Mach 1 with J-33 engine.

[Stigy]

So i started to play with planes. I got two problems using J-33.

1. I can't reach anything over 330 m/s (Surface - i don't know witch speed is Surface on navball or airspeed shown by Air intake: 420s/ms). For max thrust i have to reach mach 1.7. Now i can't get even 80 kN when it should give me minimum 80 at 0 m/s

2. I can't climb higher than 7000 meters. Even if i do climb to 7k i fall immediately.

I got 2.1 air intake (i don't know how it's calculated because the flow in one intake is like 37.0 Units - i can't check how much air is received by the engine)

Any advices ?

P.S. here are images of my plane

Edited October 12, 2015 by Stigy

[capi3101]

The J-33 "Wheesley" Basic Jet Engine won't go over Mach 1. That's by design - if you're actually getting 330 m/s with it, you're doing pretty good. But you can't expect better with it - if you want to go to orbit, get the JX-4 "Whiplash" Turbo Ramjet Engine at a minimum. The CR-7 R.A.P.I.E.R. Engine will also get you to orbit.

Air intake isn't going to matter much - these days the rule is 1 intake per engine, doesn't matter what size. Anything more is wasted mass. If you want to be sure about how much air is getting to which engine and you're okay with mods, try Intake Build Aid. Otherwise, you're going to have to slap your intakes and engines on one at a time (intake first, then engine) to ensure even distribution. Do it any other way and one engine will hog most of the air while the rest struggle to get by, and you'll wind up with asymmetric thrust.

Now, as for having problems once you get past 7k, that's going to be design and flight profile dependent. I'm guessing you're overly stable, but let's see your plane just to be sure.

Edited October 12, 2015 by capi3101

[arise257]

Swap out the Wheesleys for Whiplashes. If you don't have Whiplashes yet, you're basically screwed.

[Stigy]

Images of my plane are above in spoiler.

1. But why can't i get thrust above 80 kN ? 73 kN is max, when in engine description, it should be 80 kN already at 0 m/s ?

2. So which speed is relative to the surface and why air intake shows it different than it is ?

3. Jx-4 should get me to orbit ?! How if there is no air over the atmosphere and for orbiting i need to climb above it ?

[capi3101]

Not certain about #1 - I'd have to do research. I do know that all engines don't reach their maximum thrust at sea level; I just don't know why. Probably so they don't produce so much thrust that they tear the plane apart during take off, if I were to guess.

#2. You're seeing velocity relative to the surface when the Nav Ball reads "Surface". Now, there's a horizontal and vertical component to that, which you could figure out between the Nav Ball and the Rate of Climb indicator by your altimeter and the Pythagorean Theorem. Or you could install KER and have it tell you...

You've got a resources readout that I'm not familiar with. If it was the stock readout, though, you should know that air intake's readings work differently than the other ones. What it shows is how much air is left in an available "pool", which is depleted as you ascend. You don't start starving your engines until that bar gets down to zero, and even then the engines will keep going for a while. Still, 15,000 m is roughly the service ceiling of the J-33; it'll flame out at that point no matter how many intakes you've got on your plane.

Looking at your plane, I can see that you're doing something extremely low tech. If I may ask: how were you planning on recovering the pilot after the flight (for that matter, how did you take off without any landing gear)? I'm also not seeing any ailerons - I assume you're controlling that part of your attitude on cockpit torque alone. That could partially explain why you're having issues with the plane. The other big one is drag - those intakes are producing a lot of it. You definitely don't need three intakes for a single engine; I'd suggest cutting the radials unless you need the mass where it is, in which case I'd replace the circular intake with a nose cone. Just be careful where the CoM winds up; you want it aft but not aft of the CoL - if it winds up there, your plane with become flip happy and uncontrollable.

Which may also be what's happening, why it spazzes out at 7,000. Your plane is short - there's no other way of putting it - so it's going to be sensitive to where the CoL and CoM are as well as to changes in CoM as the fuel drains. It could be that the CoM is going aft of the CoL at that time. Try draining the tank in the SPH and see where the CoM is, and set the position of the CoL (by adjusting your main wing position) ever so slightly aft of the CoM in that position. "Refuel" and see where things are.

