Long distance airplane flight tips?

[a2soup]

I am currently designing a mission that will involve several very long distance plane flights. My plane has turbojets and lots of fuel, but I am wondering what the optimal way to fly is. What altitude and speed should I go? And how many km can I expect to get per ∆v (as calculated by Engineer redux Kerbin atmospheric stats, I know this is weird with planes)?

Right now, my big, fat plane has trouble breaking 10 km altitude and 200 m/s forward speed (largely due to its payload). I did some testing and found that the turbojets get the best Isp around 5.2 km altitude, so I was thinking of flying at that altitude. I have enough patience to do the slow flying, but is it horribly inefficient?

Thanks for any insights you might have!!

Edited November 2, 2014 by a2soup

[Starman4308]

I am currently designing a mission that will involve several very long distance plane flights. My plane has turbojets and lots of fuel, but I am wondering what the optimal way to fly is. What altitude and speed should I go? And how many km can I expect to get per ∆v (as calculated by Engineer redux Kerbin atmospheric stats, I know this is weird with planes)?

As high and fast as you can go without engine flameout: preferably ~2000 m/s at 25-30 km (and yes, that's almost orbital velocity). Once you're at that regime, you should be able to cover huge distances on tiny amounts of fuel. To get there, you're going to want to climb at the fastest rate you can manage to 18-20 km, at which point you should mostly level off and start accelerating horizontally. Be sure to have enough intakes (I think maximum efficiency in stock is 1 ramjet intake per ~2 tons of plane, though that is aesthetically ridiculous), and if you're still running short of intake air, you can dip down a bit to speed up and get to denser air.

Right now, my big, fat plane has trouble breaking 10 km altitude and 200 m/s forward speed (largely due to its payload). I did some testing and found that the turbojets get the best Isp around 5.2 km altitude, so I was thinking of flying at that altitude. I have enough patience to do the slow flying, but is it horribly inefficient?

Don't worry about Isp. Really. Turbojets are so ridiculously efficient that it doesn't really matter. What you should be concerned about is atmospheric drag, and that's minimized by high-altitude flight.

EDIT: Essentially, make like an SR-71, if an SR-71 didn't have cooling problems limiting it to Mach 3.4.

[a2soup]

Thanks, I'll go try to stuff a few extra turbojets and intakes on my plane Do you know how many intakes/turbojet are needed to run in the regime you described?

[Starman4308]

Thanks, I'll go try to stuff a few extra turbojets and intakes on my plane Do you know how many intakes/turbojet are needed to run in the regime you described?

Been a while since I've flown stock*, but I've heard that, for maximum efficiency, you want one turbojet for every 7.5 tons, and about four ram intakes for each turbojet.

*I use FAR these days, which basically means completely replacing every spaceplane and rocket you ever designed to account for its much more realistic aerodynamics.

[Alshain]

Are you using Stock or NEAR/FAR? If your using NEAR/FAR, once you get high and fast, kill the engine and glide to save fuel. In this case lots of fuel is the wrong approach, I can get more than half way around Kerbin on 2 cans of Jet Fuel and a turbojet if I glide most of it.

[a2soup]

Thanks for the help! And it's stock, unfortunately. I should perhaps start trying out NEAR to get ready of 0.90, though.

[Alshain]

It may work with stock too, I just don't know. I'd almost expect it to work better in stock since infini-gliders can be done.

[LethalDose]

Thanks, I'll go try to stuff a few extra turbojets and intakes on my plane Do you know how many intakes/turbojet are needed to run in the regime you described?

It'll probably be more efficient with more intakes, but I wouldn't increase the number of engines. Also, while you mentioned the TJs most efficient altitude is ~ 5km (I'm not sure, but lets assume it's true), you need to realize that the drag you're getting at that altitude is going to wreck any efficiency gains. High altitude flight is efficient because the air is thinner, and therefore aircraft experience less parasitic drag.

[Laie]

Right now, my big, fat plane has trouble breaking 10 km altitude and 200 m/s forward speed (largely due to its payload). I did some testing and found that the turbojets get the best Isp around 5.2 km altitude, so I was thinking of flying at that altitude. I have enough patience to do the slow flying, but is it horribly inefficient?

