SSTO Help

[MisterKerman]

I wish I was better at making SSTOs. Is there an ingredient list/guidelines I could use for requirements?

Like, number of a certain type of engines (probably RAPIERS) for a certain amount of craft mass with a certain amount of lift capable of carrying a certain amount of cargo mass? Something like that? I know the engineering is important too but it seems like just knowing what's enough or not to get the craft into space always throws me for a loop because my strategy is to make up for things by brute forcing things into space, which is sort of the opposite of what makes SSTOs good for utility purposes like shuttles and deliveries.

This is my current design. It can lift a 1.25m diameter payload sightly longer than a cargo segment for MK2 if in that configuration.

All I have is the shuttle configuration. (On mobile away from my PC right now.)

It works well enough but I'd love to be capable of bringing up a full Big Orange. That seemed to be the hallmark of a good SSTO back in the day, and I just can't see myself making a simple design that works with such a small amount of experience with SSTOs and fairly little time for trial and error.

I'd really appreciate some simplistic design tips if someone could save me the frustration and time of learning by trial and error.

[Geonovast]

Step one is to not use Mk2 parts.  They're insanely draggy.

[Aegolius13]

I don't know any rules of thumb in terms of engines per ton, or whatnot.  But a few general tips:

• Your craft looks VERY light on wing.  
• Where possible, using the BigS wings is ideal.  They have the highest temperature resistance, and can store fuel without any extra mass (i.e., the mass relative to wing area is the same for regular wings). 
• Since planes tend to have a center of mass near the back, canards at the very front of the ship are great for getting a lot of pitch authority with relatively small/light control surfaces.
• Also, if you're not already, rotate your wings to slightly point up at the front.  This is called "angle of incidence" and helps you generate lift without as much fuselage drag.
• Try to keep as clean of a profile as possible, with as few stacks as possible, to minimize drag.  For example, I'd cut the Oscar-B tanks and add that fuel to either your center stack or the Mk 1 nacelles.  The static solar panels are draggy as well; perhaps consider a fuel cell or just using batteries for short flights.  Cut the RCS thrusters if you don't need them (this thing does not look like it docks).  And reduce or remove the parachutes if possible.
• Mk 2 parts are notoriously draggy relative to their size and capacity.  That's not to say they can't be used, but if you're having difficulties, you might try a different form factor.
• I think if you make some of those changes, you'll find you won't need as many engines.  I think you could do a 10-kerbal transport with 2 or max 3 Rapiers.  This will reduce your dry weight, improving delta-v and probably mass balance issues as well.

One other random observation - do you have instability problems when you deploy those airbrakes?  They seem pretty far forward, possibly even in front of your center of mass. I'd move as far back as possible.

[bewing]

AeroGav is really big on spaceplanes, and I think the info you want is in here:

https://forum.kerbalspaceprogram.com/index.php?/topic/165435-basic-mk2-spaceplane-guide

[Aeroboi]

Minimal amount of engine is best to avoid excess dead weight in space, that means as little rapiers as possible. To achieve this you want to minimize drag. Best ways to avoid drag is using only cockpit, fuel fuselages, intakes, engines, wings and landing gear that are to be exposed to air. That means all the other parts are best to be tugged into cargo bays.
(A) In your case I would put a deployable solar panel inside a cargo bay. Why all the parachutes? You want to go to Duna? They won't be needed on Kerbin and it doesn't seem capable of reaching Duna. I never use parachutes because you can land planes safely at 50-60 m/s and Maximum wheel brakes (set to 200 in the editor) should stop most craft. If it isn't capable of stopping using wheel brakes you can use add friction to the wheels, always make sure the friction settings are a little higher on the rear wheels. Assuming your front wheel is for steering its friction should be lowest so it doesn't steer across the surface like a crazy animal.

(B) Since your capable of breaking using wheels I also see no reason for airbrakes. Are they needed in your design case? I use to pitch over and pull up violently to arrest speed close to the runway. I assume you use airbrakes for the final descent? I would discard them unless you find it is cool.

