Jet-engine & Intake velocity ranges

[Kegereneku]

This idea came to me as a simple "what if" when discussing Aerodynamic Revamp.

My suggestion : Minimal and Maximal velocity ranges for both jet engine and intake

It follow a logic of Gameplay first, Realism second.

In term of gameplay it is meant to balance air-breathing SSTO around making max speed depend directly of parts types more than their quantity, offering the same feature (game dynamic) without penalizing design variety.

In term of realism it is backed up by the need of distinct engines/intake design to operate at subsonic, supersonic or hypersonic regimes at which the aerodynamic act differently (ask if you want more explanations)

> The typical jet engine we know, Airliner, are meant to work best purely below supersonic speed.

Their intake take subsonic airflow

Then a complex turbine compress the air

(why ? Air=mass, if you can expel more mass at once you get more thrust)

An lastly fuel is used for combustion

All this happen at subsonic speed and actually most of the thrust come from Larger-fan that use energy from the engine to move air that isn't compressed or burned.

> Supersonic plane (Air-fighter, Concorde or Tupolev-144) must cross the "sound barrier" which is a big deal

Their intake (variable sometime) must first break the shockwave that happen at supersonic speed

So Air compression combustion can still happen as before.

Now since you are supersonic you can't really use Larger-fan (we tried), so it use a lot of fuel.

> "Super/Hypersonic" ramjet engine, are rarer, we can't really call SR-71 like it but its engine make a good example.

A Ramjet intake this time don't slow down the airflow, they use its particular dynamic (which mean this intake can't compress at subsonic speed)

To compress air at supersonic speed

(in our example the SR-71 switch between the two way of compressing air)

However combustion itself still happen just below supersonic speed

(because we can't do otherwise)

This process is only possible because air dynamic change at those speed.

> "Hypersonic" SCramjet

Now we are talking

Supersonic all the way. Air compression & combustion happen beyond supersonic speed

What all this mean is that common jet-liner engine/air-fighter engine would simply choke beyond the speed at which they meant to operate.

While purely hypersonic engine on the opposite are ill-suited at low speed, they just cannot work AT ALL.

Expected result :

Proposition 1

Circular intake + basic jet = efficient but subsonic speed only

Ram intake + basic jet = subsonic to early supersonic

Ram intake + turbojet = subsonic to efficient supersonic

Shock cone + turbojet = inefficient subsonic to near orbital speed

Ram intake + rapier = inefficient subsonic to efficient hypersonic

Shock cone + rapier = early supersonic to orbital speed

[Circular intake] = [Radial intake] = choke at supersonic

[Ram intake] = [structural intake] = operate at all speed

[shock cone] = choke below supersonic

In short you have an equivalent choice between Turbojet+rocket or RAPIER to reach orbit

Using turbo-jet engine for tail-sitter SSTO rocket would not suffice unless you use more set of engine.

Proposition 2

Circular intake + basic jet = efficient but subsonic speed only

Ram air intake + basic jet = subsonic to early supersonic

Ram air intake + turbojet = subsonic to efficient supersonic

Shock cone + turbojet = inefficient subsonic to inefficient hypersonic

Ram air intake + Rapier = supersonic to efficient hypersonic

Shock cone + Rapier = supersonic to orbital speed

[Circular intake] = [Radial intake] = choke at supersonic

[Ram air intake] = [structural intake] = operate at most speed

[shock cone] = choke below supersonic

In short it would be slightly harder to reach orbit using Turbojet+rocket, but once you use Turbo+RAPIER it become very easy.

Using turbo-jet engine for tail-sitter SSTO is less likely to be efficient unless you use more set of engine.

How ?

Notes :

Number given purely for demonstration, I didn't bother recalculating Kerbin local Mach number.

- Considering the turbojet as a dandy turbo-jet+scramjet

- I'm using the RAPIER as a makeshift Scramjet engines in case SQUAD isn't planning more engine type.

Jet engine :

• Basic jet (0 to 700m/s) / (0 to 500m/s)
  
• Turbojet (0 to 2000m/s) / (0 to 1500m/s)
  
• RAPIER (0 to 2200m/s) / (1300 to 2700m/s)
  

Intake :

• [Radial intake], [Circular intake] = (0 to 300m/s)
  
• [Ram air intake], [structural intake*] = (0 to 2000m/s)
  
• [shock cone] = 1000 to 2700m/s)
  

*the choice to consider [structural intake] as all-range is to allow a plane design with a [shock Cone] without requiring a [Ram air intake],

Postscript about altitude and pressure :

Normally altitude and temperature change engine efficiency, in our case, simplifying with drag alone shouldn't change the balance. Unless the new aerodynamic model is really, really very different.

Also I don't know for other but I consider that 1 annular intake should be able to fully feed 1 similarly sized jet-engine at their optimal altitude.

Edit :

Another thing for sure, we NEED a definite altitude limit for each intake to avoid the need to spam intake.

You would only need more intake to reach said-above optimal altitude. But a well designed spaceplane would switch to its high-altitude intake before that.

