My jet engine grief - am I too stupid to get it?

[ShadowZone]

I can't seem to get spaceplanes right. No matter what I do, those buggers never reach a stable orbit. And a lot of that has to do with how I am seemingly unable to plan for the performance of jet engines.

Let me start by saying that I have no idea how those work in regard to performance profile etc. Cold air comes in, hot air comes out, plane flies. You need fuel for that. That's the extent of my jet engine knowledge.

Then I build a big ass plane, strap a lot of engines on it, launch it... and as soon as I reach 2000m or so, the engines start losing thrust. Why? What gives? A plane that could reach Mach 1 easily at about 500m ASL loses speed to such a degree at around 5000m it starts dipping downwards.

Why?

I would really like to know how I could make my planes more predictable, especially at high altitudes and high velocities.

So... help pls?

No, I do not want to install FAR/NEAR

[KerbMav]

No, I do not want to install FAR/NEAR

Which would not "help" you, quite the opposite ...

I cannot really explain why, but it is just the way physics work.

Which one of the engines are you using? They are aimed at different flight profiles.

Edit: I have misread your altitudes as speeds, but my answer still basically fits.

Edited November 11, 2015 by KerbMav

[Alshain]

Then I build a big ass plane

First mistake. Your first Spaceplanes shouldn't be big. The bigger they are the more complex they get.

strap a lot of engines on it

Second mistake. Don't just throw thrust at it, you only need to get up to around 1200 m/s at 20ish km. You don't need to go from 0 to 1200 in 10 seconds. More engines = more weight.

as soon as I reach 2000m or so, the engines start losing thrust.

2000m? That's barely off the runway. You must not have enough intakes. Note that intakes have changed in 1.0.5, read the descriptions, some bring in the most air at subsonic and others at supersonic. Since you want to go to space, you want supersonic intakes.

Why? What gives? A plane that could reach Mach 1 easily at about 500m ASL loses speed to such a degree at around 5000m it starts dipping downwards.

It shouldn't be losing speed at 5000 meters unless your AoA is too high or you don't have enough thrust or air, or perhaps you just have too much mass because it is over built. It's unclear. Perhaps a picture would help. But again, I would start small till you get the hang of it.

Edited November 11, 2015 by Alshain

[lobe]

You may reach supersonic 500 asl, but it doesn't matter because most engines but juno and wheesley will do that, and not much more at that altitude. If you are tring to ssto pre-rapier, you should be supersonic around 7000-10000 asl, depending when you want to ignite the rockets at. The ramjet and rapier will last until 20000-30000 asl until oxgen deprivation. The flight profile should be 20-30 degrees pitch up to 10000 approxiamtely, then smooth out to 5-10 degrees to gain speed until burn-up or 20000 atleast.

Edited November 11, 2015 by lobe
incorrectly spelt deprivation

[Streetwind]

I can't seem to get spaceplanes right. No matter what I do, those buggers never reach a stable orbit. And a lot of that has to do with how I am seemingly unable to plan for the performance of jet engines.

Let me start by saying that I have no idea how those work in regard to performance profile etc. Cold air comes in, hot air comes out, plane flies. You need fuel for that. That's the extent of my jet engine knowledge.

Then I build a big ass plane, strap a lot of engines on it, launch it... and as soon as I reach 2000m or so, the engines start losing thrust. Why? What gives? A plane that could reach Mach 1 easily at about 500m ASL loses speed to such a degree at around 5000m it starts dipping downwards.

Why?

I would really like to know how I could make my planes more predictable, especially at high altitudes and high velocities.

So... help pls?

No, I do not want to install FAR/NEAR

I wrote a thing a week or two ago which people tell me was very helpful to understanding how jet engines work. Perhaps it will help you too.

[Pixel of Life]

Are you using RAPIERs? They're ramjets, they lose thrust very quickly as air density decreases (as long as airspeed stays constant). Keep an eye on the thrust values when you accelerate through Mach 1 - if the thrust is going down, you should pitch down because the increase in airspeed isn't enough to compensate for the reduction in air density.

