Fireball on ascent

[seaces]

Hi,

So I am at the stage where I want to make sure I design efficient rockets but also extract maximum out of my rocket with a good ascent profile.

My usual approach to rocket design that lifts certain payload to LKO is to have first stage around 2.0 TWR, second stage around 1.2-1.5 and by the time second stage is done I should already be in LKO. Using this design, my throttle profile is to keep my throttle set 100% all the way until AP is around 75km and then I coast and burn once again until I get LKO.

The flight profile is as soon as the rocket get to about 50m/s and its stable of launch pad, angle to 60deg instantly, wait for prograde marker to catch up and then instead of staying in the prograde marker I "pull the marker" by flying rocket on the very edge of the marker all the way till 50deg. I continue slowly pulling my prograde marker and aim to be around 45-35deg at about 10km. Then I check my time to AP and ensure that is going up and also verify in Kerbal Engineer that my horizontal speed is catching up with my vertical speed. I keep pushing my rocket till about 20deg but by that time I am already at stage 2 and will probably start coasting any moment. Sometimes I find myself up in space with still first stage not finished let alone second stage fired which was made in the first place for upper atmosphere and orbit insertion.

Thing is my rockets on the way up with Kerbals in them burn like a massive fireball, legit sometimes I think that my rockets burn more going upwards to space then when they are coming down to Kerbin. Poor Kerbals have really fiery and aggressive launch.

Anyway, does burning like this all the way up create massive drag inefficiencies, more then gravity? Some people say burn horizontally as much you can and get that horizontal speed, forget about fire but still I think this much drag would count for something?

To burn or not to burn.....

[Echo__3]

Gravity loss is a far bigger concern than drag for "reasonably" aerodynamic rockets. It sounds like you could be more aggressive with your gravity turn, especially with a starting TWR of 2.0. While you can get into orbit more efficiently in terms of ∆v, you probably will be better served by not having so much dry mass in engine weight. I have found that a starting TWR around 1.3 to work best for me.

Have you experimented with different launch profiles? I have done some experimenting and found a fairly aggressive gravity turn to work best.

 

 

[Streetwind]

2 hours ago, seaces said:

Thing is my rockets on the way up with Kerbals in them burn like a massive fireball, legit sometimes I think that my rockets burn more going upwards to space then when they are coming down to Kerbin. Poor Kerbals have really fiery and aggressive launch.

It's a consequence of the scaling difference between the game and IRL.

Kerbin is less than 10% of Earth's diameter. But, the atmosphere is roughly 35%-40% as thick. In other words, Kerbin's atmosphere is way taller compared to Kerbin's diameter than in real life.

As a result, RL rockets clear the atmosphere far steeper and far earlier than KSP rockets do. The vast majority of the sideways speed a RL rocket gathers is gained outside the atmosphere. In KSP, the vast majority of the sideways speed is gained inside the atmosphere.

And thus, because you're going that fast (relatively - KSP's physics constants are tweaked) inside the atmosphere, you get compression heating and reentry effects even on the way up.

 

Edited September 9, 2021 by Streetwind

[DeadJohn]

6 hours ago, seaces said:

... first stage around 2.0 TWR ... get to about 50m/s and its stable of launch pad, angle to 60deg instantly ...

What you are doing is efficient with KSP physics, but not "realistic" in terms of how real rockets fly. You are doing a very aggressive turn and that causes you to gain "too much" velocity (compared to real rockets) while still in thick atmosphere.

Consider experimenting with less TWR, don't turn as sharply, mostly follow prograde on the way up with some keyboard input to steepen the gravity turn.

[king of nowhere]

depending on the rocket, it may well be the most efficient launch path.

i suggest you experiment with less steep turns, and see how much deltaV you have in the end.

[Blaarkies]

Having a low TWR 2nd stage (0.5-0.8 TWR) can do surprisingly well if you point it 10-20° above prograde (when 50km+ altitude). The steering/cosine losses on a small angle is tiny, and is outweighed by less dry mass (from having less engines).

This is really easy to do if your 1st stage yeets it into a fast horizontal trajectory like you do, it gives the 2nd stage a lot of time to build up the last bit of speed to orbit.

[Cavscout74]

Personally I aim for closer to a 1.4-1.5 TWR first stage.  For stock scale, usually that will clear or nearly clear the atmosphere and my second & higher stages I will stick with 0.5-1.

[seaces]

Thank you to all of you for tips given, gonna give them a try.