Having issue with my SSTO

[Jestersage]

In my continue refinement of SSTO (and to make it replace my rockets), I am attempting to bring an orange tank in my SSTO to 151 km x 151 km.

What happen is that it rolled off one side before disassemble rapidly. Even if by some flying miracle, it never managed to gain enough speed reach orbit.

AFAIK, I didn't have anything offset that majorly. What could be the cause?

I will post a pastbin sometime.

 

EDIT: The pastbin: http://pastebin.com/wJAdUUiL

Edited December 4, 2016 by Jestersage

[Vanamonde]

Pictures of the craft are often enough to diagnose problems. How about some of those? 

[AeroGav]

31 minutes ago, Vanamonde said:

Pictures of the craft are often enough to diagnose problems. How about some of those? 

Craft files are better still though.   A dry CoM problem is not always obvious from the picture, nor an occluded intake, incorrectly attached parts etc.

Then there's the two dozen pairs of four way thruster blocks attached to the fuselage but clipped inside "to reduce drag"  hey why can't i go over mach 1? thing

[bewing]

Raise your wings above your CoM to help with roll stability. Other than that, to get to orbit you need good engines, enough fuel, and a good ascent profile. So we'd need to know all those things.

[foamyesque]

2 hours ago, AeroGav said:

Craft files are better still though.   A dry CoM problem is not always obvious from the picture, nor an occluded intake, incorrectly attached parts etc.

Then there's the two dozen pairs of four way thruster blocks attached to the fuselage but clipped inside "to reduce drag"  hey why can't i go over mach 1? thing

 

I'll have you know I've broken Mach 1, ASL, carrying 6 4-way RCS, 4 linear RCS, 8 vernor RCS, 7 struts, 2 radial attachment points, 2 Clampotron Srs mounted on those radial points, 8 radially attached Clampotrons, a Thud, a ladder, and 4 solar panels. >=|

Edited December 3, 2016 by foamyesque

[Jestersage]

Okay, my pastebin is up. Check OP.

The goal of this SSTO is of follow:

1. Orange Tank challenge
2. Cargo ramp open at the front. Ramp pointing down to allow loading and unloading of wheeled vehicles.
3. Easy flight with minimal user control on pitch/roll/yaw. Expected atmospheric flight will be at 10 degrees above horizon.
4. Minimal goal: 80km with 1k dv left for possible Munar injection. Optimally 1.1k dv at 100km (Possibly unrealistic because if this is good, it will be replaced by a lab+science package -- 1 is just to show it can be done and is a hallmark of good SSTO)
5. Re-entry: Not planned yet. Can't even enter orbit at this point!

Possible misaligned parts, but as far as I know, the only clipping-hiding parts should be a jr dock port, the crew cab, and a drill o matic

Edited December 4, 2016 by Jestersage

[AeroGav]

First attempt.

Before flight, I was a bit concerned about the side pods being ahead of CG, along with the crew cabin, and creating too much surface area up front and the ship maybe having a yaw issue - but that was not the case.  It's a bit pitch unstable when you start getting to higher speeds, but i clearly managed.

RCS build aid thinks the engines are thrusting above CoM , giving a nose down tendency under power.   As the engines fade over 20km, perhaps that's causing the nose to rise?

The lack of cones on the back of engines is causing some excess drag.  As are the non-angled (and possibly slightly small) wings.  And 3 mk3 fuselages in parallel isn't exactly keeping the frontal area down, but it works so...

Also I'm not sure those radial intakes or diverterless supersonic intakes do much for you.   One shock cone can apparently feed two rapiers and the pre-cooler and adjustable ramp (f15 style)  intakes are good on a one for one basis, but the other types of intake fade badly after mach 3 and can't supply rapier past mach 4 no matter how many you spam.

[Jestersage]

1 hour ago, AeroGav said:

First attempt.

Before flight, I was a bit concerned about the side pods being ahead of CG, along with the crew cabin, and creating too much surface area up front and the ship maybe having a yaw issue - but that was not the case.  It's a bit pitch unstable when you start getting to higher speeds, but i clearly managed.

RCS build aid thinks the engines are thrusting above CoM , giving a nose down tendency under power.   As the engines fade over 20km, perhaps that's causing the nose to rise?

The lack of cones on the back of engines is causing some excess drag.  As are the non-angled (and possibly slightly small) wings.  And 3 mk3 fuselages in parallel isn't exactly keeping the frontal area down, but it works so...

