What happened to engine stall and the atmo?

[Ozzallos]

So last time I played Kerbal in .90 I had a fairly awesome little number that used 8 rapier (those damn raptors) engines in a vertical launch system to directly insert a ship into orbit. She could climb through more or less constant acceleration and hang on to atmospheric breathing to about 23k before switching over to the rockets. There wasn't much fuel left by the time I got her into orbit, but the ship could refuel once she was there, de-orbit on chutes afterwards with minimal retro burn in final decent, then relaunch again back to orbit. It was a pretty sweet system until, well, now.

Now the very same ship and other similar models built from the ground up in 1.0.4 can barely make it to 3k and 200m/s before their speed starts to bleed away to the point where the ship can no longer maintain its ascent. The ship lifts off without issue and I've got more than enough air intake, which makes my the stall so much more mysterious. I'd consider it a drag issue, but again, I lift off just fine. Below is a pic of the ship in its former glory, quite obviously space worthy.

The Star Scream Orbital Return Vehicle, build 10

Single Stage direct to orbit system.

Any thoughts? What happened?

Edited October 31, 2015 by Ozzallos

[FancyMouse]

Atmo starts to use the scaled standard model instead of blind exponential.

Jet engines are completely reconfigured.

Better unlearn atmo knowledge in 0.9, including the old deprecated 10km 45-degree turn launch advice, and start learn the more correct thing from scratch - we all went through this process when 1.0 first came out.

[Ozzallos]

Atmo starts to use the scaled standard model instead of blind exponential.

Jet engines are completely reconfigured.

Better unlearn atmo knowledge in 0.9, including the old deprecated 10km 45-degree turn launch advice, and start learn the more correct thing from scratch - we all went through this process when 1.0 first came out.

Doesn't explain how I can launch, accelerate, then stall out at only 2k using the 'more correct' model, but sure, I'll press the i believe button.

And Hnn. Just found a model that works... Using the Mk3 cockpit.

Sorry, but I'm calling shenanigans

[The_Rocketeer]

Yeh, jet thrust is now proportional to intake speed, so it's not really appropriate to put them under vertical lifters unless they're really light and ur TWR is plenty high to get good vertical acceleration.

[Chaos_Klaus]

As you get higher, the air gets thinner. You have to compensate for that by going faster and thereby pushing more air into your intakes. If you never gain enough speed, you won't get enough thrust.

[Streetwind]

Jet engines work differently. For starters, they no longer have an Isp curve and a thrust curve - instead, they have no Isp curve and two thurst curves. One is altitude-dependant, one is speed dependant.

Since you are climbing in altitude and the air gets thinner, the engines gradually lose thrust along the way. The RAPIER engine, which I assume you meant when you said raptor, drops from full thrust at sea level to half thrust at 1/3rd atmosphere density. Now I don't know where 1/3rd is on Kerbin, but it's probably at or below 10 kilometers due to the exponential scaling.

Then, you need to understand that both the Whiplash and the Rapier are no longer bog standard turbojet engines. Much rather, they are turboramjets. That's what you get when you take a ramjet engine and build a full-bypass turbojet into it so that it forces air into the ram combustion chamber. Ordinarily a ramjet develops zero thrust at standstill, so it couldn't take off; but with the help of the turbine, you can force it to work a little bit and thereby let it get you up to takeoff speed. However, the turbine is an anemic crutch and cannot possibly realize the ramjet's full potential. For that, you need speed. Around mach 0.5, it begins slowly picking up performance, though usually you don't notice it then because you're losing performance to climbing altitude at the same time. At 300 m/s, transsonic wave drag begins to set in and further makes your craft feel sluggish and stuck behind the "sound barrier". But if you can push through, and get to 400 m/s, then the atmosphere lets you go again... and at the same time, now that you're supersonic, the ram effect really kicks in. By the time you're going mach 1.4, your RAPIERS are already getting a 4x thrust multiplier, catapulting your craft forward like a bat out of hell. And it only keeps increasing for a good while. The RAPIERS peak at mach 3.75, where you are getting 8.5 times your base thrust worth of power output (still modified by altitude though). After that it begins to decline, but it's not until mach 5 and beyond that it really begins dropping off.

Since speed and altitude both apply multipliers to the base value, the maximum thrust is obviously achieved the faster you go at lower altitudes. When building planes, you want to climb to a point where the atmosphere is thin enough to have reduced drag but still thick enough for the engines to produce sufficient thrust; then level off to accelerate through the sound barrier and get the ramjets going (sometimes even by pointing the nose down and diving a litte); then climb again until just before the engines start seriously suffering from lack of atmospheric density; then level off again and accelerate to the point where the engines are past their peak speed and lost thrust to the point where you are almost not getting any acceleration anymore. Then you pull the nose up for the final time and switch to rockets in order to go for orbit.

With your jet driven rocket vehicle, you are not getting good results because you are climbing vertically. Not only does a vertical climb minimize your acceleration (gravity steals 10 m/s²), but it also maximizes your climb rate into less dense atmospheric regions. So you are getting to a point where you are rapidly losing thrust from altitude before you ever had a chance to build enough speed for the ramjets to get going. Turboramjets unfortunately make extremely poor vertical climb engines - unless you give them SRB assistance to get through the sound barrier really early, they won't lift anything anywhere. And even then you might discover that you have to switch to closed cycle as early as 12-15 km already.

EDIT: geez, this thread had one reply when I started typing!

[Ozzallos]

Guess that explains it. Now how do I mark answered...

Thanks for the replies, all.

[KerikBalm]

thrust decreases as air density decreases.

This is offset by thrust increasng with higher velocity (to a point)

Also, the TWR of jets was decreased.

Also, drag coefficient is not constant. There is increased drag around mach 1 (roughly 300-350 m/s)

You are too high going too slow. You fail to break mach 1.

You ascend to where the air is too thin to maintain a 1:1 TWR

Also, do not intake spam.

1 intake per engine should suffice

[Ozzallos]

Thanks to this thread, I've determined that this build is still quite viable with some changes to the launch profile. Actually, the fact that they now behave as actual ramjets was the biggest piece of the puzzle. It does require some engine toggle jockeying, but I'm finding it's imminently doable.

Switch all engines to F/O mode.

Vertical launch, immediately roll to orbital insertion heading.

Burn to 250m/s, switch to Ram Air mode.

Climb to flame out altitude. 20km was easily obtainable now.

Switch back to F/O mode, insert orbit.

The problem was less that I was climbing vertically and more that- or at least I suspect -the engine was suffering compressor stall. Launching in F/O mode front loads enough velocity and thereby intake pressure to get the rapiers up and running. Vertical launch definitely aggravated the problem. A solid booster can be used for the initial velocity front loading as well, but defeats the stand alone purpose of this design.

Super useful help here resulting in a breakthrough. Thanks

*Addendum

It isn't just a certain speed, but a pressure threshold. I'm running a 12 engine build that can switch over at about 250m/s and burn all day but the 8 engine wallows at the same speed. I'll pay attention to how many g's I'm pulling next time, but I'm pretty certain that acceleration is a huge factor on whether the engines have enough pressure to produce. ...2g's seems to be the minimum, with an airspeed as low as 150, but not lower than 100. That alone saves a ton of fuel.

Edited October 31, 2015 by Ozzallos