Having Trouble With Heavy SSTO

[Kenny Loggins]

Hello, I've been having issues with my large cargo SSTO. I can't seem to get it to orbit. I'm having both balance issues and issues actually establishing orbit. Although, it has come a long way since I started building it a few months ago, and it gets much closer to orbit than it used to, and it doesn't flip backwards immediately upon takeoff anymore, so that's cool. Anyway, any help would be greatly appreciated!

https://kerbalx.com/kennyloggins/Zeus-Mk1

^Pictures and craft file^

[AeroGav]

1 hour ago, Kenny Loggins said:

Hello, I've been having issues with my large cargo SSTO. I can't seem to get it to orbit. I'm having both balance issues and issues actually establishing orbit. Although, it has come a long way since I started building it a few months ago, and it gets much closer to orbit than it used to, and it doesn't flip backwards immediately upon takeoff anymore, so that's cool. Anyway, any help would be greatly appreciated!

https://kerbalx.com/kennyloggins/Zeus-Mk1

^Pictures and craft file^

I only have an hour of playtime tonight , so can have a proper look at your craft on Sunday.

First things after downloading it -

Drag -

• The mk3 engine mount has 3  x 1.25m nodes and a central 2.5m node in the middle.   By not using the 1.25m nodes you are creating "flat plate" drag with them.  Put whiplash on these 3 nodes perhaps.
• the central 2.5m node you have attached a quad adapter to.   Swap to a tri coupler instead , they have a lot less drag for only 33% less engines

Overall it probably has too many whiplashes , you only need about one per 30 tons if your craft is slick.

 

Weight and Balance -

RCS build aid shows that you have most of the fuel up front, CG moves aft as the tanks drain and it becomes unstable when empty.

Also your cargo bay is at the front, when full you will be nose heavy, when empty, unstable.

Move the pax to the front of the main fuselage, behind the cockpit,  put the cargo bay around CG and try to get a bit more fuel near the back.

My guide may have some info that helps you though it's a big read

Or try these two craft i made on KerbalX,  one is a liquid fuel only cargo lifter mk3,   the other is a heavy passenger liner

https://kerbalx.com/AeroGav/Lusitania

https://kerbalx.com/AeroGav/partridge

[AeroGav]

Alright, it's bedtime for me.

This is as good as I could get it , within an hour.

https://www.dropbox.com/s/0c0otfzirpvbubu/Zeus Mk1.craft?dl=0

Goes to space ok, when empty.

You had a huge amount of quad RCS blocks clipped into cockpit.   This does not prevent them causing drag !

The side pods weren't really worth it, caused a lot of drag.

Wings hardly cause any drag, fuselages, especially fat ones, make a lot.

I had to add a bit more wing, then angle the wings up a bit, to get acceptable performance.   This allows the fuselage to fly flatter to the airflow, and at higher altitude.

To fly it, set prograde hold on SAS as soon as possible.  This will make a shallow climb with the nose close to prograde.  To climb steeper, take SAS off and the built in nose up trim of the design makes the nose rise a bit more.

[bewing]

2 hours ago, AeroGav said:

Overall it probably has too many whiplashes

Heh. Those aren't whiplashes, they are Dart aerospikes.

 

[Kenny Loggins]

Wow, so I was WAY underestimating those whiplashes, I thought it was 1 engine-15 tons, gonna make some serious adjustments with that in mind. Also, the side pods are there for the passenger cabins, and the idea with the fuel is that most of it should be used in space, not to get there, hence why the balance while empty wasn't a tremendous concern at the time of design. 

Side note, how much mass can a single nerv handle? do i need to adjust the number of those I have?

Edited October 20, 2017 by Kenny Loggins

[Spricigo]

What is the design goal? what you want to do with the craft?

Crafts are tools, they are made to perform certains tasks. You may notice that tools for different task are very different, just compare a hammer, a screwdriver and a plier. Same goes for craft: a heavy lifter to LKO will be a lot different from a long range science gathering probe. What makes the tool (craft) good for one task may make it terrible for different task.

