Ideal flight profile for SSTO? Trying to crack the sub challenge.

[Klapaucius]

I've been tinkering on and off with a modified submersible plane design I already have for this challenge:

 

I can get to space no problem, but I cannot get it into orbit before running out of oxidizer.   The problem, of course, is I am carrying extra weight to make it submersible.  And anytime I add more tanks (even full ones), I need to add more weight to correct the buoyancy.  I could just drop the weight, but that seems like a cheat to me.  While there are still technical challenges to iron out, I am sure I could do a lot better in my flight profile.  I am taking off due east fairly close to the equator (using Mission Builder to place it in the ocean).  What is my idea angle as I leave the atmosphere and switch the Rapiers to rocket mode?

 

The top photo is the six engine variant (in its original form, used for the Mountain Lake Challenge Part II), and the bottom is the four.  The front engines are Panthers which are necessary for the submarine part due to their extreme gimballing and low-speed acceleration.  The rears are Rapiers and on the 6-engine variant, also a set of ramjets since those seem to give you the most highspeed atmospheric performance.

 

[bewing]

Why the heck did you use MK2 parts? They have the most awful drag characteristics for getting an SSTO to actually get to orbit.

Ditch the MK2 for MK1 and rebuild the thing. Then try flying it to orbit. And then I will tell you the flight profile to use.

[AeroGav]

39 minutes ago, Klapaucius said:

What is my idea angle as I leave the atmosphere and switch the Rapiers to rocket mode?

On a design where you're getting rocket propulsion from chemical propellants,  you have high thrust but low isp/limited fuel margins.  There is less scope for flight profile to influence the result than with the NERV oxidizer free designs that i make.   

The main thing is to make sure you're getting maximum speed out of the airplane in airbreathing mode before switching.    You should be able to get at least 1300 m/s,  even with only a pair of rapiers.    Do a level flight speed run at 17-21km for this.    Upward velocity is not worth any extra.

Once you switch to close cycle mode,   remember orbital velocity is about horizontal speed not vertical,  so on the one hand you want to keep the plane level and have the engines thrusting at the horizon.   On the other hand,  you want to get up to higher altitudes  where heat and drag are less.   However,  you also don't want to pitch up too much because that also makes drag.  l

So, after switching to close cycle mode, i'd say just pitch up gently so the nose is pointing  5 degrees above prograde ,  5 degrees is the best angle for lift/drag ratio in supersonic flight.

Given that your screenshot shows you above the atmosphere at only 1800 m/s,  (in surface mode,  that's only 1600m/s true airspeed),  you must have either climbed very steeply (and possibly not hit your true air breathing top speed) or you pitched up excessively after switching mode.      However you are quite a long way short of orbital velocity so i don't think that would have got to space no matter how flown.

2200 m/s is orbital velocity,  if you got 1400 air breathing that means a gain of 800 is needed in close cycle mode,   you are only doing 1600,  some 200 faster than air breathing velocity.

 

I think you'd be better off with a mk1 inline cockpit design (less drag, less buoyancy).   The RAPIER/Panther config i am a fan of, so stick with that.     I don't know if a NERV SSTO like mine would work better in this scenario or not.       On the plus side,  the engines aren't buoyant.    Also,  since they are damn heavy anyway,  the ballast tanks won't hurt delta V so much.   But the feeble thrust means you need good lift/drag ratio to continue gaining speed and height after the jets quit.    That means skinny mk1 fuselage, lots of big s wing parts (buoyancy!)  and requires good control of pitch angle.   Every 2 degrees the nose moves away from prograde doubles your drag.   On RAPIER close cycle power (180kn per engine) you don't notice it so much,  with NERVs (60kn per motor) you will.

It would be easier to help if you share a craft file but i'd probably struggle with the "making it sink" part !

Edited November 27, 2018 by AeroGav

[Klapaucius]

14 minutes ago, bewing said:

Why the heck did you use MK2 parts? They have the most awful drag characteristics for getting an SSTO to actually get to orbit.

Ditch the MK2 for MK1 and rebuild the thing. Then try flying it to orbit. And then I will tell you the flight profile to use.

Well, I did not know that.  Also, as I said, I started with a flying sub that I know works and works well.  The lift it generates also gives it a very low stall speed, which is necessary for water landings.  However, I will try your suggestion.  Thanks 

[AeroGav]

4 minutes ago, Klapaucius said:

Well, I did not know that.  Also, as I said, I started with a flying sub that I know works and works well.  The lift it generates also gives it a very low stall speed, which is necessary for water landings.  However, I will try your suggestion.  Thanks 

The lift is absolutely negligible,  if you press ALT F12 in game, go to Physics, Aero,  and check the box "show aero data in action menus"    and see how much drag and lift each part is making in flight, you'll be shocked at how badly they compare to wing parts.   The hard part of this challenge is the sinking bit.   How much ore does your craft carry to make it sink?  What is the empty weight?