Also I'm not sure those radial intakes or diverterless supersonic intakes do much for you.   One shock cone can apparently feed two rapiers and the pre-cooler and adjustable ramp (f15 style)  intakes are good on a one for one basis, but the other types of intake fade badly after mach 3 and can't supply rapier past mach 4 no matter how many you spam.

So that was my craft file without modification? Just need to solve it by flying well?

Anyway, I will remove the extra radial intakes and figure out how to fly the thing...

[AeroGav]

43 minutes ago, Jestersage said:

So that was my craft file without modification? Just need to solve it by flying well?

Anyway, I will remove the extra radial intakes and figure out how to fly the thing...

Unmodified.

I took off, had a look at how much nose up input SAS was using to keep it level.  Turned off SAS  and used pitch trim ALT + S, to trim the nose up to the same point - about 5 degrees above the horizon.  Let it climb to about 5km, by which point we're getting near 250m/s - time to go supersonic.  Reduced the nose up trim with ALT + W so the nose is only 2 or 3 degrees above prograde.  Kept trimming it down more as the speed built up to stop it trying to pull up prematurely.   At 440 m/s , I get it climbing again.      Around 13 or 14km I notice we're going to zoom out of the atmosphere prematurely, use prograde to gently level off.     After that i switch between prograde and SAS to keep the nose where i want it.

When heat bars appear, climb a bit.  When air gets too thin (over 21km) for it to accelerate much more, switch mode.    By this point we're doing over 1500m/s.  From this point on I maintain nose about 5 to 10 degrees above prograde.

There is only enough oxidizier to get us to about 1900 or so.    That's actually pretty well judged.   2 nukes are just - barely able to get us to orbital velocity from there before we start to fall back and burn up.

To make orbit, you have to get 1500 air breathing because oxidizer only adds another 400 velocity before running out.   And you need to be at 1900 when the oxidizer runs out or the 2 nukes are too weak to circularise.

By the way, the pointy mk2 cockpit nearly overheated.  An inline mk2 cockpit would do much better.

[Jestersage]

Thank you for solving it.

To be honest, this is just a demo model. The actual model is one that have only a lab and science stuff, and a rover, which naturally makes it much lighter and easier to fly... but I guess that's what most orange-tank SSTOs are: demo models.

[foamyesque]

3 hours ago, AeroGav said:

 

The lack of cones on the back of engines is causing some excess drag.  As are the non-angled (and possibly slightly small) wings.  And 3 mk3 fuselages in parallel isn't exactly keeping the frontal area down, but it works so...

Also I'm not sure those radial intakes or diverterless supersonic intakes do much for you.   One shock cone can apparently feed two rapiers and the pre-cooler and adjustable ramp (f15 style)  intakes are good on a one for one basis, but the other types of intake fade badly after mach 3 and can't supply rapier past mach 4 no matter how many you spam.

 

One shock cone can actually feed six RAPIERs, though it needs throttle-feathering (or supplemental intakes) to avoid flameouts before 10m/s. The pods do provide additional frontage but, more crucially, they shorten the craft which makes it considerably sturdier longitudinally (less so laterally, but one set of pods is usually fine). Those roll control surfaces are nasty, though. Any control input to them will create far more drag than necessary because of their large angles. I'm a bit surprised that the rearmount RAPIERs/nukes don't pull the CoM back as fuel burns off; have you tried a no-cargo no-fuel flight (via infinite fuel or during a re-entry cruise)?

[AeroGav]

48 minutes ago, foamyesque said:

 

 The pods do provide additional frontage but, more crucially, they shorten the craft which makes it considerably sturdier longitudinally (less so laterally, but one set of pods is usually fine). 

He would have been better off replacing the mk3 with 2.5m fuel tanks for the side pods, less drag per fuel capacity.  Mind you , 2.5m is only available in LF/O  and as the challenge is to bring this tank as far afield as possible he needs to stay mainly LF. 

The forward-most rapiers you can see (on the main wing, alongside the pods) are on nacelles.  The pair mounted on the upper surface of the wing are attached to pre-coolers , which is fine, but the bottom pair are attached to diverterless supersonic intakes.  They are useless above mach 3 so he'd be better off with another precooler or LF tank.

[AeroGav]

2 hours ago, foamyesque said:

 

. I'm a bit surprised that the rearmount RAPIERs/nukes don't pull the CoM back as fuel burns off; have you tried a no-cargo no-fuel flight (via infinite fuel or during a re-entry cruise)?

Yeah i just noticed an unloaded CG prob,    The aft section of cargo bay has irsu gear in it, you really need that stuff fwd somehow to try offset engine mass.  

