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Related: Terminal velocity table for KSP v1.0 and later?

I like to build big fast rockets. I always get wind and heating effects on my way up. Prior to version 1, that was incredibly inefficient because of the severe effects of terminal velocity, but now (version 1.0+) it's not such a big deal because of the atmospheric remodeling.

Obviously, the less time you're affected by gravity the better so going fast has a measurable benefit. So does Terminal Velocity matter anymore?

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  • Yes, although perhaps less so for launching as for landing. I lost a probe returning to Kerbin because I couldn't slow down before I had to deploy the parachutes. Come to think of it, air-brakes might be a good idea for my upcoming Duna mission. – MBraedley Aug 28 '15 at 2:28
  • I think it depends just how much thrust you are wasting due to excess friction. I have found with my designs that getting a very large boost from first stage to break free from the ground and then dropping it fast makes a bigger difference to orbit than going super fast all the way up. – nbering Aug 31 '15 at 3:41
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It is important... on the way down :)

Getting off Kerbin got significantly easier. With the same ~2300m/s of LKO orbital speed, the actual fuel requirements accounting for the losses dropped from 4500 to 3500m/s of delta-V.

All but the absolutely fastest of rockets aren't bothered by the terminal velocity on the way up. If your TWR is less than ~8, you don't need to bother. If it is, you'll be still better off going over the terminal velocity over the short distance where your drag losses are higher than gravity losses, than suffering increased gravity losses over the remaining distance. Only with TWR exceeding something of order of 12 you might consider increasing your payload.(*). With TWR exceeding 16-18 (achievable with jet engines on small craft) air heating becomes a concern.

The situation is different on the way down. The low drag and high terminal velocity means low atmospheric deceleration. You need extra drag-inducing features like airbrakes, drogue chutes or such to reach a safe speed - only the last few kilometers have a terminal velocity below 250m/s which is safe to open the main parachutes, and if you're coming in too fast, or in a heavy, aerodynamic vehicle, you'll never reach that speed before lithobraking.

(*)Obviously flawed solutions like reducing the number of boosters or throttling the engines are against the Kerbal Way.

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  • as an aside, I botched a gravity turn once and circularized at the altitude of 30km. Not even close to terminal velocity. It may matter below 20km, especially with spaceplanes that have long, flat trajectory, but then in their case, at altitudes where it would occur, you're using air-breathing engines that have so high ISp that the losses caused by unoptimal speed/pressure curve are still negligible. – SF. Sep 3 '15 at 11:07
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    +1 for "Obviously flawed solutions like reducing the number of boosters or throttling the engines are against the Kerbal Way" – Svj0hn Sep 3 '15 at 11:13
  • I like this answer. It's funny, but it can't be accurate. If a vessels has a starting TWR above about 3 it will explode due to dynamic pressure and heating. I know because I've done it... many times. Throttles are for sissies. – Coomie Sep 7 '15 at 1:24
  • @Coomie: Starting TWR of above 3 near sea level with full tanks of fuel and low payload fraction means once you've burned past 2/3 of the fuel and reached 90% vacuum ISp you have TWR of order of 15. It's even more pronounced with turbojets, A turbojet-based plane with 1.6 TWR at stadstill, sea level, will have TWR of some 18 at 10km and 800m/s. Usually though with these high-TWR vehicles a steeper trajectory, a slower gravity turn suffices - reach thinner atmosphere before gaining more horizontal speed. – SF. Sep 7 '15 at 9:19
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    Nevertheless, if you want more precise results, install Kerbal Engineer Report, customize the in-flight display to include Atmospheric Efficiency and observe it in flight. <100% means you're losing more to gravity than to air drag. >100% still doesn't mean terminal velocity, just drag exceeding gravity losses. And you'll see most of the time you barely reach 60%. – SF. Sep 7 '15 at 9:29

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