Help getting a Mk3 SSTO to orbit

[NightshineRecorralis]

Here is the craft in question:

7645 Ox, 9355 LF, 4 Batteries, 2 Solar Panels.

Originally designed to carry an orange tank or some other equivalent to orbit, but the darned thing won't get to space even when empty. Judging from the fuel layout, there seems to be plenty to orbit, so it has to be my piloting at fault. Any tips to help?

[AeroGav]

You've not given us much to go on, is this craft on kerbalX ?

I'm only just getting into cargo mk3's myself, i've finally got something in beta that looks promising .

How much speed are you getting in air breathing mode on that thing?  It's got a huge TWR for an aircraft,  and mk3 are very heat resistant, so it should go over 1400, possibly well over that.  If things do get hot, you can climb higher in your speed run, but as you get above 21km power really falls away quickly.  

Once you hit rocket mode, don't yank the stick back (what i did when i was newb).  Just gradually pitch up to 5 degrees above prograde , 10 degrees at the very most, where lift/drag ratio will be at its best.

I have some video footage of me flying stuff to orbit but they're nothing like that ship,  much more wing area , much less thrust relative to weight, and rely heavily on nuke engines to get to orbit.

the first 5 minutes is construction, then actual flying from then on, going supersonic about 6.30.    

I've also got this old passenger mk3 which lumbers to orbit like a dc3,  but again i doubt it flies anything like yours

 

[NightshineRecorralis]

@AeroGav It is now,

https://kerbalx.com/NightshineRecorralis/Kendrel

[NightshineRecorralis]

The previous generation worked fine, but I really wanted a Rapier only design, but this one isn't working out.

Edited November 9, 2016 by NightshineRecorralis

[AeroGav]

18 minutes ago, NightshineRecorralis said:

@AeroGav It is now,

https://kerbalx.com/NightshineRecorralis/Kendrel

Well, here's my WIP

https://kerbalx.com/AeroGav/Storm-Badger

Went to 29km on it's maiden flight then half the engines switched automatically, half did not, sending me into a spin.    Have just fixed action groups but was going to try and go for orbit in the morning.  I'll play with yours instead because i'm fed up of the sight of mine.

[NightshineRecorralis]

@AeroGavIndeed, I saw that, let me see what I can do with it ^^

[NightshineRecorralis]

19 minutes ago, AeroGav said:

Well, here's my WIP

https://kerbalx.com/AeroGav/Storm-Badger

Went to 29km on it's maiden flight then half the engines switched automatically, half did not, sending me into a spin.    Have just fixed action groups but was going to try and go for orbit in the morning.  I'll play with yours instead because i'm fed up of the sight of mine.

It can totally get to orbit when empty, that is for certain.

So, what gives on mine? Can you tell?

[AeroGav]

OK, i got up to mach 5 or so airbreathing then switched mode and got sub -orbital.   Had a 120km ap, but was unable to perform the circularization burn as it has no reaction wheels. 1500 lf and 3800 oxidizer remaining.   

It's a bit tricky to fly in that it needs a lot of roll corrections, and attempting to feed those roll corrections in causes adverse yaw at higher altitudes.   

It seems to have more drag and less lift than mine, but it does have good power.   I was able to get lift drag ratios of about 1.4 or so, at times less than unity. 

Tbh mark 3 and mark2 fuselages are very draggy and this isn't uncommon, normally people just brute force it with more engine.

You can try adding more wing so you're at higher altitudes/lower aoa for any given speed, or you can try angling the wings up 5 degrees so the fuselage angle of attack is lower, but that's a major rebuild.  I did both on  mine which does cause a "roller coaster" effect down low.

Have you got cones on the back of the engines?

[NightshineRecorralis]

@AeroGavNever needed cones, as with my previous builds, The reaction wheels can certainly be added in to aid stabilization. What sort of ascent profile did you use?

[NightshineRecorralis]

@AeroGavHere's my best attempt so far. She's empty, and was just 30m/s short from orbit.

[AeroGav]

Ok, these are the mods i'd do if it was my ship

1.  created a T-tail by putting big-s wings horizontally attached to vertical stab.   moves CoL aft and gives us more LF capacity, brings the dry CoM closer to the full one

2. moved the tailplanes to the front as canards

3. replaced some of the strakes with big -s ones for extra LF

4. dihedral on the outboard main wing, on the t-tail and the canards, for a bit of passive roll stability

5. replaced the ailerons with the biggest ones available (big S) and then turned their authority down to 36%.  Why?  Because a big aileron operating at a lower deflection angle, generates less adverse yaw than a small one operating at 100% when trying to pick a wing up

6. cones on the back of the engines for lower drags

7. reaction wheels...   cuts down on some of our cargo space sadly.   also batteries went inside in the name of streamlining

https://kerbalx.com/AeroGav/Kendrel2

EDIT -  i got this version to orbit with 3416 ox  and  3481 lf

i keep on nearly melting the nose cone off, trying to get more speed in the atmosphere.  Think we maxed out at 1500 and 21km this time.   Maybe the probe core should be in the safety of the cargo bay as well, at least when i'm flying

Edited November 10, 2016 by AeroGav

[Jarin]

Your design looks vaguely similar to my old Archon series that made up the backbone of my 1.1.3 space program. I just got a similar craft flying in my 1.2 plane-only career, so I know the design is still solid. It's running on 8 rapiers like yours, so I'll give this a shot to see if it's flight profile or drag that's causing the trouble.