[ShadowZone]

First mistake. Your first Spaceplanes shouldn't be big. The bigger they are the more complex they get.

Yeah, well... I got this fixed idea that I want to get 3,75m parts into orbit using a spaceplane that is basically a huge 5m diameter fairing (which would stay intact) with tanks, engines and wings arranged around that.

We're talking about 200 to 300 tons, depending on cargo

I have made successful smaller SSTOs in the past... but mostly I use rocket planes (looks like a plane but with just LF/O and no jets) for that.

Second mistake. Don't just throw thrust at it, you only need to get up to around 1200 m/s at 20ish km. You don't need to go from 0 to 1200 in 10 seconds. More engines = more weight.

I would be happy if I could get up to 10k at least...

2000m? That's barely off the runway. You must not have enough intakes. Note that intakes have changed in 1.0.5, read the descriptions, some bring in the most air at subsonic and others at supersonic. Since you want to go to space, you want supersonic intakes.

Thanks for the hint, I'm going to read up on that!

It shouldn't be losing speed at 5000 meters unless your AoA is too high or you don't have enough thrust or air, or perhaps you just have too much mass because it is over built. It's unclear. Perhaps a picture would help. But again, I would start small till you get the hang of it.

over built is a very likely scenario... see also my usual design style

I wrote a thing a week or two ago which people tell me was very helpful to understanding how jet engines work. Perhaps it will help you too.

That offers a lot of insights in how those engines actually work. Thank you! Not sure if I can translate that into improving my build.

[Streetwind]

That offers a lot of insights in how those engines actually work. Thank you! Not sure if I can translate that into improving my build.

It's more about improving your flight profile than about improving your build.

From the sounds of things, you run out of thrust before you can go supersonic. Since thrust will magically fix itself once you pass mach 1, you need to figure out how to get there. You need to find the height with the best compromise between losing thrust to altitude and lowering drag, i.e. the height at which your plane accelerates best. Then go level there, and see if you can get past 350 m/s. If level flight won't get you there, then try flying a little higher, and then diving down trying to accelerate.

If that doesn't work - if you cannot get past the sound barrier no matter what you do - then your plane needs more thrust. Not intakes usually; you should be able to rightclick jet engines now in 1.0.5 to see if their air requirements are met or not, that's how you can check if you need more intakes, but in 95% of all cases they are not the culprit. More subsonic thrust however will get you supersonic, which is what you really need.

You can achieve that thrust with extra engines - one or two afterburning Panthers, perhaps, or maybe even a pair of SRBs that you light once you're approaching the sound barrier and then drop after they burn out. SRBs are cheap and generate a lot of thrust, and rocket assisted planes are a thing IRL too.

EDIT: Oh, and one more thing... 1.0.5 build 1024 has a bug where body lift isn't being applied. It's possible that your plane fails to climb because it doesn't generate enough lift. Squad just released a silent patch tonight which fixes this issue and will improve the lift your plane generates. If your build number shows 1.0.5.1028 on the main title screen, you have that patch. (See also the announcement forum for the patch post by Squad)

Edited November 11, 2015 by Streetwind

[T-Bouw]

Also, don't just gun your plane straight up (if you are). Not all planes can keep accelerating vertically.

Remember the essence of orbiting: it's about going fast enough horizontally to keep up with the rate of falling.

[cantab]

KER in atmospheric mode will let you choose an altitude and speed. That can be useful for getting an idea of how the engines will perform before you leave the SPH.

The RAPIER and the Whiplash both have a strong ramjet effect, producing more thrust at high speeds. That means you don't want to slow down too much as you climb. Typically I look to steadily accelerate as I climb, and IIRC I'll be supersonic below 10 km, then around 15 km I start reducing my climb rate aiming for level acceleration around 20-22. That's in FAR though, stock aero is a bit different.

If that doesn't work, three other things you can try. Break the sound barrier low then climb without losing speed, but you'll be fighting a lot of drag. Climb to a decent altitude then break the sound barrier in a shallow dive, but you will have to regain that high. Or light the rockets as an ad-hoc afterburner to get past Mach 1, but that will use a chunk of fuel.