Your craft seems to be intended to do "everything" and, as result ended up being good to nothing. I think you really need to rething the objetives and narrow it down to something that you can optimize the craft to do. (e.g. cargo lifter OR tourist bus OR long range exploration)

8 hours ago, AeroGav said:

You had a huge amount of quad RCS blocks clipped into cockpit.   This does not prevent them causing drag !

Also, 4 cylindrified MP tanks clipped in the rearmost fuselage.

[KerikBalm]

11 hours ago, AeroGav said:

I only have an hour of playtime tonight , so can have a proper look at your craft on Sunday.

First things after downloading it -

Drag -

• The mk3 engine mount has 3  x 1.25m nodes and a central 2.5m node in the middle.   By not using the 1.25m nodes you are creating "flat plate" drag with them.  Put whiplash on these 3 nodes perhaps.
• the central 2.5m node you have attached a quad adapter to.   Swap to a tri coupler instead , they have a lot less drag for only 33% less engines

Weight and Balance -

RCS build aid shows that you have most of the fuel up front, CG moves aft as the tanks drain and it becomes unstable when empty.

Also your cargo bay is at the front, when full you will be nose heavy, when empty, unstable.

Move the pax to the front of the main fuselage, behind the cockpit,  put the cargo bay around CG and try to get a bit more fuel near the back.

Well, I'm going to have to take a bit of issue with the 4x vs 3x adaptor as general advice. I don't think you should compare the 3x part to the 4x part, but rather the entire stack drag. What will have more drag, 4x stacks with 3x adaptors on them, or 3x stacks with 4x adaptors on them? If one needs more power, if adding an entirely new stack to get an additional node is going to increase drag more than just going to a 4x adaptor, then you should go with the 4x adaptor.

You want to maximize the thrust to drag ratio of the entire craft, not the adaptor.

I didn't use the detailed aero displays that you did, but I did some testing just seeing how fast I could go, and I'm pretty sure the 4x adaptor was beating out the 3x adaptor designs... although it was a while ago. I also avoid major part clipping, so I'd actually make entirely new stacks (like an engine precooler with nosecone/shock instake, to allow 1 more engine to be mounted), as opposed to using a surface attach part to make a node for a rapier/whiplash, and then clip that into the others.

But I agree on the balance points, you want your cargo stored near the CoM. I would also advise moving engines forward as much as possible.

This SSTO works on a 3x rescale of kerbin (but with a 1.5x atmo rescale, but I don't think it makes such a difference, although drag is significant when you'r going through the atmo at 4km/s)

Note that many of its engines are mounted near the CoM.

Its also got an unusual feature - related to not using a carobay - where the rearmost engines move forward before reentry.

The rear part undocks from the payload, and redocks with the main ship:

This illustrates some principles...  most of these engines are near the CoM/CoL - but don't use it as a model for stock kerbin. This thing needs to get to over 4km/second in the atmosphere, but uses 100% stock parts to do so (its got 1 non stock part, a TAC-LS container to improve the time it can stay in orbit with kerbals, but that's just dead weight and extra drag for short cargo delivery flights). It reaches >1,500 m/s surface velocity, but still needs to accelerate another >2,600 m/s, losing about another 1,000 m/s at least due to aero drag while accelerating and climbing from the rapier cutoff point around 30km - this is not what a stock kerbin SSTO needs (in a stock system, this SSTO can deliver payload to minmus rather than LKO)

Here's one of my older stock system stock designs:

Although it was rather lacking in wing area, note that the distribution of engines and fuel tanks is such that the CoM doesn't really shift much. The cargobay is right in the center so that the cargo mass also doesn't really shift the CoM

 

[Kenny Loggins]

My reasoning for the quad-coupler in the back is that when i used the base tri-configuration that the mk3 engine mount was resulting in a low center of thrust and flipping my craft upwards and backwards as a result. I don't know how to fix that without a coupler to even out the thrust.

[AeroGav]

18 hours ago, Kenny Loggins said:

Wow, so I was WAY underestimating those whiplashes, I thought it was 1 engine-15 tons, gonna make some serious adjustments with that in mind. Also, the side pods are there for the passenger cabins, and the idea with the fuel is that most of it should be used in space, not to get there, hence why the balance while empty wasn't a tremendous concern at the time of design. 

