Funky new Jet engine model. SSTO spaceplanes still possible?

[hawkwing]

Problems, as I see it:

• They cut out way lower in the atmosphere, regardless of how many intakes you have. (Not promoting intake spam, just saying they CANNOT be used at 30km anymore from my testing)
  
• Thrust spikes absurdly high (950 / 1150 for turbojet / rapier) at moderate altitudes as speed increases
  

This is causing planes that behave as I'd expect only in the lower atmosphere, suddenly turn into a 15g fiery deathtrap in the middle-atmosphere, then cut out again shortly after.

Now I have a lot of experience building spaceplanes in the old model, so maybe I'm just stuck in the old way of thinking. And if the jets are supposed to get a sizable nerf to how high up or fast they can operate at, then fine. But the massive increase in thrust (over 10x!) as you approach the intended operating altitude/speed of the engine is just absurd. At 1000kn a single turbojet is putting out almost as much thrust as two skipper engines!

The code behind this system seems great - like it could really support an awesome balance for jet engines. But the numbers just don't make any sense!

[Bobnova]

On earth, which is ~8x the size of Kerbin if I recall correctly, you have ~0.162PSI of air pressure at 30km. This is opposed to 14PSI at sea level (more or less).

If you scale things based more or less on the Realism Overhaul numbers (where "space" starts at ~120km(~140km?) or so rather than 70km) that 30km altitude on Kerbin equates to 51km or so on "earth", ~167,322 feet above sea level with a atmospheric pressure of ~0.017PSI. There isn't any air to burn there to speak of, certainly not without one hell of a scramjet.

Like many of the "issues" with 1.0 I'd say this is more a case of the previous model being so incredibly terrible and allowing things that really ought not be possible.

For further reference purposes the all time record for a jet engined aircraft is 37,650 meters or 123,520 feet. Odds are excellent that it flamed out well before that though and coasted upwards.

The wild thrust spikes seem like an issue, though. I'm not aware of that being a thing with any real turbine engines, with the possible exception of the SR-71's engines. Those're odd ducks

[Armchair Rocket Scientist]

On earth, which is ~8x the size of Kerbin if I recall correctly, you have ~0.162PSI of air pressure at 30km. This is opposed to 14PSI at sea level (more or less).

If you scale things based more or less on the Realism Overhaul numbers (where "space" starts at ~120km(~140km?) or so rather than 70km) that 30km altitude on Kerbin equates to 51km or so on "earth", ~167,322 feet above sea level with a atmospheric pressure of ~0.017PSI. There isn't any air to burn there to speak of, certainly not without one hell of a scramjet.

Like many of the "issues" with 1.0 I'd say this is more a case of the previous model being so incredibly terrible and allowing things that really ought not be possible.

For further reference purposes the all time record for a jet engined aircraft is 37,650 meters or 123,520 feet. Odds are excellent that it flamed out well before that though and coasted upwards.

The wild thrust spikes seem like an issue, though. I'm not aware of that being a thing with any real turbine engines, with the possible exception of the SR-71's engines. Those're odd ducks

Yep. Good luck running a jet at 30 km.

Really, Kerbin's scale height is pretty small. According to the formula I found on Wikipedia, assuming the same atmospheric temperature as Earth, its atmosphere would have to have a mean molecular weight of 42.4 g/mol. Assuming an O2 and CO2 mixture, the atmosphere would have to be 13% oxygen and 87% CO2. Assuming an Earthlike 21% oxygen, the remainder would need to be heavier than pure CO2, meaning Kerbin needs to have significant amounts of a gas heavier than CO2. Unfortunately there aren't many gases like that: apart from weird organic compounds that won't be stable over geologic time, the only good candidates are Sulfur Dioxide, Krypton, and... wait a second... Xenon.

Huh. Does this mean we could use an ISRU converter to extract xenon from Kerbin's atmosphere?

tl;dr reason #468 why I like RSS.

[Blu_C]

Your thrust spikes may actually be the engine functioning as intended. Turbojets function better at higher speeds. You'll notice that until you hit Mach 1 they are significantly less powerful than a regular jet. I suspect RAPIER works similarly though I haven't tested it myself.

All that said, those max thrust values doe seem rather absurd to me. XD

In my experience you want to stay under around 1km/s until you are above 25k-30k. Space planes are TOTALLY possible, I have made several already that are capable of achieving orbit without problems, but you are pretty much going to have to throw away everything you thought you knew about how to do it. Personally I've stopped thinking it as building a plane and instead think of it as a rocket that just happens to get past the first 20km using jet engines. Which means bringing waaay more oxidizer than I am used to (although I am starting to consider a try with RCS engines. Sorta wish there was an inline version of those).

[eddiew]

You'll get a thrust peak from the engine when you see mach effect visuals around the plane. This is also the point at which you're getting a heat spike, which makes it a great time to pull up until the effect fades to a minor haze. Rinse and repeat for a safe, if slightly bouncy, ascent to orbit

I believe I shall keep pasting this quote until it becomes common knowledge...

