1.0.5 Heating and SSTO Ascent Profiles

[March Unto Torment]

Hey everyone,

So, I'm trying to get my new SSTO to orbit (well, I say 'new' - it's basically a rebuild of the old Valkyrie SSTO), but I'm running into some issues in the ascent stage.

Basically, the thing has oodles of thrust. That much, I know. It's capable of hitting over 2,000m/s while still in atmosphere.

However, this leads to a big problem - heating. I never encountered this before 1.0.5, but all of a sudden, I need incredibly precise throttle control and can only really open the taps above 22,000m, at which point I'm starting to run out of air anyway (something which is mitigated, but not eliminated, by my absurd number of intakes).

If I fail - and my first ascent involved many, many fails - then the thing completely disintegrates due to overheating.

My ascent profile looks something like -

1. Accelerate to 400m/s at sea level at full throttle.
2. Pitch up and slowly claw my way towards more speed, start cutting throttle at around 7,000m as it begins to accelerate.
3. Hit 20,000km doing about 750m/s (the thing refuses to stop accelerating, even while pitched up at 15 degrees).
4. Desperately try to figure out where the 'balance point' of heat vs. intake air is.
5. When feasible, floor it until I gain an absurd amount of orbital velocity.
6. Wait until engine thrust drops too low, then make a roughly thirty-second burn for orbit on closed cycle.

Am I doing something wrong? I feel as though my overheating problems are excessive. Anyone know a fix?

Best regards,

 - March Unto Torment

[Pecan]

10 minutes ago, March Unto Torment said:

Am I doing something wrong?

Lots

• Absurd number of intakes no longer has any value whatsoever
• If you have lots of thrust - launch!  Go up!  Nothing wrong with rocket SSTOs
• No reason to stay at sea level if you can accelerate in a climb.  If you can't, you don't have oodles of thrust ^^
• Why cut throttle, when the whole point is to gain speed?
• You should be well over 1km/s before reaching 20km if you're using a flat profile
• Low-20s is the max altitude you'll be able to get and still run air-breathing engines
• There is no such thing as an absurd amount of orbital velocity unless you accidentally find yourself leaving Kerbin SOI

The final burn to orbit sounds good though.

[March Unto Torment]

Ah, thank you for the advice. To address -

1. Thanks for the note on the intakes! That'll save me some weight, I reckon. Another precious few tonnes of cargo and/or dV to take to orbit once I get rid of them.
2. Not quite enough thrust for a rocket SSTO, because...
3. The engines I use (both from mods - scramjets and 2.5m RAPIER variants) have terrible thrust at low speeds. Just getting the damn thing up to 400m/s can be quite the hassle. However, after that point, I get a huge spike in thrust and all is well.
4. The reason why I cut throttle is because - as mentioned above - if I don't, I gain too much speed at too low an altitude and the thing explodes due to overheating. Hence my whole problem here.
5. Ah, thank you for the velocity advice. I'll up thrust a little during the 15km-20km phase, then, to get myself to over 1km/s. Although I'm concerned that this may induce overheating.
6. Duly noted on the subject of the correct altitude for airbreathing engines. I find myself starting to lose thrust about then anyway.
7. Regarding 'absurd orbital velocity', I say that because I'm travelling at close to 2,000m/s orbital while still at around 25km altitude. This puts my Ap at 55km, hence why such a short burn to put my Ap up to 100km, and another short burn (only 185m/s) to circularise. Come to think of it, if I held altitude at 20km, the thing would probably accelerate itself to a 20x20km orbit (although air resistance would cause such an orbit to rapidly decay).

[g00bd0g]

No more intake spamming needed. 2 shock cones are enough for 3 rapiers! You can right-click on an engine and it will say "fuel requirements met" or something. Any more air than needed does not help. Rapiers have terrible low speed thrust now. This launch profile works reasonable well for most 1.05 spaceplanes.

Launch profile:

Takeoff
Accelerate to 350m/s
Establish climb approx 20 degrees, should still be accelerating slowly
Go prograde to slow climb to near level at 10k alt
Accelerate to ~1000m/s
Establish positive climb ~10-20 degrees
Should be around 1400-1500m/s at ~20k alt
Switch to rockets when jets are under ~1/2 thrust

It is important to accelerate quickly and then climb quickly to get out of the atmosphere before parts start overheating. Also a nice steep launch profile will reduce drag losses spent leaving the atmosphere. But launch angle is always a tradeoff between accelerating on efficient jets vs drag losses coasting through atmosphere.

Basically you need a high TWR and to accelerate and climb hard once you are in the "danger zone" (>1000m/s). If parts are blowing up, climb faster

[cephalo]

This version is a bit weird with the heating. If you want to go fast in the atmosphere, you can replace a normal nose cone or intake with a small heatshield. That will make your craft rapier proof in terms of heating, and you can ultimately go faster in spite of the extra drag by taking advantage of the superior thrust. I made a small spaceplane this way and it sure looks dumb but it gets to space with more dv than my older designs.

Edited December 16, 2015 by cephalo

[Vegetal]

The heat shield is quite blunt, not very good for planes in general. You might want to use a fairing instead, they have immense heat tolerance.

[AeroGav]

1.  As has been said, less intakes now.    Personally i'd swap the shock cones for pre-coolers, since they are zero drag and one pre-cooler can do one rapier.

2. The main saving from cutting out the intake spam is not weight, but drag.   The old drag model did not take account where parts were placed, and was proportional to mass.   The new drag cubes do take account of placement.   Lengthening the fuselage by adding extra sections in line will not increase drag, but putting nacelles or sponsons parallel to the main fuse in order to get an attach point for more intakes significantly increases drag.  Same with radial mount stuff - more frontal area, more drag!

3.  Try climbing steeper if going too fast for your altitude, rather than throttling back.  

 

My approach now is not to exceed Mach 3.7, or 1120 m/s, in airbreathing mode.  Why?  Because that's the airspeed at which they produce max power.  If the goal is to get as high as possible on RAPIERs before switching, hold this speed and use excess power to climb.   When you can no longer climb to a significant extent, switch to closed cycle.

Some of my designs can get all the way to 29.5km on airbreathing alone , at which point the RAPIERs shut down no matter what your airspeed or what intakes you feed them with.    At this altitude, both their thrust and fuel use are tiny.   The air is very thin at this altitude so you need a lot of wing area to get enough lift without flying an excessively nose up attitude.  If you need more than 5 degrees nose up to stay airborne, you are suffering increased lift induced drag and would benefit from a larger wing.

However, this larger wing may create too much drag to break the sound barrier at sea level.      You may want to climb subsonic to 10km or more before going into Chuck Yeager mode.

 

Incidentally, if you do end up designing a craft that way,   it can make sense to take things to their logical conclusion and skip oxidiser altogether.   Instead of flying like a jet, then flying like a rocket,     I just fly like an airplane all the way up, optimising my lift:drag ratio and steadily gaining energy.      I attach NERV to the engine mount nodes, then attach RAPIERs to the NERVs, offset them back slightly so they don't occlude each other, then finally attach tail cones to the RAPIER to minimise drag then offset deep into the engines to allow them to still make thrust.      

I use ModularFuelTanks to replace the oxidiser with extra LF stowage (and all those big-S wing parts provide additional tankage ofc) and Tacfuelbalancer to work around the NERV's inability to draw on wing tanks (and to unbalance the a/c by draining the fronts first).