How to get a rough idea of the ideal altitude of a aircraft?

[goldenpeach]

The question is pretty simple: how can I get a rough idea of the ideal altitude of a plane(in term of fuel efficiency).

Like, what the math(which mean I want to know it before I even use the plane(if possible)) to do to know the best altitude if I want to do a long flight?

And if I want to go fast?

Thank you for reading!

[Loligoat]

In terms of speed and efficiency, you want turbojets at a high altitude. The exact height depends on your ratio of intakes:engines, but ~12k to ~25k is generally good. There probably is an exact mathematical formula for it, but I don't bother. Just experiment with different planes and altitudes and see what works best. I'm sorry this isn't exactly what you're looking for, but I hope it helps.

[goldenpeach]

Don't worry, it help.

Anyway, one of this day I should make a Python script that will tell a "good" altitude by estimating the speed of the craft at a particular altitude(using the mass, drag etc..) and dividing it by the fuel consumption(I'll have to find out how to know it).

[mhoram]

Fuel consumption of Engines depends on their Specific Impulse (ISP).

The ISP of Rocket-Engines is linear in the pressure.

ISP of Jet engines is a bit more complex to figure out. If you want to get into the details, I recommend this thread:

http://forum.kerbalspaceprogram.com/threads/67606-Fuel-consumption-as-a-function-of-atmospheric-pressure

Planes that contains different engines running at the same time have a mixed-ISP value. It can be calculated as described here:

http://wiki.kerbalspaceprogram.com/wiki/Specific_impulse#Multiple_engines

[Franklin]

KSP's atmospheric/intake model isn't as advanced as all that. Your engines are going to run just as efficient with your intake reading 10.0 as it is reading 0.1, so your only challenge to efficiency (aside from the plane itself) is how deep in the atmosphere you are.

So do what everyone does, spam intakes so you can fly as high in the atmosphere as possible and ignore everything you know about flight.

Super-efficient plane:

[goldenpeach]

Ok, so the ideal altitude is the highest altitude I can get with decent maneuverability?

[Franklin]

pretty much. with stock model as it is higher is always better. it's something we've fought about for ages.

[mhoram]

Ok, so the ideal altitude is the highest altitude I can get with decent maneuverability?

It is a little bit more complex than that.

Have a look at the fourth picture in this post.

http://forum.kerbalspaceprogram.com/threads/67606-Fuel-consumption-as-a-function-of-atmospheric-pressure?p=939065&viewfull=1#post939065

It might well be that the influence of ISP is much lower than the atmospheric drag, but I never did any calculations about that.

Basically you want low Drag and high ISP for fuel-efficient travel in the atmosphere.

[goldenpeach]

Ok, I'm too lazy to do the chart for each designs I make.

I will just fly high.

[cantab]

Indeed, flying high helps. As does not flying too fast (unless you're trying to go suborbital) since you'll incur greater drag that way. Though with the turbojet's that's offset to a point by increasing Isp with speed.

[Friend Bear]

Doesn't sound like the most ideal answer but it is pretty good.

If you want speed, get your air intake to .5ish. If you want to enter space, get to .1 and gun it at one elevation.

[Progressm]

Nothing new to add as such, but generally my SSTO spaceplane returns to KSC from orbit end up a bit short rather than a bit long... sometimes by a fair margin (think the previous continent) and with very little fuel left.

It's usually not a problem, because flying high is so efficient; at 30km ish altitude I just run the jets a bit and I'm back on track to KSC with just a few units of fuel spent...but if I descended to say 10km instead and tried to make it home, it would take forever and I would run out of juice.

[goldenpeach]

Doesn't sound like the most ideal answer but it is pretty good.

If you want speed, get your air intake to .5ish. If you want to enter space, get to .1 and gun it at one elevation.

Well, the plane must be able to support that: The fastest plane I have(2.2 m/s, 30km high) must have at least 0.14 air intake.

At that point, the engine start to have different thrust but it's small enough to be compensated by the rudder.

oh, and if you want to know, my plane have 12 ram air intake for 3 turbojets; I do a circumnavigation in 40 minutes with a lot of fuel to spare.

[Friend Bear]

Well, the plane must be able to support that: The fastest plane I have(2.2 m/s, 30km high) must have at least 0.14 air intake.