[Klapaucius]

1 minute ago, AeroGav said:

The lift is absolutely negligible,  if you press ALT F12 in game, go to Physics, Aero,  and check the box "show aero data in action menus"    and see how much drag and lift each part is making in flight, you'll be shocked at how badly they compare to wing parts.   The hard part of this challenge is the sinking bit.   How much ore does your craft carry to make it sink?  What is the empty weight?

That would explain my early lifting body failures 

The four engine variant only has two medium ore tanks.  It's got enough thrust to force itself down.  The six engine has one extra set. 

[AeroGav]

OMG !  I've had some success !

Thanks to @Klapaucius  for the ore hint.    That was something i was missing.

I threw this together in a hurry,  and it succeeds on two counts :

https://www.dropbox.com/s/4dqyxyax6ee60qi/Batwing.craft?dl=0

Sinks...

Goes to space

Main fuselage stack consists of shock cone, then a diverterless supersonic intake, then the ore tank (full), then inline cockpit, then a rapier.   NERVs and Panthers held to wing with NCS adapters.

What it cannot do is land in the water without damage.  Always busts its nose cone off  (unless you use unbreakable joints cheat).  Then again, nothing in the challenge rules say it has to land in the water directly.  Could use its wheels to land then taxy in for a dip.         Launch from underwater is a wild ride...  zooms up like a cork and immediately starts flying.

 

 

[Klapaucius]

1 minute ago, AeroGav said:

OMG !  I've had some success !

Thanks to @Klapaucius  for the ore hint.    That was something i was missing.

I threw this together in a hurry,  and it succeeds on two counts :

https://www.dropbox.com/s/4dqyxyax6ee60qi/Batwing.craft?dl=0

Sinks...

Goes to space

Main fuselage stack consists of shock cone, then a diverterless supersonic intake, then the ore tank (full), then inline cockpit, then a rapier.   NERVs and Panthers held to wing with NCS adapters.

What it cannot do is land in the water without damage.  Always busts its nose cone off  (unless you use unbreakable joints cheat).  Then again, nothing in the challenge rules say it has to land in the water directly.  Could use its wheels to land then taxy in for a dip.         Launch from underwater is a wild ride...  zooms up like a cork and immediately starts flying.

 

 

That's so awesome!  Have you posted it to the challenge thread yet?

 

Concerning your water landing, what is your stall speed?  I can usually get mine down no problems by coming in low and just as I am about to stall, pitching up.

Another option is a lot of chutes.   This is video of my run from the Lake Challenge II.  I used chutes to knock off the velocity and avoid trying to do a standard landing in that lake.  It might be a good solution for you.

 

 

[AeroGav]

Stall is just over 40 m/s when full of fuel and ore.    I could add more wing to reduce stall, but that might make it too floaty.       Your mk2 design wins here because the cockpit has a 45m/s impact tolerance, the shock cone on the front of mine only handles 12m/s iirc.    So you'd need floats or hydroplanes to keep it out the water until enough speed been lost.    However , while a cockpit at the front of the ship is good for water impact,  for the heat of the ascent/re-entry,  it's not so great.

TBH,  one chute might be all you need.   Just something to keep the nose pitched up for a half a secod more, by the time it goes into the water drag from the body flopping in has brought the speed down.

[Klapaucius]

38 minutes ago, AeroGav said:

TBH,  one chute might be all you need.   Just something to keep the nose pitched up for a half a secod more, by the time it goes into the water drag from the body flopping in has brought the speed down.

The chutes were only for that challenge so I could just float down evenly. Normally, I can land without them in less confined spaces.

[bewing]

1 hour ago, AeroGav said:

the shock cone on the front of mine only handles 12m/s iirc.    So you'd need floats or hydroplanes to keep it out the water until enough speed been lost.

12 m/s is the vertical speed tolerance. A shock cone can easily handle a 60 m/s splashdown, just so long as you are flying almost horizontal.

[FleshJeb]

1 hour ago, bewing said:

12 m/s is the vertical speed tolerance. A shock cone can easily handle a 60 m/s splashdown, just so long as you are flying almost horizontal.

Just for everyone's edification, the relevant lines from physics.cfg are:
buoyancyMinCrashMult = 0.1 // The downwards component of velocity is used, not the whole velocity, when seeing if a part is destoryed when splashing down. However, that component will be clamped to no lower a portion of total velocity than this value
buoyancyCrashToleranceMult = 1.2 // Multiplier to crash tolerance used when checking if a part is destroyed on splashdown

[Thorn_Ike]

It depends between craft, depending on TWR, CoM/CoL ratio, etc. but usually i take off from KSC and pitch up to 25 degrees up, which reduces drag, but makes sure you're going a decent speed. I suggest you try that, and pitch down if you're not going fast enough. Be going at least 400m/s once you hit 8000m.