Terminal speed (how to determine)?

[linuxgurugamer]

it's been said a lot that you shouldn't go faster than your terminal speed while taking off. I realize that the terminal speed changes depending on the altitude, but how can you determine that you are approaching it?

I'm using DAR & FAR, as well as about 90 other mods.

I can see the visual effects on the ship, but don't know how to relate that to the terminal speed.

Thanks in advance

Edited February 19, 2015 by linuxgurugamer

[cicatrix]

If you use FAR it's impossible to reach the terminal velocity as far as I can understand. You can use Flight Data from FAR to see the exact current TV

[John FX]

In FAR it`s not so much terminal speed (where you would have to thrust upwards at 2G to feel 1G, a case that would never happen in FAR) it`s the angle of your craft and how much Dv you are using to get to orbit.

I find that if I launch too shallow I spend too much time in the lower air and that uses Dv. If I launch too steep I use too much Dv to circularise and this is not ideal either.

I would say your efforts would be better used finding the launch profile that is best for each of your craft rather than chasing terminal velocity (which will change depending on the AoA of your craft anyway)

[Red Iron Crown]

I'm not an experienced FAR user, but it seems odd to me that there is no terminal velocity in it. Do objects falling in the atmosphere not reach an equilibrium speed at some point?

[Kryxal]

I think it's less that it doesn't exist and more that your rocket doesn't reach it on the way up ... at least, for reasonably aerodynamic designs.

[capi3101]

it's been said a lot that you shouldn't go faster than your terminal speed while taking off. I realize that the terminal speed changes depending on the altitude, but how can you determine that you are approaching it

Do you use KER? If so, look for a figure on one of the displays (I think it's Surface) labelled "Atmospheric Efficiency". It should be a value expressed as a percentage. When your craft is travelling at the current terminal velocity for the altitude it's at, that value will be right at 100% (or close enough to it to spit). Much below 100% and you're losing delta-V to gravity; much above 100% and you're losing delta-V to drag.

If you're not using KER, well, it can be done by hand - the formula is: V = sqrt ((2Mg)/(rho*A*Cd)), where V is terminal velocity, M is the mass of the craft, g is the acceleration due to gravity, rho is the density of the atmosphere, A is the "projected area" of the craft and Cd is the coefficient of drag. Trouble here is that g and rho are both dependent on altitude, and since you're in craft burning fuel, M is also time dependent. You'd be looking at a really nasty differential equation. But it is an option.

[Renegrade]

FAR has a terminal velocity - it's just higher than with stock air. I believe the current iterations have lowered it - I know I've actually been past it on some occasions (if you're doing sub-orbital hops just to peek into space, you want to be at terminal velocity for maximum altitude with minimum rocket, since it's all gravity drag otherwise).

There's a button ("Flt Data") that brings up the Flight Data panel - you can see terminal velocity there.

Here's a test flight specifically to push the terminal velocity. As you can see, I'm past it already (Terminal V in the FAR Flight Data window).

Based on my VOID data, my acceleration should be about 41.46 - instead, I'm getting 28.48, or a difference of about 12.98 (slightly over 1g). I'm losing more to aero drag now than gravity.

In stock, I'd be WAY past terminal velocity in that shot (it's around 200-ish at that altitude) and experiencing enormously more drag.

For an optimal flight-to-orbit with FAR though, you don't want to be going quite so fast - remember the objective is to go east, not up. Pushing the terminal velocity isn't compatible with the curve you want to draw.

[cantab]

Yup, just open the FAR flight data and it's there.

Keep in mind that it depends on your rocket design and its orientation. For example a typical simple rocket will have a much higher terminal velocity when travelling nose first than when travelling engine first.

Generally speaking unless you either have crazy TWR or a very draggy build you won't get near terminal velocity in FAR on ascent anyway.

[linuxgurugamer]

Thank you all for answering. I wasn't aware that FAR provided the info.

Re. my ships hitting it, I didn't know. But some of my ships have enough accel to kill Kerbals.