Edited November 10, 2016 by Jarin

[Jarin]

1 hour ago, NightshineRecorralis said:

Here's my best attempt so far. She's empty, and was just 30m/s short from orbit.

Okay, I loaded up the craft file in my sandbox save and blind-launched from the runway without entering the SPH.

Spoiler

Got it into orbit with 300m/s to spare, but dear god did it fight me for every inch. The simplest change I'd make to your design is less wing, and then give the wings you DO have some angle of incidence. Gvies you extra lift without having to angle the entire body so you have less drag. Once you have Big-S parts, you really want to minimize the use of old-style wings as much as possible. Unless you just really want that visual design, in which case you just over-engineer in the name of style. Still angle those wings a degree or two regardless, though.

You'll note in the picture above that I was actually short of LF rather than Ox, which is the reverse of what you want. Reducing drag should reduce the LF needed to get to orbit, which should help there, but you might still consider a Mk3 LF tank behind the cockpit.

Other minor tweaks:
-Move the tailfins out of the way of the engines.
-Get a radiator on that thing somewhere. Just one of the small deployable ones is all you need, but trust me, even if no heat bars are showing, that plane is still cooking there in that picture. If you only went one orbit and tried to land at KSC with it, you'd still have most of that internal heat and it would burn hard on the way down.

Non-tweak commentary:
I love the landing gear design. Even if it bounces awkwardly on the runway, it's got style.

 

Edit: Crud, I almost forgot to actually tell you the flight profile that got to orbit. Just run flat and level after take-off until you're in the 500-600m/s range, so your rapiers are at full thrust, before starting to climb. Then just keep yourself at about a 10-15 degree climb. You should continue accelerating all the way up, hitting 1300~ish before the engines start winding down. Adjust your climb rate accordingly. Watch that spedometer, and the instant the m/s stops climbing due to altitude, flip the engines. Then just set SAS to follow prograde until AP is in space.

Edited November 10, 2016 by Jarin

[NightshineRecorralis]

3 minutes ago, Jarin said:

Snip~

Okay? Less wing, but which should I remove? I don't have that much pitch authority in the first place. The tailfin I can see, but the radiator isn't needed. Used some cheats (Not the heat one) to orbit and came down through with a full load of fuel half an orbit later. Got some bars into the yellow but she was sound all around, so I'll pass on that one. I'll tweak the LF content, for sure.

Also, would you mind showing me your ascent profile? Cuz mine sure as heck is inefficient.

[Jarin]

1 minute ago, NightshineRecorralis said:

Also, would you mind showing me your ascent profile?

Edited to give the ascent profile just before you posted. Mostly I'd remove all the non-tank wing parts amidships. If you need pitch authority, move those Big-S tailfins further back and up. 

[NightshineRecorralis]

2 minutes ago, Jarin said:

Edited to give the ascent profile just before you posted. Mostly I'd remove all the non-tank wing parts amidships. If you need pitch authority, move those Big-S tailfins further back and up. 

Thanks a ton, mate! I'll go change a few things and see if I can work it out. If not I always have other SSTOs to use

The wheel config was mostly for looks XD, the large three wheeled one looks awkward cuz it juts out the rear. Also, two wheels per side had a tendency to break, so a little touch of necessity there.

[Jarin]

5 minutes ago, NightshineRecorralis said:

Less wing, but which should I remove?

Here's what I've got on mine:

Spoiler

And again with the engine nacelles shoved aside so you can see

 

[NightshineRecorralis]

@JarinI hope you don't mind if I use that design on an upcoming SSTO ^^

[Jarin]

Borrow away. The Archon was the result of shameless theft from @mk1980 and @bewing, so it's tradition.

Edit: Also, the above ascent profile is what I use for basically every SSTO I build. All that changes is how long I accelerate at sea level. Less draggy craft go faster, lower, before pitching up.

Edited November 10, 2016 by Jarin

[NightshineRecorralis]

@Jarin Well, I did finally get it into orbit with some tweaked wings and proper engine timing ^^

Still not a big margin, though, I wonder if I'm actually gonna use this as a Cargo SSTO, perhaps with some proper fuel ratios...

