Estimated burn time not displaying

[Shotza]

Hi

I'm just starting out with KSP, watched a lot of Scott Manley's stuff and am currently trying to consistently get into a good orbit.

The problem I'm having is after I set my manoeuvre node the estimated burn time is not giving me a figure until I get to the node and fire up my engines. Although the approach and theoretical orbit looks good and tight, only after I fire my engine it tells me I have a couple of minutes burn time, there's no figure before I fire so it's impossible to do a half and half burn, the time is too long anyway, I'd expect well under a minute.

I realise I must be missing something, can anyone tell me where I'm going wrong.

Thanks in advance for any input.

Edited June 9, 2015 by Shotza

[Invader Jim]

Do a really quick engine tap on and then off and that usually will make it display time. I bet you could even use RSC thrusters to do it if you don't want to use the main.

[Shotza]

Thanks Jim, is this a bug or by design? For some reason I thought it would know the thrust values of the engines and give you a time without having to do this.

[haltux]

Estimated burn time is not computed from rocket features, it is computed from actual acceleration and target delta-V, once this acceleration has started. Which mean that it is estimated once you have started to burn. This is expected behavior.

The usual way to deal with that is, for long burn, to do a quick burn before the manoeuvre node, let's say 2 minutes before, and then you get the information required to precisely start your actual burn when you should (half the time of the burn before the node).

If your burn is really too long, you can (if you want) do it in two parts. A first burn with half the required delta-V, which make you stay in elliptic orbit with a high Ap, then you do a complete orbit and burn again when you are back at your initial position to actually leave your planet.

Edited June 9, 2015 by haltux

[Snark]

Yeah, the problem is that the game is more simpleminded about your acceleration ability than you might think:

- It doesn't know what sort of acceleration your ship can do until you actually accelerate. (For example, it doesn't look at "okay, here are your currently active engines, here's their rated thrust, here's your current mass, I'll just do some trivial math." Why it doesn't do that, I have no idea. Seems like it would be really easy to code.)

- Even after you activate the engines, it only remembers the info for the duration of the current "flight session". If you switch to another ship and then come back, it forgets. (Again, no idea why... even if there's some good reason it couldn't calculate, surely asking it to remember one number wouldn't be asking too much.)

- And it only remembers the highest acceleration you've actually done, not what you could do. e.g. if you switch to a ship and thrust at 25% throttle, it'll only remember that; the estimated-burn-time isn't smart enough to realize "you could accelerate 4 times faster than that if you floored it."

- And it doesn't allow for things that affect your acceleration, e.g. you do some thrust, it remembers your acceleration, then you decouple some dead weight. It doesn't realize you can accelerate faster now.

...So all of that makes for an experience like you've observed. A brief tap of the engines at full throttle will do the trick (at least, until you switch away from the current vessel and it "forgets" again.)

Edited July 2, 2015 by Snark

[Shotza]

Thanks for the info. I've just jumped into KSP and see that I've got a lot to learn! I'll keep chipping away and watching the tutorials

[Shotza]

Brilliant, that's given me a much better understanding of where I'm going wrong (or not as the case may be!)

[Pronoes]

- And it doesn't allow for things that affect your acceleration, e.g. you do some thrust, it remembers your acceleration, then you decouple some dead weight. It doesn't realize you can accelerate faster now.

This is an absolute PITA. Try launching an unmanned satellite to get geo sync with heavy fuel load and doing a flight computer pro-grade burn when out of contact. OMG. 45 seconds you say? OK, setting for 45s.... Wait, wait, wait. Computer kicks in, starts burning, the seconds on the "estimated time" on maneuver node go by exponentially faster as the ship loses fuel, while my flight computer is just using "normal/real" seconds. So 22.5 seconds into burning, computer wants to burn for another 22.5s, but estimated burn time is down to around 17seconds already...

[moronwrocket]

Get Kerbal Engineer Redux, and Kerbal Alarm Clock. KER will give you burn times, even if KSP doesn't "know" them. KAC can now set alarms 1 minute before 1/2 of KER's burn time, instead of just the node. Makes life soooo much easier.

[pincushionman]

If it uses your last known acceleration and your current mass, that would explain why it seems to tick by more quickly toward the end of high-mass-fraction burns.