Edit :

I took aside the [intake fuselage], [engine nacelle] and [radial engine body] as mostly "buff" because they were made like that.

Frankly myself I would prefer to remake them entirely.

Hypothetically speaking we could use the concept for :

- A fuel-to-thrust buff, call it post-combustion or something and it would give -if activated- a massive boost of thrust to a linked engine (ideal for tail-sitter).

- a mk1 internal compressor, which would give a linked [shock cone] the ability to compress air at subsonic speed, but deactivate near hypersonic. Giving the ability to fly from ground to orbit with only a [shock cone]+[Rapier] at the cost of some mass (its mostly design flexibility but with savant tuning I bet we can make it privileged for a task), Realistically the RAPIER would have one.

- let's go exotic and say cryogenic cooling for the last one, if you don't know what I am referring to, google "Sabre Engine", such system is also what would make the RAPIER work. In our case, it could use electricity to also give a buff, but without using fuel this time.

Edit : About design

About the possibility that it might be confusing for people.

The term : Subsonic / Supersonic / Hypersonic seem straightforward enough to put in the part name to explain the difference.

Giving the speed range in meter per second seem also ok as it can be seen on the Navball

Edited January 16, 2015 by Kegereneku
addition, correcting a bumber, about intake spam, can't believe I let this error, adding explanation on real jet-engine

[Captain Sierra]

I like the engine curves you created here. That said, I disagree some with the shock intake having such a high minimum velocity for efficiency. Its penalty for being so good is the mass. I'd make the argument that intakes should have progressively less drop off as they go up in speed and tier.

Additionally, what are your recommendations for the intake fuselage and the engine nacelle?

[Kegereneku]

About the [shock Cone] : Nothing say we have to keep its mass as a penalty when we can use a game mechanic and twist other parameter.

In any case, we can rework the number. What's important is that it ask for a fair speed beyond the reach of Basic-jet and not too easy to reach with turbojet.

I didn't suggested a "progressive" solution where intake work at all altitude with various efficiency for two reasons :

# First in term of gameplay it would simplify greatly design and piloting without handing-out miracle-engine at all, it give a clear demarcation for the speed of sound, and lastly because the limited number of parts available do not actually allow the fine-tuning of intake-area vs drag to make a design both sleek & efficient.

Knowing what you can count on would also simplify more exotic design such as rocket-ramjet-plane (or tail-sitter turbo-rocket-plane) and lastly, give greater meaning to "technological progression" by making physically impossible to air-breath your way to orbit without the right technology.

# Second it happen that real intakes must in fact be distincly shaped in order to work at 3 main regimes.

- At subsonic speed you compress air and burn fuel at subsonic speed, no big deal.

- Around supersonic speed, to compress air normally you must first slow it down to do so, hence different intake.

- Faster you can compress speed at supersonic speed, but with a special intake that can't compress below. Then you can slow it down to burn it at subsonic speed.

- Finally at hypersonic speed you need this special intake and a special engine to compress & burn fuel at supersonic speed.

This is what got me to abstract a game-mechanic around 3 regimes. In reality there's more than that, but if we went too realistic we would require variable [shock cone] and also some exotic cryogenic-MHD engine (joke : the working of both are mutually exclusive) to even get close to orbital velocity using only air-breather.

For your question :

I took aside the [intake fuselage], [engine nacelle] and [radial engine body] as mostly "buff" because they were made like that.

Frankly myself I would prefer to remake them entirely.

Hypothetically speaking we could use the concept for :

- A fuel-to-thrust buff, call it post-combustion or something and it would give -if activated- a massive boost of thrust to a linked engine (ideal for tail-sitter).

- a mk1 internal compressor, which would give a linked [shock cone] the ability to compress air at subsonic speed, but deactivate near hypersonic. Giving the ability to fly from ground to orbit with only a [shock cone]+[Rapier] at the cost of some mass (its mostly design flexibility but with savant tuning I bet we can make it privileged for a task), Realistically the RAPIER would have one.

- let's go exotic and say cryogenic cooling for the last one, if you don't know what I am referring to, google "Sabre Engine", such system is also what would make the RAPIER work. In our case, it could use electricity to also give a buff, but without using fuel this time.

[KrazyKrl]

I think that a simplification of each intake having a maximum operating altitude(defined by atmospheric pressure), and a maximum speed(Air speed * Air Density) would be plenty. Air breathing engines in KSP need work anyhow; they currently work nothing like actual jet engines.

Edited December 27, 2014 by KrazyKrl

[Kegereneku]

Well, I didn't focused on atmospheric pressure because it is already covered by the game (and keep my suggestion simple), and using pressure alone is also how we ended up with air-intake spamming.

In any case it goes without saying in my suggestion that pressure would also balance the spec of engines.

Normally Terminal Velocity shouldn't create deadlock where a turbojet can't accelerate or climb (no intake spam possible here) at the altitude required to start a RAPIER. I took large range for that.

- - - Updated - - -

Since the subject is getting very popular now let's bump !

(I have no shame)

Edited January 14, 2015 by vexx32