[Jarin]

46 minutes ago, NightshineRecorralis said:

perhaps with some proper fuel ratios...

After reducing drag, the next step towards making this a proper cargo craft is increasing carried fuel overall. The tyranny of the Rocket Equation will come into effect if you scale up too much, but enough LF to comfortably reach 1400m/s before the air-breathers cut out will help a lot. That'll be the point you want to tweak for. 1400 on air, then just enough LFO to get where you need to go and back. Extra LF for landing maneuvers as you prefer (I tend high here, since I always want to be able to hit the runway, even if I over or undershoot). Feel free to slip in a third pair of Big-S wings somewhere.

Edited November 10, 2016 by Jarin

[AeroGav]

6 hours ago, NightshineRecorralis said:

@AeroGavNever needed cones, as with my previous builds, The reaction wheels can certainly be added in to aid stabilization. What sort of ascent profile did you use?

Jet engines like the Whiplash have very low drag, similar to nose cones,because they don't have a rear attach node.  Unfortunately the RAPIER has a rear attach node and if left unoccupied, the game treats it the same way it would if you left a fuselage stack with no tail cone, flat plate drag, and you have 8 of them.

Flight profile -

With any spaceplane, after liftoff I have a look at how much angle I'm needing to keep between where the nose is pointing and prograde, in order to get enough lift to not fall back to the ground.    Kendrel needed a lot of nose up input and was flying quite a big nose angle, so i figured it was intended to go supersonic right down on the deck before trying to climb.

It was obviously much happier once up to 500 m/s so i started to climb to avoid fighting drag in the thick air down low.   Once we got into thinner air above 10k I wanted to level off and pick up more speed, unfortunately the climb had gotten so steep it ended up coasting past flameout altitude even though i'd set Prograde assist at  12km.  The rapiers autoswitched to closed cycle - had i let them continue , we'd have only gotten 800m/s out of air breathing mode.

So, I switched them back manually and just waited for it to arc over for another dip in the atmosphere.   We bottomed out at around 14km doing around 1200m/s,  then went into a more controlled climb.   When heat bars started appearing, i let it climb a bit more.  Eventually milked it up to about 1500 m/s at 21km, by which point we weren't getting a lot of power from the engines,  looking at the aero data gui thrust was barely any more than drag , we couldn't really accelerate except by shallow diving.    At that point, switch to close cycle and hold 5-10 degree AoA until our Ap goes above 70km.

 

The modded version i uploaded on kerbal X was a lot happier in low speed flight, so i let it cilmb subsonic

to about 7 or 8 km, then i hit the prograde button.   It ended up coasting up to about 10km,  accelerating supersonic with the nose level to the horizon. As before, i overshot flameout altitude and had to double did or triple dip  the atmosphere

[Jarin]

37 minutes ago, AeroGav said:

As before, i overshot flameout altitude and had to double did or triple dip  the atmosphere

This is why I tend to do more of my acceleration low, even with drag losses. I've always had trouble with other ascent profiles. Either I bounce like you were, or I manage to level out and accelerate at 10km but have trouble getting the nose up again without losing a lot of hard-won velocity.

I will definitely have to try the small nosecone thing for rapiers, and see what kind of performance boosts I can wring out of it.

[AeroGav]

28 minutes ago, Jarin said:

This is why I tend to do more of my acceleration low, even with drag losses. I've always had trouble with other ascent profiles. Either I bounce like you were, or I manage to level out and accelerate at 10km but have trouble getting the nose up again without losing a lot of hard-won velocity.

I will definitely have to try the small nosecone thing for rapiers, and see what kind of performance boosts I can wring out of it.

The bouncing gets better if you dance between SAS and prograde to try hold the nose at a more "average" level.  It's annoying as hell and wastes time, but the parabolic flights over 30km aren't really costing any fuel or drag, we're just coasting on prograde with the engines flamed out.   I tried my storm badger with a full load and it seems less skittish.  Has the game got a lot easier in 1.2 or something?

hmm 130 tons loaded.

I'd reckoned on 1 rapier and 1 nuke per 30 tons of launch weight, but the ship was overall lighter than expected.

Do I try make a 4 engine version, or just stretch the cargo bay ?

 

Edit - also @Jarin it's going to depend on the design of your craft esp. wing loading when best to do your acceleration

If you have more wing area, you can go higher while staying subsonic because your induced drag will be less (lower angle of attack),  but you won't do so well trying to accelerate at sea  level because you have more parasitic drag

Edited November 10, 2016 by AeroGav

[AeroGav]

36 minutes ago, Jarin said:

 

I will definitely have to try the small nosecone thing for rapiers, and see what kind of performance boosts I can wring out of it.

Not the small nosecone.   Use the 1.25m part, then offset it inside the engine so it's not in the way.

The cone must be the same diameter as the attachment node of the engine or most of the benefit is lost.