Large MkII SSTO Flight Path

[Whyx]

Okay folks,

ive built a ship based off of Matt Lowne’s Duna SSTO design and all I’ve done is replace the extra passenger space with science equipment in a cargo bay and a MKII docking port, which in total weighs less than the MkII passenger pay used by Matt.

i can’t get the blasted thing into orbit. My current flight path is:

fly 10 degrees on takeoff till I get to 300 ms^1.

pitch up gradually to about 35 degrees (I start to lose speed very quickly if I pitch up any more as my thrust isn’t very large)

once I get above 10000m I flatten out my trajectory.

this is where my problems start, even in a power dive I cannot accelerate beyond 350ms^-1 with 4 RAPIERS I have to activate my 2 NERV engines to get myself past 400.

once past 400 I have no problem getting up to 1200ms^-1 but my engines soon overheat so I have to switch to closed cycle. My closed cycle uses all my oxidiser so fast I can’t get my aopsis past 35000m and then I’m left with just NERV which can’t accelerate fast enough to get my aopsis high enough.

i really don’t know what I’m doing wrong 

[Gordon Fecyk]

Are you talking about this SSTO? Matt has five or six of them that could reach Duna, but this is the first Mk2 one I found:

 

Here's some stupid things about the stock aerodynamics model:

• Things attached to the inside walls of a cargo bay get treated like they're attached to the outside walls, drag and all.
• Mk2 parts are draggier than they look, and require careful attention to stay prograde as much as possible in atmospheric flight.
• Drag is worst on the edge of Mach 1 in that transonic region and you need to push past it to resume accelerating.
• Mach 1 is not the same speed in m/s at different altitudes, so that transonic region isn't the same either.
• Nukes are poor below 20 km up, but start working OK past that. Still anemic thrust though.

I'd suggest trying to get the original SSTO flying first, before replacing parts on it. Make sure you have Matt's ascent profile working. Things may also have changed between KSP 1.0.5 and 1.3.

[One quick test later] I grabbed Matt's Duna Phantom and flew it in stock aero. Sure enough, any angle of attack other than zero is a detriment to this craft's performance. It needs to be kept pointing as close to prograde as you can get it to get any speed beyond Mach 1 out of it.

I could get it to orbit though, with (I think) just over 4500 m/s delta-v remaining (4505 / 8680 LF and 110 / 1320 OX). This ship is almost all fuel. I also almost burned the nose off of it, which is another stupid thing: The Mk2 nose cockpit is awful in stock aero. 

My ascent profile looked like this, and I didn't watch Matt's video aside from verifying the design so I didn't follow his ascent.

1. Ignite rapiers, hope to hell it climbs after falling off the end of the runway.
2. Climb between 5 and 10 degrees pitch angle, keeping as close to prograde as I could.
   (Pitch angle is not angle of attack by the way; pitch angle is relative to horizon while attack angle is relative to prograde.)
3. At around 5000 m it started exceeding Mach 1, but I had trouble climbing. So I climbed slowly, making sure to pick up speed.
4. Level off above 10 km, pick up speed as you did. I was quite past Mach 1 so transonic didn't give me as much trouble.
5. Ignited Nukes at 20 km (Or was it 18? High enough though.) Still had a very small angle of attack but I was climbing steadily.
6. Switched Rapiers to closed cycle once their thrust dropped below 80 kN. Didn't exceed more than 4 degrees angle of attack, but steadily climbed.
7. I think my AP was at or near 50 km when the Rapiers cut out. The two front tanks had oxidizer switched off; I think this was for a boost at Duna takeoff.
8. Hit SAS Prograde Hold and use full Nuke thrust until AP hits 75 km. Got very hot on the way up.
9. Close my orbit at AP with under 70 m/s, or only a 16 sec burn.

But then this is after a year and a bit of learning how to fly KSP aircraft into space. Don't be discouraged because some expert's design isn't working for you yet.

Edited December 4, 2017 by Gordon Fecyk
Test results

[GoSlash27]

Whyx,

Once you've reached 300 m/s, try leveling out your climb until you attain 380 m/sec, then gradually increase pitch as it accelerates. Try to level out around 20km altitude to maximize velocity from the air breathers, then go to the nukes and climb out.

Best,
-Slashy

[AeroGav]

Yeah, that craft was built more than a year ago, quite a few things have been revised since then.  Some parts have got draggier, others less draggy, aerodynamic heating is now much more sensitive to placement of the part (bad news for pointy cockpits).  In terms of Matt Lowne's craft in the picture,  the pointy cockpit would have to go, and the solar panels are now draggy even when stowed, and should be moved inside a service bay (or have an RTG in service bay).

I did put a guide in the Tutorials section on making SSTOs, it's only one game version out of date !

 Keep the Mk2 parts to a bare minimum, they are so awful for drag.    Ideally, only the cargo bay should be mk2.   Mk1 can even be used for cockpit and crew cabins provided there's a service bay, fuel tank or intake ahead of them to absorb heat.     Definitely do not keep fuel in mk2 parts, not worth it at all.   The best thing to put fuel in is a Big S strake , failing that big S wing, failing that, use a mk1 size tank.

You need to use a lot of wing area and angle the wings up at their attachment point to the fuselage to make a mk2 design viable.   This enables you to fly with the nose on prograde lock to minimise drag from the fuselage parts.

I have some vaguely similar craft on KerbalX that are fairly recent if you want to study them -

https://kerbalx.com/AeroGav/Lamb

That nose cone is now a poor choice, i should replace with shock cone.   Has a clamp o tron and cargo bay.    

https://kerbalx.com/AeroGav/Coimbra-PA-X-LR

This one has a rapier clipped into a Panther,  the two engines complement each other well (one is good at low speed, the other high speed).   Can reach Minmus but a return from Duna would need a refuel.    The reaction wheels are now somewhat high drag, though not compared to all the mk2 cabins it has !

My most recent SSTO  avoids most of these faults and combines a decent amount of delta v as well as crew capability.  But it's a mk1.  Also as a "barebones" craft, it lacks lift thrusters for landing offworld.

https://kerbalx.com/AeroGav/MK1-Griffon-Deep-Space-Crew-Shuttle