Side note, how much mass can a single nerv handle? do i need to adjust the number of those I have?

It all depends on how drag-resistant your design is,  but as a rule of thumb,   one Whiplash is ok for up to 30 tons,  one NERV for about 15 tons.    If the craft is very slick you can push a nerv to 20-25 tons perhaps.

Alternatively,  I quite like using one Panther and one Rapier per 60 tons ,  instead of two whiplashes.    The Panther's strong low speed performance gets you to the point where the RAPIER starts to get a good ramjet boost going.      One panther and one rapier weighs 3.2 tons, which is slightly lighter than two Whiplash (3.6 ton)  and will give higher top speed at higher altitude.   The Whiplash fades very quickly after 900 m/s, the  Rapier is strong till 1400+.

,

Quote

and the idea with the fuel is that most of it should be used in space, not to get there, hence why the balance while empty wasn't a tremendous concern at the time of design. 

Even with no cargo bays and small crew cabins, and a very efficient design, you're likely to burn at least half your fuel getting to  space.   A ship like yours with huge cargo bay and lots of cabin,  is going to be mostly empty by the time it hits 70km.    Also,  how are you going to land it at the end of your mission, if it won't fly in an atmosphere when empty?

7 hours ago, Kenny Loggins said:

My reasoning for the quad-coupler in the back is that when i used the base tri-configuration that the mk3 engine mount was resulting in a low center of thrust and flipping my craft upwards and backwards as a result. I don't know how to fix that without a coupler to even out the thrust.

SQUAD thought of that when they designed their tricouplers.    You may notice, the "bottom engine" is twice as far below the middle of the mount than the  upper pair are above it, which evens out the torque.   To prevent the top two enignes interfering they are separated quite a bit left to right.     Tri couplers are perfectly balanced - make a rocket with one, you will see it flies straight.

[AeroGav]

One more thing I've noticed,  is that chemical fuel spaceplanes have very powerful engines in rocket mode (RAPIER in rocket mode is 3x the thrust of a NERV),  so they are not so critical about drag, provided they can get over mach 1.  In rocket mode, you just boost straight up to quickly get out of the atmosphere, then finish off accelerating to orbital velocity. 

However, they need lots and lots of fuel, so you need big bulbous fuselage pieces to carry it all.

NERV only ships have much weaker thrust in rocket mode.    They need low drag or they fail to accelerate at all.       They need plenty of wing so they can make enough lift to stay high up where drag is less, whilst keeping the nose pointing close to prograde.          The fuselage needs to be as small as possible. 

But,   the NERV engines use so little fuel you can go to space using only the stuff that's in your wings.   The fuselage should be used for crew and cargo, with most of the fuel in wings and strakes.

 

Edited October 20, 2017 by AeroGav

[Spricigo]

6 hours ago, AeroGav said:

 In rocket mode, you just boost straight up to quickly get out of the atmosphere, then finish off accelerating to orbital velocity. 

I think "straight up" was an unfortunate choice of words. As long you have enough upward moment to not fall back into atmosphere before circularization, keep your focus on building up horizontal velocity.

 In some sense that last step to orbit is a race against gravity. 

6 hours ago, AeroGav said:

 

[Chemical] need lots and lots of fuel...

...

But,   the NERV engines use so little fuel ...

Let's be clear there. For a given target deltaV, it defends on two factors. Isp and mass fraction. Nervs have a clear advantage in Isp and chemical have a clear advance in mass.

So "what engine consume more fuel?" is yet another of those questions to be replied with "depends". 

[Kenny Loggins]

Alright, I've made quite a few changes to the design, and it comes MUCH closer to orbit than before. 

https://kerbalx.com/kennyloggins/Zeus-Mk2

Any input on the new design?

[AeroGav]

12 hours ago, Kenny Loggins said:

Alright, I've made quite a few changes to the design, and it comes MUCH closer to orbit than before. 

https://kerbalx.com/kennyloggins/Zeus-Mk2

Any input on the new design?

i actually got it into space on my first try, but I did change the staging order.     You'd set things up with action groups so the Whiplashes are shut down when the NERVs are started.   Why not just leave the Whiplashes on until they quit ?  Every little bit of thrust they provide is helping, even if you are operating above an altitude where they provide sufficient thrust on their own.