Another way to minimize drag is to minimize the frontal cross section. If the spaceplane travels through the center of the prograde reticle the cross section is lowest as the fuselage will be dead center in the direction of travel.
This never happens if the wing chord line is perfectly horizontal parallel to the fuselage. This is how most people attach their wings, which is straight on.
Luckily you can put the wings on a incline (Wing incidence) so that the spaceplane can stay in level flight or even climb while staying on prograde to minimize all excess body drag.
This in turn means less required engine to go through the sound barrrier.

(C) Additionally always try to configure wing incline on the fwt and aft wings/control surfaces so that it stays level flight without key controls. If the incline fwt <> aft is mismatched the spaceplane will either tip over or pull up natively, this means any of the available control surfaces has to be extended which causes additional drag if this balance is mismatched.
This balance is further affected as you burn fuel and CoM shifts. I try to build my spaceplanes so wet and empty mass is at the same place. IME you have to be experienced to know where and how to organize your engine mass and fuel mass in relation to the CoM.

The general rule to further reduce body drag is that if you need more then one engine at the end of a 1.25m stack is that you'd better use a adapter like a bi-coupler rather then bolting 2 new stacks on the sides as the added cross section of a added stack will be greater then the extended surface of i.e. any engine adapter.
For that reason I would discard the MK0 liquid fuel tanks and use the larger 400LF MK1 size at the front or back at any of the available 1.25m stacks. Alternatively you can use the BiG-S strake that holds 100 LF. It also has the lightest dead mass in relation to it's wing lift so it's the best wing piece out there to be used if you want the maximum wing lift for the least amount of mass.

(D) MK 1 fuselages are the least draggy on the frontal cross section. MK2 may look prettier but it is draggier. Because it is draggier you are normally required to move your main wings further to the back to compensate for this in order to achieve a controlled re-entry. Using MK1 your are less draggier and you generally have your wings moved more to the front as you wont have to deal with the drag of a Mk 2 cockpit fuselage on the front.

(E) Why two Mk2 drone core? isn't 1 sufficient? It isn't like one of them breaks randomly lol

(F) Why 4 reaction wheels? Isn't 1 enough? In a spaceplane you are constantly under aerodynamic stress. This stress is so great that the reaction wheel torque is completely futile. It would fly as good as it would without any of them. Furthermore, according to the size of your spaceplane 1 reaction wheel is enough to tumble freely in space, so why even 2, let alone 4?
The only reason I can see is if you have problem during landing or takeoff. During landing and takeoff the airspeed is very low so then the reaction wheels will start to have effect. As you increase speed the torque generated by control surfaces will become greater. For this reason a aircraft becomes more maneuverable as it increases in speed which is why you are capable of controlling your vessel using your control surface only past a certain speed while it wouldn't be able to takeoff or land if it hadn't the available reaction wheel torque.

(G) I would always go rapier unless you have very draggy cargo that is enormously large. In such a case it might be difficult to breach the sound barrier. Whiplashes have more thrust at transonic speeds so using them will get you past 400m/s. Usually a combination of whiplashes and rapiers is used. (1) so that you have closed cycle mode on the rapiers so that you wont need additional engines, another tip is to amount only the least amount of whiplashes in relation to rapiers to achieve the maximum air breathing speed. Another reason might be to use whiplashes if your aircraft cant take off past the runway. But IMO that is a design mistake as any sized spaceplane should be able to lift off the runway using any engine layout by properly placing landing gear.

P.S. : 1 rapier per 20 Ton is to be expected at the top of the table. Most people struggle doing 10-15 Ton as it is crucial to learn how to reduce drag. I expect 25Ton is the utmost using a proper balance of all the drag inducing parts and a proper climb<>dive<>climb.

 

[Aegolius13]

1 hour ago, Aeroboi said:

The general rule to further reduce body drag is that if you need more then one engine at the end of a 1.25m stack is that you'd better use a adapter like a bi-coupler rather then bolting 2 new stacks on the sides as the added cross section of a added stack will be greater then the extended surface of i.e. any engine adapter.