[Jestersage]

10 hours ago, AeroGav said:

Yeah i just noticed an unloaded CG prob,    The aft section of cargo bay has irsu gear in it, you really need that stuff fwd somehow to try offset engine mass.  

CG issue is definitely with ramp, right? Even if I go with rear ramp, the engine will still be problematic...

Edited December 4, 2016 by Jestersage

[AeroGav]

14 minutes ago, Jestersage said:

CG issue is definitely with ramp, right? Even if I go with rear ramp, the engine will still be problematic...

I am working on a ramp loading cargo mk3 right now , they aren't easy.

Your front ramp design allows you to put engines on rear fuselage but then you need something (that isn't fuel or cargo) at the front to balance the ship when it's empty.    Unfortunately you can't really do that with irsu stuff because that will stop vehicles driving through the ramp to cargo bay.   I did a quick try with the small converter moved to the top , aft of the passenger cabin, and moved drills and ore tanks down to the side pods.  Bit yukky though.  The small converter is really wasteful of ore and electrical power.

I am not sure why this thing has ISRU.      It won't be able to reach another body to mine anyway, a medium kerbin orbit is all it has delta v for.   Ramp loading cargo ssto are probably supporting a base with IRSU.  Your first vehicle payload can be a tanker truck to transfer fuel from the base to the plane so it can make the return trip.    

I did also try making the side pods 2.5m and considered putting a 2.5m converter at the front on the left side, and a hitchhiker , drills and RTG generators in a service bay on the other, then removing the top cabin.  But I soon realised i was in over my head and gave up.

Having some engines on the back is ok  so long as you got some wing/side mounted engines that are shifted well forward to balance them, esp. the heavy nukes.

For example, on this one

The centre of mass is in the middle of the cargo bay.   There's 1 tail mounted and 4 tailplane mounted RAPIERs,  but we also got 6 NERVs mounted well forward on the front wing, which keeps the weight distribution where it needs to be.

 

[Jestersage]

I guess it's going to suck no matter what. The alternative will be similar to the "platypus lifter b" that you mod recently.

[AeroGav]

35 minutes ago, Jestersage said:

I guess it's going to suck no matter what. The alternative will be similar to the "platypus lifter b" that you mod recently.

I must admit I'm having a problem with all my craft looking very similar atm.  I don't know if that's lack of imagination on my part or Kerbal physics forcing certain layouts on us.

I've taken the Platypus lifter off KerbalX now, didn't think it was good enough.   It's been replaced with a clean sheet design, but it ended up looking rather similar to the Wyvern non-ramp cargo by the time i'd corrected all the balance probs.

[foamyesque]

8 minutes ago, Jestersage said:

I guess it's going to suck no matter what. The alternative will be similar to the "platypus lifter b" that you mod recently.

 

No, you've actually got a fairly solid base here and, if you tanked it all the way up, can easily hit Minmus orbit; probably land, if you removed some of the excess oxidizer. I spotted a couple of non-aero design issues as I was fiddling, though:

 

1. Your ISRU has no power source, as far as I can tell;

2. You've got no means to detach your orange tank;

 

[Jestersage]

7 hours ago, foamyesque said:

1. Your ISRU has no power source, as far as I can tell;

2. You've got no means to detach your orange tank;

Oh, I got that a long time ago. Actually it is able to go to Mun if I use the standard configuration, which just comes with a lab module with the RTG, thermal, and solar panels (hence the lack of electrical generator). I am doing the orange tank version just to confirm that it is a good and useful SSTO design.

Mk1 and Mk2 are easy to make (unless we are talking about Duna, but I can't even fly to Duna on standard rockets yet). A useful Mk3 is a different story.

[AeroGav]

5 hours ago, Jestersage said:

Oh, I got that a long time ago. Actually it is able to go to Mun if I use the standard configuration, which just comes with a lab module with the RTG, thermal, and solar panels (hence the lack of electrical generator). I am doing the orange tank version just to confirm that it is a good and useful SSTO design.

Mk1 and Mk2 are easy to make (unless we are talking about Duna, but I can't even fly to Duna on standard rockets yet). A useful Mk3 is a different story.

Mun is way harder to reach than Duna.   The Delta V of making a propulsive landing sees to that, as well as the need to perform a capture burn..   Once you got your AP  up to Mun levels it takes very little extra to leave Kerbin SOI, and from there it's only 150 m/s to Duna.     You can aerocapture into Duna (about 16km PE)  then land dead stick.... if your airplane can manage a Duna landing that is.