Turns out the key is to follow the Mach effects, without blowing up. Basically, the closer you are to Mach effect territory, the more air you will have for your engines and the higher their thrust will get (the effect is quite dramatic, actually). If you go too slow, they won't give you enough oomph, so pull down AoA again until you get mach effects and a decent acceleration, then go up basically as fast as you can. But, once you get close to the thermal barrier (about 1km/s at 20kms, less if lower) you have to pull up to go higher, faster and thus avoid burning up.

So far this flight plan has consistently worked on every dumbass thing I've made that actually had enough TWR and fuel to get to LKO

Regarding 1MN from a single turbo... I've never seen anything close. 250kN is the best I've gotten out of both the working turbo/LV-45 spaceplanes I've put together. I sort of want to call pics or shens

[hawkwing]

All of that I'd be fine with, except that the amount of thrust gained on these small turboramjet and rapier engines is ridiculous.

The implementation of the system in code makes sense to me, but the config value numbers are just silly. No 1.25m engine should be capable of 1000kn of thrust, at ANY operating condition. Let alone jet engines, which should fundamentally have far LESS thrust than a rocket of comparable size.

[EtherDragon]

SSTO Spaceplanes Possible?

[Blu_C]

http://i.imgur.com/6QHYq6T.png

Heh, that second one of yours is almost identical to the first one I got to work in 1.0 for myself (replace the docking port with a cargo bay and the crew compartment with more fuel though).

[EtherDragon]

Heh, that second one of yours is almost identical to the first one I got to work in 1.0 for myself (replace the docking port with a cargo bay and the crew compartment with more fuel though).

I have seem some really cool 1.0.1 Space-Plane designs. What's really cool is that they are all different enough to not be cookie cutter! The second one was me making one for RAPIERS at first, then seeing if I couldn't retro-fit it with Turbojets and Reliants, the kind of tech one might have part way through Career Mode, as RAPIERS are a top tier item (1000 science to unlock! Whew!).

[NathanKell]

The wild thrust spikes seem like an issue, though. I'm not aware of that being a thing with any real turbine engines, with the possible exception of the SR-71's engines. Those're odd ducks

Funny you should mention that; the turboramjet (note: turboramjet) engine does appear to be modeled on an "only moreso" version of that engine, i.e. one that switches over much more heavily to ram mode at high speed (rather than only bleeding some of the air around the turbine, on the Blackbird's J58).

It's worth mentioning that, indeed, the jets are very, very OP compared to real life engines. The Blackbird's J58, that engine which topped out somewhere around Mach 3.5 (we presume), produced only 150kN static thrust (compared to the TRJ's 180) on 2.7 tons (compared to the TRJ's 1.8 tons). And the TRJ can provide useful thrust past Mach 4. Besides, the J58 was about 1.4 meters in diameter, not 1.25m. Oh yeah, and the TRJ is about 10x the efficiency (Isp) of the real engine.

Now some of that is presumably because we can't make planes as well-streamlined as the Blackbird. But let's recall that the Blackbird had about 60 tons takeoff weight, on 2 J58s (300kN static thrust afterburning, total). Scaled, that's like using only two TRJs for a 72 ton craft. Try that. Please try that.

[Scarecrow88]

I've found that you need to seriously modify your ascent profile to get SSTOs to work in 1.0

[panzer1b]

although I am starting to consider a try with RCS engines. Sorta wish there was an inline version of those

200 isp engines, GL.

The little monoprop attitude control thrusters (the rcs block and the linear thingy) have better ISP. Those O-10s are now worthless when every single other engine is better suited for flight.

Anyways, as for SSTOs, they still work, but forget achieving 400km AP and 50km PE with nothing but jets+airhogging. Right now a single RAM/engine is plenty, and you can probably even get away with a single side mounted intake too (like those precooler things). Also, the absolute max ejection velocity ive managed myself is 1600ish m/s, any more and your TWR just wong cut it even if you spam engines. Im guessing with inf fuel and just a pod+engine+nose cone, you can pull off more, but i dont use inf fuel unless im screwing around in DPM. The hard limit is the engines failing at 30ish KM, and while you can get the highest velocities down near 10-15km, if you try to gun it at 1500m/s that low you will fry (using stock heating amounts).

Finally, wings are all but useless for anything but landing, ive switched to nothing but 3 delta-deluxe wings for every SSTO thats under 15t and a control surface in the front, more then enough lift and it doesnt add excessive drag either.

Edited May 2, 2015 by panzer1b

[Slam_Jones]

Is it still possible to get to orbit as an SSTO without RAPIERS? In 0.90, I was able to offset a Turboramjet and a 48-7S into each other and used that to get to stable orbit (2.2km/s @ 28km on jet, switch to 48-7S for the rest). Clearly the 48-7S has been nerfed, but is such an idea still possible? Maybe with a LV-909 instead of the 48-7S?

I really like non-Rapier spaceplanes. Almost to the point I consider Rapiers too​ easy.

[Anquietas314]

Is it still possible to get to orbit as an SSTO without RAPIERS?

Yep, totally still doable. Here's my (seriously half-assed) SSTO spaceplane using turbos and LV-T45s:

Granted it has no payload and doesn't get to orbit with much room to spare, but the ascent profile's nice and simple at least: hold ~40 degrees at full throttle all the way up, switch to rockets after the turbos fail. Airbrakes for reentry ftw

Edited May 2, 2015 by armagheddonsgw