Having trouble building a good heavy SSTO.

[CrimsonEclipse]

I want to be able to put an orange tank (36t) into a 100-200km orbit, but so far all my designs fall short in some regard.

My smaller planes have good balance fueled/dry, but as I go bigger and add more engines, it becomes harder to keep them stable during re-entry.

Additionally, for the bigger planes I'm having trouble getting enough TWR to do my insertion burn when laden (I typically run out of LOX for the rapiers before I have a circular orbit but after my apoapsis is in space).

My most successful designs so far (last one is my best small lifter/personnel shuttle): https://imgur.com/a/vY4sDgk

I've probably done a dozen or more iterations but none have worked to a satisfactory level, and I don't know what I'm missing. If there's an obvious flaw with my designs, I'd appreciate being appraised of it.

Some extra miscellaneous questions:

1. Are the "Tail Connector A/B" pieces appreciably more aerodynamic than the "Advanced Nose Cone A/B" and the Shock Cone? They're very pointy, but also kinda heavy so if anyone knows that'd be great.

2. What do you guys reckon is the optimal mass/rapier?

3. What is the optimal wing shape, and how much lifting area/t is recommended?

Please rip my designs to shreds. I desperately need the more efficient price/t of a good heavy lifter SSTO so I can build my Duna mission in orbit.

[bewing]

Welcome to the forums.

You do understand that putting an orange tank in orbit with an SSTO is considered to be an expert-level SSTO design problem?

To become an expert at SSTO design, you need to concentrate on 1) reducing dry mass, 2) reducing drag, 3) optimizing your flight profiles. Your designs really have too many engines, and getting rid of them would probably help. Using wing parts that hold fuel helps to reduce mass. Getting rid of tailfins reduces drag and mass. Avoiding MK2 parts reduces drag. Using a shallower ascent allows you to swap more inefficient rapiers for very efficient nukes. Moving your engines forward would probably help with stability. Using more shock cones and fewer other intakes would probably help with drag and mass. Adding just the right amount of incidence to your wings helps reduce drag and improves your flight profile. And then there's the trick/cheat of adding inverted cones to the back ends of your rapiers, to reduce drag -- do you know that one?

And no, the Advanced Nose Cone is best for drag. But the shock cone is very close behind and produces all the intake air you need.

AeroGav figures 30 tonnes per rapier, IIRC. But that's the absolute max. I admit I like my SSTOs to perform a little better than that.

There is no optimal wing shape. All wings of all shapes are treated identically. Lifting area per tonne is a religious question. You will get an endless argument about that one. There is the "spaceplanes have wings for a reason" camp -- which promulgates large wing areas, shallow, slow ascents, and nearly horizontal "speed runs". And then you get the "just set your AoA to 30 degrees and blast your way to orbit" crowd. They say that wings are mostly for decoration, so you should have the very very smallest one you can get away with.

 

Edited November 11, 2018 by bewing

[Rocket In My Pocket]

4 hours ago, CrimsonEclipse said:

Please rip my designs to shreds. I desperately need the more efficient price/t of a good heavy lifter SSTO so I can build my Duna mission in orbit.

@bewing already covered the SSTO stuff quite well so;

How big is this Duna mission that you need multiple orange tanks of fuel in orbit? What exactly are you sending there?

Whatever it is, it sounds a bit ..excessive; no offense.

Finally, SSTO's are great and all but brute forcing it with a rocket is always viable as well. A large enough rocket could easily send a half dozen or so orange tanks to orbit for a one time cost.

Oh, and welcome to the forums!

Edited November 11, 2018 by Rocket In My Pocket

[BRAAAP_STUTUTU]

you need to make sure that your crafts COL is at all times behind the COM, otherwise your plane will want to fly backwards

make sure that the COL never gets too far behind however, especially at take off, where you want to have it as close to the COM as possible to not just drop into the sea.

Don't overdo it with air intakes, you only need a couple once you get going

add small nosecones to the back of the rapiers to reduce tail drag

a duna SSTO would need LV-N's to ensure you have enough deltaV to get back

ideally your cargo has its COM at the same spot as the SSTO's COM

Avoid drag as much as you can

make sure the crafts TWR is high enough as to not waste too much fuel getting up to optimal climbing speeds

[CrimsonEclipse]

1 hour ago, bewing said:

Welcome to the forums.

You do understand that putting an orange tank in orbit with an SSTO is considered to be an expert-level SSTO design problem?

To become an expert at SSTO design, you need to concentrate on 1) reducing dry mass, 2) reducing drag, 3) optimizing your flight profiles. Your designs really have too many engines, and getting rid of them would probably help. Using wing parts that hold fuel helps to reduce mass. Getting rid of tailfins reduces drag and mass. Avoiding MK2 parts reduces drag. Using a shallower ascent allows you to swap more inefficient rapiers for very efficient nukes. Moving your engines forward would probably help with stability. Using more shock cones and fewer other intakes would probably help with drag and mass. Adding just the right amount of incidence to your wings helps reduce drag and improves your flight profile. And then there's the trick/cheat of adding inverted cones to the back ends of your rapiers, to reduce drag -- do you know that one?

