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Astronut33

RSS Ascent Profile

Question

I recently installed realism overhaul with RSS and have been wondering what the optimal ascent profile is. I’ve made it into orbit a few times but each time it was a different ascent and a different orbit. 

What would be best for a rocket with around 9.5-10k dv, 1.5-1.7 TWR, and end up in a 150km orbit? Like what should my pitch be at different altitudes and speeds, etc. 

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The answer is as straightforward as it is pointless: "it depends."

As a ballpark figure, I've tried to get my initial gravity turn such that, some time later, I'd cross 30km with a pitch of about 30° above the horizon. This became a sort of universal rule for all my rockets regardless of TWR. From then on, "whatever it takes".

While you cannot throttle you engines, you can control your ascent by pitch. By 50km, the atmosphere is thin enough that you can point your rocket any which way with reckless abandon. Keep an eye on climb rate, the other on time-to-apoapsis and pitch up and down as necessary.

Early on, you want time-to-apoapsis not to get too low, though precise numbers depend on the TWRs of your upper stage(s). With a very long-lasting upper, it may be reasonable to let TTA drop to very low numbers (maybe even negative, iow, you're falling) as long as you're confident that the high TWR near burnout will eventually bring it back.

Towards the end, you should pitch so that your climb rate becomes nearly zero: that, and cutting the engines at just the right moment, will make a circular orbit.

Hitting a certain altitude takes some experience, both overall and with the particular LV you're flying. I suggest you aim for somewhat higher orbits to begin with,180+km, and then circularize at whatever altitude you end up.

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I tend to try to aim my initial gravity turn for 45 degrees at 20 km altitude. You sort of get a feel for how to start the turn to achieve that; with high-thrust first stages, you may be starting your turn at 30 m/sec or less, whereas low-thrust rockets may see you turning at 60-80 m/sec.

For low-thrust upper stages, especially long-burning engines like the RL-10, you'll often wind up with a fairly slow gravity turn, and then keeping your nose pitched up about 20-30 degrees above the horizon for several minutes. I know with my Titan* series of launchers, I don't dip the nose below 30 degrees until roughly 8 minutes into flight.

*Only tangential relation to actual Titan rockets. The upper stage is a triple RL-10 burning for 470 seconds, the core sustainer stage a pair of hydrolox-variant LR-87s burning for 300 seconds, and has one to six H-1 based boosters burning for 150 seconds. Hydrolox stages have wonderful specific impulse, but the thrust tends to be poor, and since you're losing mass slowly (high Isp), TWR doesn't climb as fast as it does with kerolox/hypergols.

Also, I have one rocket that I don't pitch over until at least 200 m/sec velocity is achieved.

Spoiler

Power sliiiiide!

kUmIBbN.png

 

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Thank you for the responses, I’ll have to try these. I for sure was going way to steep on my ascent, I’d be at 45 degrees around 50k ://

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