Regarding drag

• may as well replace those structural fuselages with mk1 liquid fuel tanks, since they are the same drag but actually hold fuel
• the solar panels make a lot of drag even when stowed, better to put them in the cargo bay then open up in orbit.  
• You are not using the central 2.5m attachment node of the mk3 engine mount, leaving it empty creates a lot of flat plate drag.   Put a 2.5m tricoupler on here and move some of those radially attached engines off the rear passenger cabin onto this tricoupler.    The engines that are directly mounted to the mk3 mount can be offset outward with the offset tool to make room for all of this.

Edited October 22, 2017 by AeroGav

[AeroGav]

Here's my version of Zeus mk2, I've put a tricoupler on the unused attach node, moved some of the engines on to that.    The structural fuselages are replaced with a pair of liquid tanks with mirror symmetry either side of the engine mount,  and type b nose cones with 4-way symmetry mounted around the mk3 engine mount.   We have space for 1 extra NERV than before, but the drag is greatly reduced from the mk3 mount  - 7kn instead of 57kn

This gives us an extra 500dV in orbit.    

More importantly, acceleration and climb are stronger (you'll notice from the timer, we got to orbit at least 4 minutes sooner) so you could add more liquid tanks in the cargo bay if you needed extra fuel, and it would have the power to lift it.

https://www.dropbox.com/s/ookvbdlnamk1nf8/Zeus Mk2 bis.craft?dl=0

I would say that the location of the cargo bay is still not ideal,  with a payload it's going to shift the CG fwd and become nose-heavy.  But this is turning into quite a nice airplane all the same.

[AeroGav]

I've made a third version of the OP's ship.         I moved both passenger cabins to the front and the cargo bay to the back.   This puts the centre of mass in the middle of the cargo bay.  There is now more fuel aft then up front, so a short mk3 liquid fuel fuselage goes between the cargo bay and passenger cabin.      The wing is unchanged and our CoM significantly further forward, so i added canards.    These are angled up so it maintains a slight positive pitch angle when left to its own devices.   You can fly this with Prograde hold set.   

 

Action group one deploys the inboard elevons , which depresses the nose a few degrees - useful for keeping the deck angle to something sensible at low altitude,  for levelling off to break the sound barrier,  and for the speedrun.

Action group 3 toggles the nukes on and off, in case you want a temporary burst to get through the sound barrier.   OR just trigger them with staging at 1350m/s to turn them on and leave them on.

I also swapped out two Whiplashes for Panthers and the other three for RAPIERS (air breathing mode only).

Took it for a spin,   it can bring spacestation components to orbit along with all those passengers, so it does have "jack of all trades" utility.   Barebones atm though, no RCS, no electrical system fitted.

https://www.dropbox.com/s/hpy0j41tqnargsj/Zeus Mk3.craft?dl=0

 

[Kenny Loggins]

Quote

I also swapped out two Whiplashes for Panthers and the other three for RAPIERS (air breathing mode only).

 

Does this significantly change the performance of the craft? I understand the Panthers are better for getting up to speed but are the RAPIERS worth it in air-breathing only? I was under the impression they were less powerful/efficient than Whiplashes, and made up for it by having a rocket mode.

Also, I'm having issues getting to orbit with the craft as Zeus mk2, but only because I am struggling to keep it stable when I change flight angles. For example, when I pitch up to make better use of the nukes. it tends to yaw left or right, and I struggle to correct it, and lose speed while fixing it. At first, I thought it was because in the thin air the control surfaces aren't particularly effective, and the nukes do not have gimbal, so I added some reaction wheels to aid in maneuverability outside the thick parts of the atmosphere, and it certainly helped a bit, but it did not make enough of a difference to stop the craft from reacting violently to attitude changes. It is worth noting that this was a problem I struggle with using my smaller SSTO as well, but the gimbal from the RAPIERs is enough to force the craft to remain relatively stable. Any advice pertaining to this for my spaceplanes?

