggle to fit enough engine under the rocket to achieve a TWR above 1 (Source: I have played around in Kerbol 3.7 with stock parts only). The engines simply weigh way too much for the power they output, the tanks simply weigh way too much for the fuel they contain. If you use a smaller system rescale like KScale2, that isn't much of a problem; if you use a larger system rescale, like 64K or RSS, you generally load up custom engines and maybe even custom tanks to address exactly that issue and keep the game from becoming unplayable because of it. Kerbol 3.7 straddles the border. You can make do with stock parts, but you'll struggle with very low TWRs in all situations.

[Jouni]

One thing to keep in mind is that in KSP, engine weights and tank dry masses are artificially high, to avoid completely trivializing the task to reach orbit on stock size planets. You may think Isp is the main factor that governs how easy it is to reach orbit, but that's not the case. If you play Kerbol 3.7 with stock parts alone, you will frequently encounter a situation where you struggle to fit enough engine under the rocket to achieve a TWR above 1 (Source: I have played around in Kerbol 3.7 with stock parts only). If you use a smaller system rescale like KScale2, that isn't much of a problem; if you use a larger system rescale, like 64K or RSS, you generally load up custom engines and maybe even custom tanks to address exactly that issue and keep the game from becoming unplayable because of it. Kerbol 3.7 straddles the border. You can make do with stock parts, but you'll struggle with very low TWRs in all situations.

The old stock parts were nicely balanced for 64K (or the 6.4x Kerbol System, as it was called back then). Typical payload fractions were 4-5%, which made rockets look much better than the current 20-25%. With simple designs, you could have up to 100-120 tonnes of payload on a single launch. I'm not sure what planet size the current stock parts are balanced for, but it's probably somewhere between 3.7x and 6.4x.

From what I can remember, the real issue wasn't with TWR, but with the lack of raw power. The biggest engines were only 3200 kN back then, so you needed a lot of boosters/parallel stacks. Once I installed the SpaceY part pack (which was generally balanced with the stock parts), launches became trivial.

[NathanKell]

You probably want an orbit of at least 100x100, probably more like 150x150. Even so, it's fine to circularize after apoapsis. Burn such that your initial apoapsis is like 180km, and then burn with positive pitch once you pass it, canceling negative vertical velocity once you reach desired altitude.

[NFunky]

Hey all, sorry I haven't checked these forums for a while. Just wanted to say a big thank you to all you guys for your suggestions. I've combined a few of them and am now quite handy at orbiting pretty much anything with at least 5500 dV, basically no matter the TWR.

I now have been flying a shuttle with one Mainsail fueled by asparagus staged external tanks, so my TWR only increases the whole way. With that much thrust, I find I can fly an incredibly shallow trajectory and actually get a nicd 75x75 without too much trouble, but if I fly it wrong, or need to incline too far away from 90, I end up having to complete orbital insertion with monoprop (very low TWR), so I've been using the "pulling the AP along with you" technique a bunch. And of course, my main heavy lift rocket requires a combination of many of the techniques you guys suggested.

Thanks again