At that point, the engine start to have different thrust but it's small enough to be compensated by the rudder.

oh, and if you want to know, my plane have 12 ram air intake for 3 turbojets; I do a circumnavigation in 40 minutes with a lot of fuel to spare.

I don't know if it's a glitch or something, but Ram intakes always bum out on me at low altitudes. The front and mod intakes work wonders.

[Kerbart]

In reality there's a lot more to that. Given that it takes the extra fuel to gain altitude, you'll see that in real life jets on long flights will start cruising at a lower altitude (say FL350) and only climb to the optimal altitude (say FL420) after burning off a good amount of fuel.

So if you look into optimizing the crap out of your flight path you might want to look into the optimal profile and not just "altitude."

[Progressm]

Hmm given that a decent TWR jet in KSP reaches 10km from standstill on runway under a minute and things only get quicker from there, I would say in most cases "go high and cruise" is a good rule of thumb... sure, there's more to it but for all intents and purposes is better to go above 25 km if you are going any further than the island airfield in Booster Bay.

Edited October 1, 2014 by Progressm

[O-Doc]

If you just want to cruise around to locales on Kerbin I've found you can push a one basic jet engine with 2 radial intakes to around 550m/s at 18km altitude with the throttle setting at one third. That's pretty fuel efficient. Granted, you need a fairly light plane which means not alot of fuel to burn(1t/engine). I would say you can still do a circumnavigation with this setup.

[cantab]

Flying the Aeris 3A stock plane I tended to cruise around 8 km, with the throttle set real low, like 15% I think. Much higher and the Isp starts to roll off badly. I flew from KSC out to the pyramids, KSC West, then back to KSC, so not too bad for range especially as takeoffs are thirsty.

[Mesons]

Like, what the math...?

Brilliant! You've made my day!

Don't the air-breathers' thrust change with altitude as well? This could complicate things and result in a lower most-efficient altitude than "as high as you can manage."

[Yakky]

Lots of good tips posted by many people here. But here's the way I think of it, which keeps things blindingly simple:

In almost all practical cases, your craft will be operating at its most efficient when it's going the fastest. That's because fuel consumption is proportional to time (assuming constant throttle), and going faster makes you cover more distance per unit of time. And how do you go fastest? You go fastest by getting up where the air is very very thin and there's not much air resistance to slow you down. So the most efficient altitude in KSP is generally the highest altitude your craft can sustain, and going as fast as you can there. This is true whether you're playing with stock aerodynamics or FAR/NEAR.

While it's true that a KSP jet engine's Isp changes with altitude, in practice the speed pickup significantly more than compensates for that under practically all circumstances. I'd actually be curious to see an example of an aircraft where this isn't the case.

A spacecraft in orbit above the atmosphere has infinite "fuel efficiency" (if you want to think about it that way)... it will go as many miles as you like with no thrust at all. So the closer you can approximate that ideal, the better your efficiency is going to be.

There might be exceptions to this rule of thumb, but in practicality they are very rare. One obvious exception is when your trip is short and it's not worth climbing to a high altitude. I honestly can't think of other exceptions off the top of my head but would be curious to hear of others I may have forgotten about.

Edited October 1, 2014 by Yakky

[cantab]

Speed only helps if you can get it for "free", which you can by climbing to a higher altitude. But at any given altitude, doubling the throttle is likely to double the fuel consumption but less than double the speed, therefore giving worse fuel efficiency. Though varying engine Isp with speed may affect things.

[Franklin]

Speed only helps if you can get it for "free", which you can by climbing to a higher altitude. But at any given altitude, doubling the throttle is likely to double the fuel consumption but less than double the speed, therefore giving worse fuel efficiency. Though varying engine Isp with speed may affect things.

It's all about terminal velocity, baby. Which further emphasizes the best place to be is always high.

http://wiki.kerbalspaceprogram.com/wiki/Kerbin#Terminal_Velocity_Table

Any fuel spent trying to capture speeds above the terminal velocity of any given height is compoundingly inefficient. Get up high and then begin your horizontal stretch.

Edited October 1, 2014 by Franklin

[Progressm]

Also, even if you did find optimal scenarios where you could save a drop or two of fuel by cruising at a low altitude, it is hard to justify the time expense when you can easily have all the range you need in a third of the time (say 500m/s at 17km vs 1500m/s a 27km).

Interested to hear of exceptions but in general I would opt for fast and efficient rather than slow and maybe just slightly more efficient.