Planes - ow much engines per kg

[kiwiak]

So, how do i figure out how much engines shoudl my plane have?

[Hodo]

There is no exact number you need for a plane. The simplest answer is... As long as your plane can get off the ground and climb to an altitude you want to fly it at, you have enough engines. One basic jet engine is plenty for most small planes.

[kiwiak]

There is no exact number you need for a plane. The simplest answer is... As long as your plane can get off the ground and climb to an altitude you want to fly it at, you have enough engines. One basic jet engine is plenty for most small planes.

Ok, so how much jets per kg for SSTO?

And how many intakes - i saw some planes with intakes all over a place, how do such intake spam helps?

[Hodo]

Ok, so how much jets per kg for SSTO?

And how many intakes - i saw some planes with intakes all over a place, how do such intake spam helps?

I usually build my SSTOs with a TWR of at least 1.5:1 for the jet engine stage. And at least a .7:1 TWR when on Rocket power.

The most powerful of my SSTOs has a TWR of 2.3:1 under jet power and a 1.3:1 rocket power.

[Jouni]

I prefer my planes to be able to climb and accelerate vertically. As the thrust of a turbojet is around 110 kN at sea level, that usually means having one turbojet per 7-8 tonnes of launch mass. With basic jets, you can go with something like one jet engine per 10 tonnes, but the rocket stage has to be huge.

Having one ram intake per turbojet allows you to reach orbit with less than 500 m/s of delta-v from the rocket stage. Increasing the number of intakes makes it possible to reach orbit with even less delta-v from the rocket stage. The ascent is also easier with more intakes, as you can climb faster at high altitudes without risking engine flameout. Adding more turbojets also makes the ascent easier, because the intakes work better at high speeds.

[Hodo]

It is not effiecent to climb that way with SSTO space plane. It is best to climb at a 45deg angle till you get to arround 12-18km alt then bring it down to a 10-15deg climb and gain as much speed as you can under the jet engines before you have to switch over to the rocket engines at around 25-30km alt. (unless you airhog, then in that case you can fly till around 60km)

[capi3101]

Rule of thumb I go with is one turbojet engine per 12-15 tonnes of total craft, or one engine per 9-12 tonnes of payload mass (assuming that an engine, a pair of wings, the fuel for the engine and its intakes come in somewhere in the neighborhood of 2.5-3 tonnes or so). Nothing wrong with having more thrust, of course.

In full, the general guidelines I give are:

1 Turbojet per 9-12 tonnes payload, leaning towards 12 if you like to go airhogging and 9 if you don't.

200-250 units of liquid fuel per turbojet. You can get away with less if you're a halfway decent pilot.

One pair of swept wings or delta wings per six tonnes of aircraft.

No fewer than three ram intakes per turbojet engine. Higher intake to engine ratios generally afford you more tonnage per engine.

[Jouni]

It is not effiecent to climb that way with SSTO space plane. It is best to climb at a 45deg angle till you get to arround 12-18km alt then bring it down to a 10-15deg climb and gain as much speed as you can under the jet engines before you have to switch over to the rocket engines at around 25-30km alt. (unless you airhog, then in that case you can fly till around 60km)

Turbojets behave in a different way, when you use more than the absolute minimum number of them. With one intake per engine, most of my SSTOs using turbojets get to 33 km at full thrust. Once I tried manually reducing the thrust after 32 km, and that got my light spaceplane to 43.6 km before flameout.

In general, the efficiency of SSTOs depends on what you want to achieve with them. If you are carrying crew or cargo to LKO, you want a powerful jet stage for a quick ascent. On the other hand, if you are going beyond LKO, jet engines are just dead weight, so you don't want to use any more of them than necessary. I personally prefer to use specialized lifters and landers at high-gravity planets, and transfer stages to move them around, but that's up to individual taste.

[LethalDose]

Ok, so how much jets per kg for SSTO?

And how many intakes - i saw some planes with intakes all over a place, how do such intake spam helps?

Spamming the air intakes helps by increasing intake air at high altitudes. Jet engines require fuel and atmospheric oxygen (aka the "intake air" resource). The intake air produced by the intakes is proportionate to air speed and atmospheric pressure. As your altitude increases, pressure decreases, and you need more intakes and more speed to supply the air to your engines.

Therefore, spamming the air intakes keeps your efficient jets burning longer at altitude, allowing the spaceplane to go faster before needed to switch to the less efficient rocket engines.

Designing SSTO's is much more of an art than a science because the way each pilot flies is different which leads to huge changes in design requirements and preferences. It requires a lot of trial and error. There is no "best way" or magic asparagus staging for spaceplane design.

Just make sure your CoM is in front of your CoL when your tanks are full and empty. That is the one unbreakable rule of SSTO/supersonic aircraft design.

[kiwiak]

OK, let me ask related question, how do i check how much wings do i have to add?

Is there some formula to know how much lift surface per kg i have to put on plane?

If i dont have enough lift surface i wont be able to gently glide over surface and land without smashing wheels, wont I?

[Claw]

OK, let me ask related question, how do i check how much wings do i have to add?

Is there some formula to know how much lift surface per kg i have to put on plane?

If i dont have enough lift surface i wont be able to gently glide over surface and land without smashing wheels, wont I?

Capi101's post above has some generic guidelines for your question.

I use similar design criteria as Capi. Also, I think I fly a "typical" SSTO profile, but I know there are a few things I do differently. So like others have said, you may need to modify these slightly to suit your taste/flying style/ability.

Engines: I aim for 1 TurboJet per 12t of craft, I don't usually go much above this, but in no case above 15t

Wings: 0.5 to 0.8 lift rating per ton, up to a maximum of 1.0 of lift rating per ton

Intakes: 1 to 3 intakes per TurboJet

Liquid Fuel: 100 to 200 units per TurboJet

The simplest way I've figured to use these basic design criteria is to match them all up from left to right, starting from the top and working down the list. (Essentially, the higher the TWR, the less other stuff you need.)

-For example, start off guessing that you want a fairly light, single engine craft aiming for around 8 tons. This is on the low side, so you can get away with around 0.5 lift rating/ton, 1 intake, and somewhere around 100 units of fuel.

-If you build a heavier, 12t version, you'll want around 0.8 to 1.0 lift rating per ton, 3 intakes, and 200 units of fuel.

-A 16t space plane would need 2 engines, but has a TWR similar to the first example so it can use similar design criteria.

Truthfully now that I'm more experienced, I get away with less lift rating and less fuel. But I found it's a good place to start. For stock airplanes, TWR seems to be the easiest measure to gauge the rest of your design around.

A good practical example of this design criteria is already in your KSP game. Go look at the Aeris 4A. It's a 17t plane, uses 2 TurboJets, 8.6 points of lift rating (0.5 per ton), 2 ram intakes (with a couple radials), and 150 units of liquid fuel. It turns out the liquid fuel is enough to get into orbit, but runs short if you plan on a powered return. (So it could use a little more.)

Anyway, that's just what I use. I could post a giant wall (or more giant-er) explaining how I came to all that if you want to know, but it seems to work well for me.

[Hodo]

The amount of wings you need is also a a matter of taste and style. I use FAR so I often have to use a realistic amount of lifting surface on the aircraft. But I have made a few that have no wings and they have flown fine, granted at a MUCH higher speed than most of my conventional designs. But Claws formula of .8 lift per ton is a pretty good number to start with. If you find that you can do with less by all means.

After all this is one of my SSTO aircraft, and it has no wings and is not a tail lander.

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