And no, the Advanced Nose Cone is best for drag. But the shock cone is very close behind and produces all the intake air you need.

AeroGav figures 30 tonnes per rapier, IIRC. But that's the absolute max. I admit I like my SSTOs to perform a little better than that.

There is no optimal wing shape. All wings of all shapes are treated identically. Lifting area per tonne is a religious question. You will get an endless argument about that one. There is the "spaceplanes have wings for a reason" camp -- which promulgates large wing areas, shallow, slow ascents, and nearly horizontal "speed runs". And then you get the "just set your AoA to 30 degrees and blast your way to orbit" crowd. They say that wings are mostly for decoration, so you should have the very very smallest one you can get away with.

 

Thanks for the detailed response!

So for a 150t (laden) craft, I should be using only 5 or 6 rapiers? My nice small 60t SSTO has that many, but it performs really well so I guess I'll give it a try.

I suppose cutting down the excess fuel would also help a lot.

How many nukes should I have for something with this much payload? 3-5 seems right to me but idk, if I'm overestimating how many engines I need then I'm not sure.

When you suggest getting rid of tailfins, do you mean removing them entirely, or just streamlining them down to the absolute minimum? Like a single structural wing type A with a control surface? Or is the stability not worth the extra mass?

Is this better? Threw it together in like 5 mins taking into account what you said, haven't flown it yet (though I probably will have by the time the moderators get around to approving this post. Don't know why the system is treating my account as new, pretty sure I've posted here once or twice before).

On a somewhat unrelated note, how do you turn off the grid snap for the offset tool so that you can get a smooth translation of the part?

1 hour ago, Rocket In My Pocket said:

@bewing already covered the SSTO stuff quite well so;

How big is this Duna mission that you need multiple orange tanks of fuel in orbit? What exactly are you sending there?

Whatever it is, it sounds a bit ..excessive; no offense.

Finally, SSTO's are great and all but brute forcing it with a rocket is always viable as well. A large enough rocket could easily send a half dozen or so orange tanks to orbit for a one time cost.

Oh, and welcome to the forums!

I haven't really planned it yet, but I figure an orange tank is the largest single component I'd ever need to launch so having a vehicle capable of putting one in orbit seems like a safe bet.

That said it probably will be excessive as hell.

I already have a reusable LKO rocket, but it's a pain to judge the landing correctly, and I wanted to try and build an SSTO for fun anyway.

36 minutes ago, BRAAAP_STUTUTU said:

ideally your cargo has its COM at the same spot as the SSTO's COM

Ooh, hadn't thought about that before. Thanks for the tip.

Edited November 11, 2018 by CrimsonEclipse
Forgot to ask about offset tool, added new design.

[bewing]

44 minutes ago, CrimsonEclipse said:

How many nukes should I have for something with this much payload? 3-5 seems right to me but idk, if I'm overestimating how many engines I need then I'm not sure.

Just enough to do the job.

Quote

When you suggest getting rid of tailfins, do you mean removing them entirely, or just streamlining them down to the absolute minimum? Like a single structural wing type A with a control surface? Or is the stability not worth the extra mass?

A single structural wing (or maybe BigS strake) with no control surface. Control surfaces have mass and drag and therefore need to be minimized. But you probably need something back there to provide yaw stability. Finding just the bare minimum of drag at the back end to keep things stable is an art.

And be careful of the "occlusion" effect. If an engine is directly in front of another part of your spaceplane (such as a wing), then its thrust gets set to zero.

And you turn off grid-snap by holding down Shift.

Edited November 11, 2018 by bewing

[CrimsonEclipse]

2 hours ago, bewing said:

you turn off grid-snap by holding down Shift.

 Nah that just lowers the increment of the grid. 

After some searching it would seem that turning off angle snapping next to the symmetry button is what I was looking for. Thanks anyway. I think if I can get the angle of incidence on these front wings right I can probably put my payload in orbit now. 

Edit: Success! One orange tank with assorted RCS doodads in a 100km orbit. Thanks for the help everyone.

Edited November 11, 2018 by CrimsonEclipse

[Pecan]

4 hours ago, CrimsonEclipse said:

I figure an orange tank is the largest single component I'd ever need to launch so having a vehicle capable of putting one in orbit seems like a safe bet

Good plan.  Aim for a round 40t and you can build almost anything out of only a few modules.

Now, all you contributing experts - explain to me *again* what you think the difference between a SSTO and a Single Stage To Orbit rocket is.  I'm happy to explain to you as often as you like what a spaceplane is.

