What the heck is goin on with rapiers?

[Red Dwarf]

So I've been working on an SSTO lately, and it works pretty good... for the most part. Sometimes it only takes a minute or three to get to orbit, and sometimes it takes waay longer. I've noticed that sometimes the thrust on the RAPIER engines I'm using is way low. Some ascents I get 200 thrust right out of the gate, and I push the speed of sound the entire way up. Other ascents, the thrust never goes over 100, and gradually dwindles as the flight goes on.

Has anyone else experienced this inconsistent performance, and is there any way to make it not happen?

[Randazzo]

Your flight profile matters a lot. Are you getting this difference in performance while flying the same profile?

RAPIER engines have more thrust at super-sonic speeds, so if you try to climb too early or too hard, they'll never cross that threshold and really start pushing. This was a huge part of why I couldn't get an SSTO working at first, I was pushing it into too steep of a climb.

[Red Dwarf]

Yeah, I've noticed that the ascent counts for a lot, but it seems more random than not. Some of my best ascents pushed 45°, and I was getting mach affects. Other times I would do that same ascent and drop out of the sky.

[herbal space program]

It of course all depends on how many tons per engine you're trying to push, but if you're towards the high end, you definitely need to speed up to ~300m/s right off the runway at a very low climb angle, and then pitch up but never so much you go below maybe 280m/s. Doing that made all the difference for me.

[GoSlash27]

Red Dwarf,

The thrust of the RAPIER is dependant on airspeed and air density. If you're not maintaining the same speed, you're not going to generate the same thrust, so you're going to fall behind more. This is aggravated by climbing, where the air is thinner.

If you're not going as fast as you should be, you have to fix it by arresting your climb until you're back on track.

I don't use fixed angles for my climb, I instead shoot for airspeed/altitude benchmarks. This keeps everything consistent.

Best,

-Slashy

[322997am]

I use turbojets and nukes for sstos. It is just so much more practical. No oxidizer packing and turbojets have more thrust than rapiers.

[Chaos_Klaus]

Aside from the airspeed, note that air breathing engines have a certain altitude at which they magically stop working.

[Yakuzi]

Has anyone else experienced this inconsistent performance, and is there any way to make it not happen?

I've been having similar issues with craft with Mk2/Mk3 cargobays and Nao has provided an indepth description of this issue in the Stock Payload Fraction Challenge:

Sigh... >BUG WARNING< guys, when using cargo bays, make sure drag values are correct for it and nearby parts.

Check Nao's post for more details. Nao also mentioned in a subsequent post that if you change your root to a non cargobay part, it may solve the problem. Let us know how you go Red Dwarf.

Edited July 1, 2015 by Yakuzi