#3. The JX-4 won't get you to orbit on its own; you'd have to couple it with a rocket and use action groups to turn them on and off independently of one another. The JX-4 it will get you up to 20,000 and 1,000 m/s, a "launch" point that's a bit closer to space (making the rocket-driven portion of the flight less fuel intensive). And of course the RAPIER switches from air-breathing to rocket mode.

Edited October 12, 2015 by capi3101

[Jouni]

The Wheesley is capable of breaking Mach 2 since one of the 1.0.x patches. You just need to minimize drag and fly level at a low altitude until you break the sound barrier.

[The_Rocketeer]

The J-33 "Wheesley" Basic Jet Engine won't go over Mach 1.

I'm certain this is false. Unfortunately I'm not at my computer to produce a screenshot, but the Wheesley will operate well over Mach 1.

The key to speed with any KSP jetplane is minimising losses to drag. You don't need more than a single air intake on the front of your fuselage.

I'll try to post some screenshots a bit later.

Edit: ninja'd

[Stigy]

Thx for reply.

It's low tech because i test this engine. I wanted this plane to be very light. There is landing gear hidden in cockpit and tank. Tail Fins are controlling surfaces.

Now after few tries i managed to get to 11000 m.

1.I still don't understand why i can't get over mach 1. Air intake is ok, mass is low, if it's so crapy engine why there are mach 1.7 in the description ?

For J-33 vs. JX-4 - I understand it like: the bigger engine the more thrust you got but TWR depend on mass. So they should act similarly If TWR is equal. Thats why i can't understand why (witch parameter shows this) J-33 is so crapy.

2. Second thing is that i don't know how much air intakes I need - engine needs 1.8/sec but in all intakes descriptions there are "Amount" value. But it is not amount/sec so i don't know how to calculate air intakes to match engine requirements.

[The_Rocketeer]

Mach 1.7 is just the peak performance benchmark, i.e. what is the specified limit of the engine. It doesn't mean anything, except as an imaginary border - if you're going faster than Mach 1.7, you are no longer at optimal thrust.

Your speed limitations are almost certainly related to drag. The shock cone intake in the pic above is among the lowest drag nose parts in the game, and at low tech you don't have anything like it. Still, you should be able to beat Mach 1 if you design a streamlined plane similar to the one above/without too many radial parts.

Unless using a VTOL, TWR is almost meaningless with jet engines because thrust and weight change rapidly with airspeed and altitude.

You need one intake. Just one.

[Stigy]

I manage to do 400 m/s and 13000 m altitude. Thx for advices.

Still I don't understand how the air flow is calculated. If engine need 1.7 air/s (whatever unit is that) how is it possible that it's irrelevant what intake i use (with amount of 0.6 or 0.9) ? Does anybody know how is it calculated ?

Another thing is that normally the higher you go the air becomes thiner and it's easier to gain speed due to lower drag. As i see it's not working like this here.

[Captain H@dock]

Another thing is that normally the higher you go the air becomes thiner and it's easier to gain speed due to lower drag. As i see it's not working like this here.

But as the air is getting thinner, you also loose thrust. Try to climb at a constant speed, and check the tooltip of the engine: Thrust is decreasing.

To gain speed, you need drag to decrease faster than thrust does, which is dependent on several parameters (on top of which i'm not).

[UnusualAttitude]

Another thing is that normally the higher you go the air becomes thiner and it's easier to gain speed due to lower drag. As i see it's not working like this here.

Yes, drag does reduce with altitude, but - as it should - so does the thrust of your engine as the air gets thinner.

So, for a given constant altitude, thrust increases with speed up to a certain point, after which it starts to drop off, until drag equals thrust and you stop accelerating. The thing about the J-33 is that it rarely reaches its max theoretical thrust before drag prevents it from accelerating further; unless you have a really compact, low-drag design (as demonstrated by Jouni).

I really wouldn't bother trying to calculate air requirements, at least not for the J-33. I have never experienced a flame-out due to lack of air with a single circular intake per J-33.

Yes; in most cases the J-33 is only good for puttering around or for long distance flights thanks to its great efficiency. You won't be braking any speed records with it.

[ForScience6686]

The higher you go the less air is available for the engine. The wheesley is designed for simple jets, nothing real high or fast. Use it for survey contracts on kerbin then move on to the whiplash.

[Stigy]

But as the air is getting thinner, you also loose thrust. Try to climb at a constant speed, and check the tooltip of the engine: Thrust is decreasing.