Say it with me, "A spaceplane uses aerodynamic lift as its primary means of flight when in an atmosphere.  It is an aeroplane with the additional ability to reach and maneuvre in space, although not necessarily orbit.  It may launch or land vertically.  It may or may not be a SSTO.  It may or may not use jets.  It will need rockets or another vacuum propulsion system."

PS: No, I'm never going to let it go.

Edited November 11, 2018 by Pecan

[bewing]

20 minutes ago, Pecan said:

explain to me *again* what you think the difference between a SSTO and a Single Stage To Orbit rocket is.

When the OP posts a lot of pics of spaceplanes and says "SSTO", the inference is obvious and adding lots of useless pedantic jargon confuses everyone, wastes time, and derails the topic.

 

[bewing]

3 hours ago, CrimsonEclipse said:

Edit: Success! One orange tank with assorted RCS doodads in a 100km orbit.

Congrats!

[Pecan]

2 hours ago, bewing said:

useless pedantic jargon confuses everyone

Nope, still say wilful misuse of terms is what confuses people and correct terminology clarifies things.  Specify what is required, design to requirements.

OP asked about SSTOs, showed a plane.  Shouldn't stop a sensible thinker from saying "Hey, lots of SSTOs aren't planes".  Easiest way to lift 40t with a Kerbin SSTO?  Not a plane, that's as simple as it gets.  Want to complicate things with a spaceplane design ... ok, now it's a different subject.

[Rocket In My Pocket]

14 hours ago, Pecan said:

Nope, still say wilful misuse of terms is what confuses people and correct terminology clarifies things.  Specify what is required, design to requirements.

OP asked about SSTOs, showed a plane.  Shouldn't stop a sensible thinker from saying "Hey, lots of SSTOs aren't planes".  Easiest way to lift 40t with a Kerbin SSTO?  Not a plane, that's as simple as it gets.  Want to complicate things with a spaceplane design ... ok, now it's a different subject.

Well maybe if you are dealing with a computer, specific terms are a must.

People however have a terrific knack for inferring your meaning from clues. It can be a bit of a double edged sword if they assume wrong, but I think everyone here got the gist of what the OP was asking for, which was advice on a spaceplane style SSTO.

I mean let's face it; the vast majority of SSTO's are also spaceplanes. They are far more commonly discussed than SSTO rockets at least. If you say SSTO I'm going to assume it has wings unless you specify otherwise, lol. In my mind, and I'm sure in the minds of many others; that's the default assumption, just based on the popularity of the subject.

Edited November 12, 2018 by Rocket In My Pocket

[Pecan]

8 hours ago, Rocket In My Pocket said:

 that's the default assumption, just based on the popularity of the subject.

1. Don't take me too seriously, you'll just get annoyed at the pedantry
2. On the other hand I've been fighting this fight for more than 3 years
3. My biggest objection is "SSTO to <wherever>" since SSTO, by definition, means To Orbit.
4. But, since oldbies insist on not correcting newbies, newbies keep coming along and learning wrong from day 1.

*Sigh* woe is me, but I have no-one but myself to blame for caring.

[AeroGav]

On 11/11/2018 at 3:41 PM, CrimsonEclipse said:

Thanks for the detailed response!

So for a 150t (laden) craft, I should be using only 5 or 6 rapiers? My nice small 60t SSTO has that many, but it performs really well so I guess I'll give it a try.

I suppose cutting down the excess fuel would also help a lot.

How many nukes should I have for something with this much payload? 3-5 seems right to me but idk, if I'm overestimating how many engines I need then I'm not sure.

When you suggest getting rid of tailfins, do you mean removing them entirely, or just streamlining them down to the absolute minimum? Like a single structural wing type A with a control surface? Or is the stability not worth the extra mass?

Is this better? Threw it together in like 5 mins taking into account what you said, haven't flown it yet (though I probably will have by the time the moderators get around to approving this post. Don't know why the system is treating my account as new, pretty sure I've posted here once or twice before).

On a somewhat unrelated note, how do you turn off the grid snap for the offset tool so that you can get a smooth translation of the part?

With regard to drag - you are looking in the wrong places.        Fuselage parts create vastly , vastly more drag than wing parts in KSP.        Never use a liquid fuel fuselage part if possible - store ALL of your liquid fuel in the wings if you can.  Weight and balance issues sometimes prevent following that 100%,   but that is the ideal.

Also, never sacrifice stability/controllability in the name of weight saving or drag saving.    See above.   The tiny amount of weight/drag saved by the fin will be lost many times over by the fuselage creating extra drag because you are no longer able to hold it at the right angle to the airflow.   On my designs,  I find that having the nose pointing an extra 2 degrees off prograde doubles drag.

