how to guesstimate the min. speed & altitude i've to acquire when i switch to rocket

[lammatt]

let's say... i have 2km/s of vacuum rocket dv and i can afford a 2.0 TWR; and i want to achieve a 80kmX80km orbit.

how can i guesstimate the min. speed & altitude i've to acquire when i switch to rocket

and to make thing simpler, let's say the jets and rocket cant operate at the same time, they work in an either-or fashion.

Edited October 13, 2013 by lammatt

[Aphox]

Are you talking about switching from jet engines?

If that's the case, just squeeze as much dV and altitude out of them as possible before you switch. Test the limits of the craft, see how high it can get before it runs out of air, and run it on air for as long as you can at that altitude until you stop gaining speed as you throttle down, then switch to rockets when you start to slow.

Assuming your craft has the control authority to keep up that long.

I've stressed jets out to the point where I've had low-circular obits before going to LFO power, something like 20x20-35x35km orbits, and then I shoot my AP into the 80's.

It really varies per craft, how much control authority you have, intake power, thrust, weight, etc. Just have to toy with it for a while. Having Mechjeb has stats for current and needed intake air for the throttle level you're at. Don't know if Kerbal Engineer has those stats yet, but it didn't the last time I used it.

[lammatt]

Are you talking about switching from jet engines?

If that's the case, just squeeze as much dV and altitude out of them as possible before you switch. Test the limits of the craft, see how high it can get before it runs out of air, and run it on air for as long as you can at that altitude until you stop gaining speed as you throttle down, then switch to rockets when you start to slow.

Assuming your craft has the control authority to keep up that long.

I've stressed jets out to the point where I've had low-circular obits before going to LFO power, something like 20x20-35x35km orbits, and then I shoot my AP into the 80's.

It really varies per craft, how much control authority you have, intake power, thrust, weight, etc. Just have to toy with it for a while. Having Mechjeb has stats for current and needed intake air for the throttle level you're at. Don't know if Kerbal Engineer has those stats yet, but it didn't the last time I used it.

appreciate your input...

but my question was how can i guesstimate the threshold

it's easy to do trial-and-error, but to make an educated guess is harder but can save a lot of tedious trials.

and i am actually looking for the how-to-guess instead of the trials.

i think i can handle some very basic symbols and math calculations (my math training ended when i was in year2 in university, and my physics training ended in year3, but i know absolutely nothing about fluid dynamics more difficult than GCE-ALevel or AP level and i was a chemistry graduate and i teach science in highschool)

i would appreciate the knowledge on the thought process how to guess the switching threshold (if it happens to be understandable within my scope)

Edited October 13, 2013 by lammatt

[Aphox]

Without some factual metrics on how the game's atmosphere actually works with intake air, I'm afraid I don't have any maths off the top of my head that can help here.

Intake air decreases subtly through the thicker bits of the atmosphere, up to about 10-15km, and then starts to decrease rather drastically. Keep in mind that engines become more and more fuel efficient the higher you get up to a certain altitude, and start to produce more thrust, and require more and more intake air at a pretty quick rate.

From my experience, using 2 ram air intakes can get a dual-turbojet up to around 15-20km before flameout. I don't want to suggest intake stacking unless you're into it, but I try to stay away from it and keep my designs "realistic" when I can. But it goes without saying that more intakes = higher altitude = higher isp = higher thrust = higher velocity, etc.

Something I'd really appreciate seeing is a detailed curve on how intake air is really handled and when it starts to fall off for each of the game's intakes. To my knowledge, ram intakes are the best of them all and produce the most intake air, while the spoiler scoops and nacelles are at the bottom of the chain.

In order to guesstimate how far you can really get on a set amount of intakes, I'd do some testing now that might save hassle in the future. Use one engine, and see how far up one intake gets you, then two, three, etc, and try to guess the math behind using two engines, three engines, and see if you're right. While doing the testing I'd make climb rate pretty low, because the flameouts can happen fast, and if you watch your intake air, you can throttle down lower and lower and keep gaining speed the higher you go until your TWR starts to bite you. Until someone can dissect the engine and get raw numbers I think that's the best way of finding out what the limits of the intakes are.

P.S. someone please correct me if I'm talking out of my butt. These are all just things I've noticed from playing.

[WafflesToo]

Lines up with my experience Aphox. Nice to see I'm not the only one bothered by that little exploit.

[Specialist290]

Without some factual metrics on how the game's atmosphere actually works with intake air, I'm afraid I don't have any maths off the top of my head that can help here.

Intake air decreases subtly through the thicker bits of the atmosphere, up to about 10-15km, and then starts to decrease rather drastically. Keep in mind that engines become more and more fuel efficient the higher you get up to a certain altitude, and start to produce more thrust, and require more and more intake air at a pretty quick rate.

...

Something I'd really appreciate seeing is a detailed curve on how intake air is really handled and when it starts to fall off for each of the game's intakes. To my knowledge, ram intakes are the best of them all and produce the most intake air, while the spoiler scoops and nacelles are at the bottom of the chain.

While I don't have much experience with spaceplanes myself, from the way you've described how the intakes appear to work, I'd venture to guess that air intake performance is somehow connected with atmospheric pressure at a given altitude, which is something I've looked into before. In particular, the 10-15km range you gave happens to neatly line up with the altitudes at which air pressure is between 0.1-0.01 that of sea level pressure, which is indeed quite a drastic change. The chart at that link might serve as a helpful reference for more extensive testing.