What is your Maximum Air-Breathing Altitude in an SSTO?

[Dogface]

I have built two successful SSTOs. One with the original parts, then another with the lifting body parts came out. Now I am building an SSTO with the v1.0 parts. In my previous SSTOs I could get to 20-22km before I ran out of intake air, and had to switch to rockets. Now I am having a hard time getting past 17km. So, I am curious what the limit is in v1.0 for air-breathing engines.

What is the Maximum Altitude you have attained in an SSTO* before you had to switch from Air-Breathing engines to rockets?

* This question only applies to successful SSTOs (circularized an orbit and returned to Kerbin).

Edited May 17, 2015 by Dogface

[Pecan]

69km, where there simply is no more oxygen.

Generally with spaceplanes, other SSTOs flew about the same launch profile but I generally couldn't be bothered to nursemaid them through the upper atmosphere.

[CalculusWarrior]

In 1.0, the limit for jet engines is no longer intake air, but thrust. Real-world jet engines have reduced thrust with increasing speed and altitude, as there is not enough air to combust and it is moving too quickly. The same is true with KSP now.

This means that the old days of slapping many intakes onto a craft are over, instead the more engines you have, the faster you will go. Just from a few test flights I've found about 25 km to be the limit, where I start to lose speed because the engines are losing thrust.

[Dispatcher]

Well, I do consider these test flights successful, though not within your parameters. These are engine comparison tests and I think you will find the data on the air breathers interesting. Take a look at the graph:

https://c2.staticflickr.com/6/5330/17586916398_fe69d2d033_o.png

I think the Whiplash turbojet is well named.

[Dogface]

In 1.0, the limit for jet engines is no longer intake air, but thrust. Real-world jet engines have reduced thrust with increasing speed and altitude, as there is not enough air to combust and it is moving too quickly. The same is true with KSP now.

This means that the old days of slapping many intakes onto a craft are over, instead the more engines you have, the faster you will go. Just from a few test flights I've found about 25 km to be the limit, where I start to lose speed because the engines are losing thrust.

That is interesting. I am glad of that upgrade, the old SSTOs crammed with intakes was really silly.

[THX1138]

I manage 29 km/700 ms-1 achieving orbit in just over 5 min.

Edited May 18, 2015 by THX1138

[Red Iron Crown]

A bit under 30km/1400m/s is the upper limit I've found, and they're not delivering much in the way of thrust by then.

[Murph]

I'm certain you probably can push beyond it a little, but 25km @ 1000–1200m/s, nose 10–15° above horizon, prograde 5–10° above horizon, seems like about the practical limit for the turbo ramjet, and it's going to be near impossible to maintain that. Even achieving roughly those numbers is going to require a fairly specific flight profile, as the power is mostly gone above 20km, it's relying on the power available and speed achieved at 15–20km.

Everything above around 20km is pretty much into diminishing returns, where you have to add a fairly huge amount to achieve relatively little.

N.B. that there are far more relevant parameters than altitude. I can achieve over 70km with just turbojets, but that won't achieve orbit in a reasonable manner, it's a very vertical ballistic trajectory with engines dead past around 25km. If you're trying to create a space plane that can achieve stable orbit, the speed, nose angle, and prograde vector at around 25km are all critical.

[Jaeleth]

Around 24km. That's the height I switch mode for my rapiers and switch off my whiplash