As regards to the general approach -

An SSTO consists of 3 major components - fuel,  engine,  and payload.     Getting a decent payload fraction to orbit means keeping engine mass down,  which in turn requires good / lift drag ratio to avoid excessive "gravity losses" (that's a rocket term really, the spaceplane equivalent doesn't have a name but is complicated by the fact they have wings)

For a pure chemical SSTO I'd say the optimum number of engines might be 1 panther and 1 rapier for every 40 tons of takeoff weight.

So,  if you got a 150 ton ship,   i'd say you might want to try 3 rapiers and 3 panthers,  or 4 R / 4 P,   or   3P  and  4R.            You could try switching the inner pair of rapiers to close cycle while the outboards stay airbreathing as long as possible.

For a nuke/liquid fuel cargo ssto

Airbreathing engines i'd still say fit panthers and rapiers in a 1 to 1 ratio.   You can probably push that out to 50 tons per rapier and panther,  because you'll be able to turn the nukes on to help bust you through the sound barrier.     Nukes have very weak TWR so you will need one nuke per 15 to 20 tons of takeoff weight.    On the upside,  they are so efficient you can get to space with only a 25%-30% fuel mass fraction.    However,  due to the low TWR you need a really good lift/drag ratio to be able to fly in the upper atmosphere, after the jets quit.    

As regards that second design you linked,    it has tiny wings for its weight (might be ok on a pure chemical ssto)  and large and very draggy fuel tanks (not something you want on nuke).  Also,  beware of that space shuttle style engine mount.  It has a central 2.5m engine attachment node which you aren't using, and unused nodes create huge "flat plate" drag - the game basically treats it as a missing nose cone on a 2.5m rocket stack.   So put a 2.5m cone on the back of this mount (clip it inside for cosmetic reasons if you like) or put a 2.5m triple mount on this point.

Re:  Weight and Balance

If you have not done so already,  get  RCS build aid.    It shows a red ball in the VAB / SPH which is your centre of mass when empty.    Huge CoM shifts are a problem in cargo sstos especially.   To get your dry CoM further forward ,  put your lightest engines at the back (panthers, 1.2ton each )   ,   rapiers further forward (2tons) and shove the nukes as far forward as you can (3 tons ) 

 

I'm not saying you should emulate this,  as this design uses every trick i can think of to make a good payload fraction pure liquid fuelled cargo ssto.     The space shuttle engine mount actually has all 3 1.25m nodes occupied  as well as a triple 1.25m mount on the big central mount,  i've used offset tool to give a different cosmetic appearance.   Also you will notice  there's three types of engine hanging off the back there,  i had to attach them individually and offset them till RCS build aid said zero torque.

https://kerbalx.com/AeroGav/Andromeda

[FinalFan]

8 hours ago, Pecan said:

1. Don't take me too seriously, you'll just get annoyed at the pedantry
2. On the other hand I've been fighting this fight for more than 3 years
3. My biggest objection is "SSTO to <wherever>" since SSTO, by definition, means To Orbit.
4. But, since oldbies insist on not correcting newbies, newbies keep coming along and learning wrong from day 1.

*Sigh* woe is me, but I have no-one but myself to blame for caring.

re: 3.  I think your pedantry is slipping!  "SSTO to wherever" is simply a further restriction; it not only gets T.O. but also to elsewhere.  If you really want to go down that road, SSTO is arguably not a thing but a property of a thing, i.e. you can have an SSTO rocket, and that rocket will be SSTO, but you arguably don't have "an SSTO" per se.  However, SSTO also meaning a rocket or spaceplane that is SSTO is good enough for me. 

[Pecan]

4 hours ago, FinalFan said:

re: 3.  I think your pedantry is slipping!  "SSTO to wherever" is simply a further restriction; it not only gets T.O. but also to elsewhere.  If you really want to go down that road, SSTO is arguably not a thing but a property of a thing, i.e. you can have an SSTO rocket, and that rocket will be SSTO, but you arguably don't have "an SSTO" per se.  However, SSTO also meaning a rocket or spaceplane that is SSTO is good enough for me. 

Linguistically it works both ways, in English (sorry everyone else!).  You can have a vehicle which goes To Orbit using a Single Stage and you can have a Single Stage vehicle that goes To Orbit.  Interestingly there would seem to be a difference there - the former allows onward progression, possibly discarding silly things like wings instead of dragging them through space.  So yes, I agree.

That going To Orbit does not prevent a vehicle then going further is an old point but To Orbit is then tautological except in certain unusual situations (which I'm too tired to explain now; I've been playing Witcher all night ^^).  Suffice to say vehicle routes are more usually defined by their destination than their intermediate waypoints.  In any case, SSTO says nothing about what may continue after reaching orbit.  A disposable SSTO rocket which releases a payload which continues to some planet is still a SSTO.  Hence 'SSTO' is a ridiculous term for anything which goes further or does extra.