Thats right. But shouldn't more intakes manage with thin air problem ?

Still I don't understand how the air flow is calculated. If engine need 1.7 air/s (whatever unit is that) how is it possible that it's irrelevant what intake i use (with amount of 0.6 or 0.9) ? Does anybody know how is it calculated ?

I like calculating.

No one answered this yet. Does anybody know what is "Amount" unit, and Air speed unit - air intake parameter during flight (and why it's so different than surface speed ?)?

Edit:

I've observed 3 things:

1. Air flow (U) multiplied by 50 = Air Amount

2. Fuel flow (U) multiplied by 20 = Fuel consumption

3. Surface speed + 100 m/s = Air speed (air intake parameter)

I haven't found dependence between air flow and fuel flow yet. I think we might have an answer if we find this dependence.

Edited October 13, 2015 by Stigy
new observations

[ForScience6686]

Increased intakes increase drag, that is your enemy. Fuel ratios must be maintained for best efficiency, the thinner or less dense the air, the less fuel can be used for proper combustion. Even a car with a turbo loses power when running at high altitudes.

[MaxL_1023]

The amount of air getting to your engine is a mass flow rate, equal to the air density * intake area * effective airspeed. Simplistically, the faster you go, the higher you can get before you flame out as the higher airspeed counters some of the density dropoff.

More intakes don't really help however, as the extra mass and drag slows your plane down enough that you are unlikely to get to the high-speed optimal thrust zone to begin with. I recommend going with a single circular intake on your nose like the F-86 Sabre, or perhaps a ram intake on each side if you plan to try for supersonic flight. The performance of the intakes change with air speed due to effects which are complex - mainly remember that at supersonic speeds everything gets messed up relative to "low and slow".

The ramjet engine can get you to well over 1000 m/s and an apoapsis above 50 km if you time a ballistic climb to pass through your max-TWR velocity/altitude. This is basically halfway to orbit, and high enough to use a vacuum engine like a Terrier without losing too much efficiency.

[Stigy]

Mass flow rate. That's the key for questions about more intakes. Thanks.

Still - the calculations problem remain unanswered.

For intakes - I've tested j-x4 engine with several intakes. It doesn't matter which intake i chose the result is the same. Flame-out at the same altitude. So whats the point of large selection of intakes if it doesn't mater which you chose ?

[The_Rocketeer]

The intakes are very different, and choice makes a huge difference. I think you're drawing false conclusions from your experiments, or being too aggressive in your test flights to get meaningful results. To really test the engine/intake combination you need to fly absolutely as fast as possible, which will include finding an optimal altitude.

Changing air pressure, i.e. altitude, affects thrust directly since lower pressure = less thrust. It also affects thrust indirectly via intake product, which also decreases with altitude. Airspeed also affects intake produce and thus thrust, which is why the Whiplash and RAPIERs get that massive supersonic boost in thrust.

The extent to which each intake is affected by airspeed and altitude/pressure is unique to each engine . They also have varying drag coefficients. The best all round is the Shock Cone Intake, but above certain altitudes the Ram Air Intake will actually produce more intake product than the Shock Cone does. Since the Whiplash is slower than the RAPIER, I usually pair it with the Ram intake for high altitude performance, and keep the Shock Cone for the RAPIERs.

Edited October 13, 2015 by The_Rocketeer
not engine, duh! :P

[Stigy]

It looks like I have to prepare for more test... Thanks.

Is it worth to install more than one intake per engine ? I don't understand this numbers J-x4 got 4.6 air/second. It would need at least 4 RAM intakes. Am I calculating something wrong ?

[GoSlash27]

This looks like it'd be an excellent basis for a challenge.

*evil grin* Mach 1.94 at 10 km with tech level 5 parts.

Mach 2.2 at low level

It's really pretty simple: The engine can only make use of a certain intake area. It's about .0068 m^2 for the turbojet and RAPIER and about .0017 m^2 for the Wheezley. You want to feed it enough air to keep it from flaming out prematurely. Any more than that is simply adding drag.

The object is to feed the engine(s) enough air with as little drag as possible. Most of the intakes in this game are more than enough to feed an engine. I did this using a single engine nacelle for the intake.

Best,

-Slashy

Edited October 13, 2015 by GoSlash27