Edited October 23, 2017 by Kenny Loggins

[AeroGav]

Just now, Kenny Loggins said:

Does this significantly change the performance of the craft? I understand the Panthers are better for getting up to speed but are the RAPIERS worth it in air-breathing only? I was under the impression they were less powerful/efficient than Whiplashes, and made up for it by having a rocket mode.

Also, I'm having issues getting to orbit with the craft as Zeus mk2, but only because I am struggling to keep it stable when I change flight angles. For example, when I pitch up to make better use of the nukes. it tends to yaw left or right, and I struggle to correct it, and lose speed while fixing it. At first, I thought it was because in the thin air the control surfaces aren't particularly effective, and the nukes do not have gimbal, so I added some reaction wheels to aid in maneuverability outside the thick parts of the atmosphere, and it certainly helped a bit, but it did not make enough of a difference to stop the craft from reacting violently to attitude changes. It is worth noting that this was a problem I struggle with using my smaller SSTO as well, but the gimbal from the RAPIERs is enough to force the craft to remain relatively stable. Any advice pertaining to this for my spaceplanes?

Given that I got your Zeus mk2 into space first try without making any modifications,  i think we need to look at your piloting !

The key is

1. stay close to prograde for minimum drag
2. avoid violent manuvers
3. get to at least 1100 m/s before starting the nukes.    Try to level off a bit near 16 or 17km to get max airbreathing top speed,  but don't worry if you shoot past 17km when still doing less than 1000 m/s.   Just wait for the plane to dive back down again.   On an airbreathing engine, fuel consumption is proportional to thrust so although the engines loose thrust and become very weak on this high altitude excursion,  the amount of fuel wasted is negligible.  It's only really time you are wasting.   Only stage the nukes in once you've gotten over 1000 m/s.   
4. once on nuke power, refer to rule 1 and 2.

If you have a joystick, set SAS to prograde hold and turn down joystick sensitivity or control authority on the control surfaces so you are only pitching up or down two or three degrees from prograde.

You absolutely don't want to pitch up to a steep climb when on nukes.   Just maintain a nose angle that's as close to prograde as possible without actually being below it, for best lift:drag ratio.  At 36km the navball flips to Orbit mode - best switch it back to Surface to get most optimal flight path (until over 70km). 

If you are flying on keyboard , this is what i'd do to modify your craft for easier control -
Adjust the elevons on the trailing edge of the wing to neutral angle, with the rotate tool in absolute mode.   This provides built in nose up trim so the plane files slightly nose up when set to prograde hold.    

Spoiler

I created an action group on the inboard elevons so they deploy in a nose down direction.   This way i can leave it on prograde hold, and not touch the pitch controls, but by toggling this action group i can stop the climb getting too steep at low altitude, stop it flying above 17km too early, stop it climbing when trying to get through the sound barrier etc.   

 

Forcing the nose down like that is less efficient aerodynamically but something you have to do to make sure you stay low enough to get air for your jet engines.  Once you're on the nukes, efficiency is everything, and stop using this nose down trim.

 

..................I wrote quite a lot on this subject here - 

 

I also recorded a youtube video of me flying my most recent spaceplane to orbit,   you can see the general idea - close to prograde, no violent pitch changes

 

[Kenny Loggins]

I was able to get it into space, gonna make some changes and post the mk3 when it's done. 

[Kenny Loggins]

On 10/25/2017 at 6:38 PM, Kenny Loggins said:

I was able to get it into space, gonna make some changes and post the mk3 when it's done. 

Alright, took longer than I thought it would due to classes, but here it is, the Zeus Mk3: https://kerbalx.com/kennyloggins/Zeus-Mk3 

Having Issues getting into space, but I'm pretty sure it's due to piloting error more than serious construction issues. Any glaring problems with the new design? Any changes you would make? Any piloting advice? I can't seem to angle the craft in such a way as to increase the altitude once I'm past the atmosphere, it just kinda reaches apoapsis and then descends. Is it a problem with not enough acceleration? Do I need more thrust?

