Terminal velocity on Kerbal

[Gloom Demon]

I have read, that while in the atmosphere it is best to keep the rocket's speed at around the speed of terminal velocity to maximise fuel efficiency. Below that speed the rocket will fall, and above it the rocket suffers from excess drag.

However I fail to find what is the terminal velocity on Kerbal... On Earth it is 9.8 m/s, but on Kerbal?

Edited October 15, 2015 by Gloom Demon

[Champ]

I have read, that while in the atmosphere it is best to keep the rocket's speed at around the speed of terminal velocity to maximise fuel efficiency. Below that speed the rocket will fall, and above it the rocket suffers from excess drag.

However I fail to find what is the terminal velocity on Kerbal... On Earth it is 9.8 m/s, but on Kerbal?

KER shows you terminal velocity if configured properly.

BTW, i think (not sure) you are wrong about earth. By the way, it changes with altitude (atmo density).

[Gloom Demon]

If I am not confusing terms, terminal velocity is the speed at which an object falls. Speed of free fall - I am pretty sure it is 9,8 m/s from my school days.

[Champ]

No.

9.8m/s² is the acceleration due to gravity on earth.

Terminal velocity is the speed at which the resisitance of the air equals gravity. If not thrusting, you will no more accelerate (actually, you will decelerate because, as you go down, air becomes thicker, and therefore air resistance increase). Terminal velocity depends on aerodynamism and air density.

https://en.wikipedia.org/wiki/Terminal_velocity :

"the terminal velocity of a skydiver in a belly-to-earth (i.e., face down) free-fall position is about 195 km/h (122 mph or 54 m/s)."

"Higher speeds can be attained if the skydiver pulls in his or her limbs (see also freeflying). In this case, the terminal velocity increases to about 320 km/h (200 mph or 90 m/s) which is almost the terminal velocity of the peregrine falcon diving down on its prey."

[Seajay]

It used to be a pretty constant 100 m/s ish in the old drag model. Post 1.0 it depends on the aerodynamics of your craft. It's easy to find out empirically. Just launch straight up to 5km or so, turn off the engine and see what speed you hit the ground at.

[Snark]

Short answer: Don't worry about terminal velocity, it's not super relevant anymore since 1.0. Instead, what matters more is Mach 1 (a bit over 300 m/s at Kerbin sea level): a good rule of thumb is to stay under it until you're over 10 km altitude, then floor it. A useful ascent profile is something like this: Accelerate as hard as you can up to around 300 m/s. Then back off the throttle and gradually accelerate up to 350 m/s or so. When you get to 10 km, punch it again.

The longer explanation is below.

Takeaway points:

Post-1.0 is different from pre-1.0.

In other words, any advice you hear that comes from before May 2015 is pretty much worthless now.

Terminal velocity doesn't matter (much) anymore.

Before 1.0, the game used a simplistic aero model that resulted in a couple of things. One was that all ships had the same terminal velocity regardless of size, mass, or design. The other is that the math worked out simply (and coincidentally) so that "ascend at terminal velocity" was the most efficient answer. However, it doesn't work that way anymore-- the math changed, and there's nothing special about terminal velocity for efficiency. In fact, you'll generally do better for most rocket designs to climb at faster than terminal velocity for the first few kilometers.

Mach 1 really matters now.

In the old pre-1.0 model, there wasn't anything super-special about Mach 1; aerodynamic drag just climbed smoothly with the square of velocity. In the new model since 1.0, however, Mach 1 really matters a lot. When you go past Mach 1, aero drag spikes upward dramatically, so you really don't want to go faster than that when the air is thick. Mach 1 on Kerbin is a bit over 300 m/s at sea level, and climbs gradually to higher speeds at higher altitudes. Try to climb up to around 300 m/s as fast as you can, let it gradually increase to 350 m/s by 10 km, then go as fast as you can (since at that point the air has thinned out and drag isn't as important).

Shape matters now.

It used to be that everything had the same drag (and terminal velocity) regardless of shape. With the new post-1.0 aero model, shape matters. You want to minimize drag, which means you need to design your craft to be tall and narrow, with a pointy front end.

Terminal velocity and gravity are different things.

Champ gave a good explanation of this above. The "9.8 m/s2" number is the acceleration of gravity, and is identical for Kerbin and Earth (at sea level, anyway). Terminal velocity depends on mass and shape, so will be different for different craft.

Kerbin is the planet. Kerbals are the little green guys.

'Nuff said.

[OhioBob]

Mach 1 really matters now.

In the old pre-1.0 model, there wasn't anything super-special about Mach 1; aerodynamic drag just climbed smoothly with the square of velocity. In the new model since 1.0, however, Mach 1 really matters a lot. When you go past Mach 1, aero drag spikes upward dramatically, so you really don't want to go faster than that when the air is thick. Mach 1 on Kerbin is a bit over 300 m/s at sea level, and climbs gradually to higher speeds at higher altitudes. Try to climb up to around 300 m/s as fast as you can, let it gradually increase to 350 m/s by 10 km, then go as fast as you can (since at that point the air has thinned out and drag isn't as important).

I have a different philosophy on that. On a typical ascent, the drag coefficient is at its maximum while the rocket passes through the velocity range of about 300-400 m/s. I like to keep the accelerator pegged and just blow through Mach 1 and keep accelerating. The less time I hang around peak Cd, the better. And, besides, I don't think the drag losses are enough to worry about; for a well designed rocket they shouldn't be over 150 m/s total. The rocket should be given an aerodynamic shape but, other than that, I don't see a reason to fret too much over drag losses.

[Kelderek]

If you watch the numbers in KER you'll discover that while ascending upwards from the ground it would be very hard to reach terminal velocity. Even if you could get close to that speed you would likely explode from overheating. Also, the aerodynamic forces become much more pronounced at higher speeds so if you go too fast too low in the atmosphere you will have a tough time controlling your vessel. So yeah, don't pay much attention to terminal velocity any more.

[Gloom Demon]

Thanks for the help guys - this gives a whole new perspective!

Oh - and yes - I was wrong about 9,8 m/s and termianl velocity - thanks for clearing that out Champ!

[CaribouGone]

Now, the real question is: what is the terminal velocity of a kerbal in free fall?