Awkward thing going on with my SSTO "Space"plane

[eddiew]

Claymore E... three rapiers and a nerv. Accidentally shot it into a 128km orbit. But is it better than turbos and 45's? You get an extra 500dv in orbit (largely useless) and it requires a lot more science to reach.

Overall I'm not satisfied with rapier behaviour at all; they seem to have no place on medium size ships. Maybe on big cargo haulers than can have a 50/50 mix of turbos and rapiers, but when you've only got room for 2-3 air jet engines they're just not cutting it :/

[Wanderfound]

*edit* I'm starting to suspect that rapiers don't hit their power band until about 420m/s... once I can get a plane beyond that, rapier power shoots up past 200kN, but below that speed I'm lucky to get 100 out of them. Tempted to put a disposable SRB on there for some kick off the line

Those RT-5s are nice and small...

[eddiew]

You guys might be interested in this thread I've opened on the support forum:

http://forum.kerbalspaceprogram.com/threads/118686-Rapiers-seem-too-weak?p=1894152#post1894152

I dunno if it'll get any attention, but I can't currently find a compelling case to use rapiers, and that seems wrong ^^;

[Bobnova]

The stuck-at-transonic-barrier "issue" is a real life thing too. SR-71 can't break the speed of sound in level flight. It has to climb subsonic, dive to go supersonic, then use the drastically increased thrust that its partial-ramjet engine generates at >mach1.

IMO this is a good thing to see in KSP, accidentally modeling real world stuff can be an indication of an accurate model.

[eddiew]

The stuck-at-transonic-barrier "issue" is a real life thing too. SR-71 can't break the speed of sound in level flight. It has to climb subsonic, dive to go supersonic, then use the drastically increased thrust that its partial-ramjet engine generates at >mach1.

IMO this is a good thing to see in KSP, accidentally modeling real world stuff can be an indication of an accurate model.

It doesn't really work though - when you pull out of the dive, you lose all your speed (and thrust) almost instantly :/ (I tried.) I think that we're getting stuck at similar speeds is largely coincidence; pulling some wings off mine made it max at 325 instead. It's just the power/drag ratio, and the fact that rapiers aren't delivering until about 400m/s.

[eddiew]

And that's why your rapier engines suck when trying to pass mach 1. You do quite literally need three of them to equal the low-mid altitude thrust of two turbojets :/

[eddiew]

K, well my thread on support yielded a fascinating result... this, with more wing area than my design... has no problem blowing past 300m/s and on to orbit. It seems the additional lift of the larger wing, allowing for a lower AoA, improves the thrust to the point where the extra drag of that larger wing is more than negated. Essentially the flight plan here is get to 12km, drop horizontal, get to 400m/s (it just plain happens​) and then pick up the nose again. It's bizarrely easy.

...which doesn't change the fact that rapiers have a poor power curve at the low end, but it does offer some options to get around it.

[MainSailor]

K, well my thread on support yielded a fascinating result... this, with more wing area than my design... has no problem blowing past 300m/s and on to orbit.

What's the takeoff mass of that design? And it appears to be all-fuel right? That's not a payload bay behind the cockpit, unless I'm mistaken. I was mistaken. I looked at the textures and that's obviously a payload bay. I still think I'm carrying too much fuel for my payload, and then burning it up trying to get it all up to orbit.

Edited May 4, 2015 by MainSailor

[SnappingTurtle]

It doesn't really work though - when you pull out of the dive, you lose all your speed (and thrust) almost instantly :/ (I tried.) I think that we're getting stuck at similar speeds is largely coincidence; pulling some wings off mine made it max at 325 instead. It's just the power/drag ratio, and the fact that rapiers aren't delivering until about 400m/s.

I've been finding the same issue with my four-rapier, roughly 50 tonne plane like in your original post. From my own testing it seems that 250kN is about the tipping point, where above that and the major trouble will be trying not to overheat in the atmosphere because the thrust of the rapiers just explodes upwards, and below that number you will never get supersonic. Going up to 12-15 km, then doing a shallow (as small as 5 degrees) dive until the engines are each putting out >250kN (for a ~50t plane) has worked for me. From there you can climb and the engine thrust doesn't stop going up until you flameout, or throttle to less than 176kN thrust which is what the closed cycle produces. It's pretty much the first time I've had to pilot by mainly watching which way the thrust of the engines is going.

Edited May 3, 2015 by SnappingTurtle

[MainSailor]

And that's why your rapier engines suck when trying to pass mach 1. You do quite literally need three of them to equal the low-mid altitude thrust of two turbojets :/

This helped a lot. Swapped out Rapiers for turbos and 2x LV-T45's and got to orbit handily. I had to watch my speeds near flameout, I wasn't able to just kick on the T45s, I had to do the dip to gain speed and then burn towards prograde (at about 32-35km.)

[eddiew]

This helped a lot. Swapped out Rapiers for turbos and 2x LV-T45's and got to orbit handily. I had to watch my speeds near flameout, I wasn't able to just kick on the T45s, I had to do the dip to gain speed and then burn towards prograde (at about 32-35km.)

Frustrating isn't it? :/ I'm annoyed that rapiers alone often result in non-viable planes, and you end up wasting your precious centre-engine slot on a turbo in order to get through the mach barrier.

Though by 32km with turbos, I'd expect to be ballistic anyway; you should have an AP upwards of 40 by that point... if not, you've left your speed run too late. Try to get to mach 1.5 at around 12-14km, then pull up as hard as you can get away with without losing thrust

has 1 turbo and 4 rapiers, and hammers up to a 44km apoapsis on air. 25 degree ascent all the way, no fiddling around. I kick to rocket mode at about 30km when the rapiers have utterly given up on air.

[MainSailor]

Though by 32km with turbos, I'd expect to be ballistic anyway; you should have an AP upwards of 40 by that point... if not, you've left your speed run too late. Try to get to mach 1.5 at around 12-14km, then pull up as hard as you can get away with without losing thrust

No, I'm still messing with the ascent profile. I think I still have way too much fuel so my ground TWR suffers and it takes a while to burn it off.

Once I get to 35k and get the climb back, it'll happily zoom to orbit on the T45's, so much so that I've overshot all the way up to 100k apo almost every time. I probably don't have enough air intakes (I've been avoiding air-hogging this time around) because I'm seeing pretty solid flameout at 21-25k (or I just can't maintain enough velocity.)

[FlipNascar]

^Don't forget that airhogging as we knew it... Well that's a bygone era. You are not going to get any meaningful thrust out of any of the jet engines above 20,000m. Just look how quickly the thrust tapers off. 1 Ram intake per engine is plenty... Any more intakes and you're just going to add drag needlessly.

As I posted with the OP's craft... Sometimes you have to dive to break through the sound barrier. A Rapier is better for getting to orbit than the turbojet. You just have to amend your ascent profile and help the Rapier along a little with a wee descent and your ol' pal gravity.