[Pthigrivi]

All good ideas^. If you're looking for a heavy lift SSTO Id recommend starting with the big MK3 cargo bay. I use 5 rapiers and shock cones and climb pretty steadily between 10 and 20 degrees, watching that engine thrust is steadily increasing. By 10-12 k your thrust can be above 350, and if you carefully pitch up to avoid overheat you can get to 1400 m/s before your engines switch over. Remember anything extraneous sticking off your aircraft is extra drag. Simple is better. You can also hotkey most of your intakes to keep them closed until they start to starve on that high altitude push. Also remember that many wings are no longer rated for re-entry, so check those heat tolerances for anything you hope to retrieve.

Good luck!

[ferram4]

Another method for getting altitude and speed in an SSTO is to take advantage of the fact that the Panther, Whiplash and RAPIER increase in thrust as they gain speed, especially at high subsonic speeds. You can actually reach altitude and top speed faster with a low altitude race to high Mach followed by a steep, but steady, climb up to altitude.

As an example, one of my flight profiles (using an SSTO powered by 4 Whiplashes and a pair of Vectors, yes, it's that heavy) involves climbing up to 2 km (for not-crashing-into-the-ocean-safety), then accelerating up to Mach 0.9 (~306 m/s). At that point, I put it into a climb at about 35-40 degrees pitch or so, ensuring that I maintain around 300 m/s, steadily reducing the pitch angle as the engines lose thrust with altitude. After it's at ~11 km, I let it go through a shallow dive to 9 km as it picks up beyond Mach 1, and then once it reaches that altitude I put it back into a shallow, accelerating climb until it reaches around Mach 4.5 (1350 m/s, or whatever I can get it to top out at), while still keeping the altitude maybe a bit lower than expected. At that point, it's a zoom climb with the Vectors powering it to above the atmosphere and then coasting to circularize.

Incidentally, fans of the F-104 will recognize this as the minimum time to Mach 2 in that plane.

[Redshift OTF]

Also post a picture of your craft or even upload a craft file. It will help diagnose the problem.

[Joshwoo70]

I managed to fet a couple of my Heavy SSTO's into orbit with a bit of fuel left. I suggest that you use the RAPIER engines as they can switch modes.

If you want to watch a stream.. dasvaldez streams almost everyday. he somtimes go through the basics of SSTO's and stuff so be sure to check

[Tourist]

Lots of good advice here.

To reiterate what I feel the best advice has been so far:

Start small.... I too was inspired by the monstrosities others post around here, but don't run before you can walk. Get an agile MK2 into orbit before trying to launch the Galactica.

[Hobbes Novakoff]

You can also hotkey most of your intakes to keep them closed until they start to starve on that high altitude push.

IIRC, open/closed intakes don't matter anymore in 1.0 aero.

[reavertm]

Since 1.0.5 I guess there is new stock spaceplane, ironically (?) called SatelliteLauncher - and it has detachable satellite connected.

Anyone managed to actuallly launch that satellite with that aircraft? I can reach staggering ~200m/s speed at ~7km alt in level flight, I won't ever get to 10km altitude and satellite own engines are obviously too weak to reach orbit.

What am I doing wrong?

Edited November 12, 2015 by reavertm

[Vanamonde]

This question about how to play the game has been moved to Gameplay Questions.

[quyxkh]

[quote name='reavertm']Since 1.0.5 I guess there is new stock spaceplane, ironically (?) called SatelliteLauncher - and it has detachable satellite connected.
Anyone managed to actuallly launch that satellite with that aircraft? I can reach staggering ~200m/s speed at ~7km alt in level flight, I won't ever get to 10km altitude and satellite own engines are obviously too weak to reach orbit.
What am I doing wrong?[/QUOTE]

Yes, you can get the probe to Mun orbit and if I'd kept my heading better controlled Minmus. Landing on Minmus might be doable, I dunno. Open up the engine panel at launch (rclick the engine) and stay on task, 800+m/s well above 10km is doable.