[AeroGav]

1 hour ago, Kenny Loggins said:

Having Issues getting into space, but I'm pretty sure it's due to piloting error more than serious construction issues. Any glaring problems with the new design? Any changes you would make? Any piloting advice? I can't seem to angle the craft in such a way as to increase the altitude once I'm past the atmosphere, it just kinda reaches apoapsis and then descends. Is it a problem with not enough acceleration? Do I need more thrust?

One obvious problem,   of the two engines at the front, the top pair don't produce any thrust due to their exhaust being blocked by the wing.   You can easily test that yourself on the runway.  Start the top pair, the craft doesn't move.   With the bottom pair it slowly accelerates.

Solved that with the offset tool, moved the top engines two notches up, and the bottom engines two notches down, for balance.

Also, why the hell are you still setting up your action group to kill the whiplashes when starting the nukes ?  Leave them on until they shut down themselves, you're depriving yourself extra thrust 

Edited November 1, 2017 by AeroGav

[Kenny Loggins]

15 minutes ago, AeroGav said:

One obvious problem,   of the two engines at the front, the top pair don't produce any thrust due to their exhaust being blocked by the wing.   You can easily test that yourself on the runway.  Start the top pair, the craft doesn't move.   With the bottom pair it slowly accelerates.

Solved that with the offset tool, moved the top engines two notches up, and the bottom engines two notches down, for balance.

Also, why the hell are you still setting up your action group to kill the whiplashes when starting the nukes ?  Leave them on until they shut down themselves, you're depriving yourself extra thrust 

I don't use that action group for ascent anymore, just to turn off the whiplashes once I reach orbit. (Not that it makes a difference, I just don't like seeing the engines in orbit)

Also, totally didn't realize that about the top engines, better fix that XD

[AeroGav]

1 hour ago, Kenny Loggins said:

  I can't seem to angle the craft in such a way as to increase the altitude once I'm past the atmosphere, it just kinda reaches apoapsis and then descends. Is it a problem with not enough acceleration? Do I need more thrust?

It won't help having some engines that don't produce thrust.    You have to remember though, that it has a thrust to weight of less than 1, so you need to thrust horizontal and use lift from the wings to keep you airborne.  As you get closer to orbital velocity, orbital free fall will support more and more of your craft's weight, which means the wings will be able to lift you higher into thinner air where drag is less.    But you need to be able to  make enough lift without having so much drag you can't accelerate.

One of the challenges with your plane is controlling the climb under jet mode.    Under jet mode it has so much lift, it wants to jump above 17km before you've hit 1100 m/s, you have to apply quite a lot of nose down trim to stop it bobbing out of the atmosphere.   If it does, just wait till it comes back down.  Don't start the rockets till over 1000 m/s.

Once on rocket power, raise the nose gently so it's just above prograde or right on prograde.

Using CorrectCoL,  I made a slightly tweaked version of your craft you can just control the nose with action groups..   i'll upload a video so you can see me fly it

[Kenny Loggins]

I just noticed something, it appears as if some of my wings are generating negative lift. For instance it will say -5.28 kn on a wing. Is this important?

[AeroGav]

27 minutes ago, AeroGav said:

Using CorrectCoL,  I made a slightly tweaked version of your craft you can just control the nose with action groups..   i'll upload a video so you can see me fly it

The craft is here https://kerbalx.com/AeroGav/Zeus-Mk3

They key to flight is to make sure you get a good amount of speed out of the airbreathers before starting the rockets,   then to try and keep the climb rate somewhat moderate when you've finished with the speedrun.   Once on nuke power, it is all about having fine enough control of pitch that your nose is only a degree or so above prograde.  When you start getting to 5 degrees above prograde, drag doubles.   At the same time, flying with the nose below prograde is also bad.

I adjusted the airplane so it flies close to prograde without any input from the pilot.       You can fly to space with SAS set to prograde hold, and you can control climb rate by setting action groups

action group 1 - droops canards, raises nose about 1 degree above prograde

action group 2 - neutral trim, all trim surfaces retract

action group 3 - nose down trim, droops inboard elevons to push nose a couple degrees below prograde marker

video needs another 10 minutes to upload sorry, bad connection

[AeroGav]

video here -