As has been said, ISP is nothing to worry about. But maybe your patience? Your airspeed is mostly limited by terminal velocity; you can go much faster if you climb a little higher. I don't have the numbers handy, but you can expect something like 400m/s at 15km.

Incidentally, for these speeds and altitudes, ordinary jets are the better choice. The thrust of jets (turbo or otherwise) depends on airspeed: basics deliver more thrust at low speeds, while turbojets only really come into their own when you fly very fast (which also means flying very high, 20km and over). At about 400-500m/s (~15km), the two engines are even. So if you replace the turbos with basic jets, you will have more thrust from the ground up, and your vessel will weigh slightly less. The benefits should be obvious.

Also, jets eat fuel at pounds-per-minute -- if you cover twice as much ground in the same time, your plane will be more efficient. In the stock atmosphere, the benefits of moving faster outweigh almost everything else.

[Starman4308]

As has been said, ISP is nothing to worry about. But maybe your patience? Your airspeed is mostly limited by terminal velocity; you can go much faster if you climb a little higher. I don't have the numbers handy, but you can expect something like 400m/s at 15km.

Incidentally, for these speeds and altitudes, ordinary jets are the better choice. The thrust of jets (turbo or otherwise) depends on airspeed: basics deliver more thrust at low speeds, while turbojets only really come into their own when you fly very fast (which also means flying very high, 20km and over). At about 400-500m/s (~15km), the two engines are even. So if you replace the turbos with basic jets, you will have more thrust from the ground up, and your vessel will weigh slightly less. The benefits should be obvious.

Also, jets eat fuel at pounds-per-minute -- if you cover twice as much ground in the same time, your plane will be more efficient. In the stock atmosphere, the benefits of moving faster outweigh almost everything else.

I was generally under the impression that basic jets were mostly deadweight on high-performance aircraft: while they are superior for low-speed, low-altitude operations, they become deadweight at hypersonic velocities. Since turbos are almost always enough to get you off the ground and up to altitude, I usually just do straight turbojets (with some RAPIERs for SSTO spaceplanes); the minute amount of fuel saved during the ascent isn't worth lugging those superfluous engines around at the hypersonic regime.

[Laie]

I was generally under the impression that basic jets were mostly deadweight on high-performance aircraft: [...] at hypersonic velocities

That's all true, but just read the OP: hypersonic appears to be completely out of the question.

[Starman4308]

That's all true, but just read the OP: hypersonic appears to be completely out of the question.

As I understand, the OP wanted "the most efficient way to fly", and simply needed a bit of help in realizing that that is a hypersonic, high-altitude, high-performance aircraft which is a RAPIER and a little oxidizer away from being an SSTO spaceplane.

[numerobis]

If drag were the only concern, at any given altitude the slower you go, the less fuel you need: the work done by drag over a distance grows linearly with speed (the force grows quadratically with speed, but the time falls linearly). Double your speed, you need twice as much fuel.

By climbing, you reduce drag, exponentially with altitude. You can halve the drag by climbing a bit less than 3500m. At 3500m, you have half the drag you'd have at the surface; at 7km, 1/4 the drag, at 10.5km, 1/8 the drag, etc.

That would argue for going as high as possible, as slow as possible, but apart from drag you also have to fight gravity. There's basically three ways to do that: lifting surfaces, thrust, and orbiting. (Actually, four ways: but balloons aren't in the stock game) Lifting surfaces are great but they lose effectiveness just as fast with altitude as drag falls. Thrust means that slower-is-better no longer applies: now, slower means less drag but more gravity, so there's a tradeoff (which I'm not going to carefully calculate for you, because it's hard).

Orbiting is the most fun one: the higher the orbit, the less speed you need to maintain to stay in orbit, *and* the less air there is, so that's a double-whammy in terms of reducing fuel consumption. You can spin around the whole world on just fumes. Even better, burn a little more, and you can pop out of the atmosphere for a bit and avoid all that nasty drag stuff.

The big problem is getting back down.