Interesting general question - if you need exactly two engines, are you better off drag-wise with a Mk 2 fuselage and Mk 2 bicoupler, or a 1.25m fuselage and bicoupler?  I tend to avoid the latter like the plague, but not sure if the data backs me up on this.  Of course, even if drag is a wash, there are advantages to Mk 2 -- they generate some lift, have better passenger capacity, cargo bays can hold a 1.25m stack, etc.

EDIT - did a very quick and dirty altitude test to compare.  Difficult because Mk 2 lift makes it hard to keep rocket pointing straight up.  Results were very close - with the same mass and thrust, the Mk 2 stack got to 2412 meters, whereas the 1.25m  bicoupler stack got to 2,360 meters.  

 

Edited January 29, 2019 by Aegolius13

[capi3101]

OP: Are you using stock air or FAR? I'm assuming stock, in which case my advice isn't going to be terribly useful to you. But I'll offer it anyway...some of it is still applicable to stock.

So, first thing's first - like a rocket, start with your basic concept and build the payload. Payload in this case includes anything other than fuel tanks, engines, wing and landing gear. Grab the mass of that payload and divide it by 0.15; assume that when your plane is said and done, it's going to weigh about that much and plan accordingly.

If you're doing Whiplashes, you're going to want one engine per 30 tonnes of theoretical plane. With RAPIERs, one engine per 17 tonnes. Panthers, I haven't tried to get into orbit there, so go with what other folks tell you.

Theoretical fuel load: Assume here that you're going to want somewhere in the neighborhood of 1,500 m/s of dV for the part where your plane becomes a rocket; that'll get you to orbit, futz around a bit and come back. For most purposes, that's a wet-to-dry-mass ratio of 1.6, and I usually reverse the ratio (instead of 45% fuel and 55% oxidizer, go with 55% fuel and 45% oxidizer; this will give you some gas for the atmo part of the flight). If you're wanting an interplanetary spaceplane (say something you want to get to Laythe with), include the fuel for that with your payload design.

 From there, you build your fuselage, engine and intake assemblies. For the wings, I use a set of formulas that work in FAR; the wing loading is designed at 0.5 tonnes per meter, aspect ratio at 2:1, and uses a composite setup dependent on Procedural Wings that gives an approximation of the wings used on the Shuttle. I generally give the plane one set of airbrakes per engine, and set up my gear in a tricycle pattern with the brakes and friction turned up all the way in the back, with no brakes and 2.5 friction up front as well as 1.25 damper. I usually go with no steering on any of the wheels; in my experience steering is more trouble than it's worth.

Hope that's moderately helpful.

 

 

Edited January 29, 2019 by capi3101
threw in some actual advice.

[Starhawk]

Some very general guidelines I used to use for smaller SSTO spaceplanes:

1 Rapier per 20 tonnes

1 lift per 4 tonnes

20 to 50% more LF than Ox  (depends heavily on design and flight profile)
 

These numbers change a bit for heavier craft.  My current medium-heavy SSTO spaceplane carries a Jumbo 64 (orange tank) to 350 km orbit.

It has 6 Big-S wings, fully fuelled, 6 rapiers and a bit more LF than oxidizer (6380 and 5500 units respectively not counting the orange tank) and masses 132 tonnes.
 

Some design tips that I find very useful:

Engineer the craft so that the wet and dry CoM are in the same place, or as close together as possible (the mod RCS Build Aid helps immensely with this)

Add incidence to your wings

Separate pitch and roll authority on different control surfaces - yaw control surfaces are unnecessary

The smaller the craft, the more drag matters

Happy landings!

[AeroGav]

1 hour ago, Aegolius13 said:

Interesting general question - if you need exactly two engines, are you better off drag-wise with a Mk 2 fuselage and Mk 2 bicoupler, or a 1.25m fuselage and bicoupler?  I tend to avoid the latter like the plague, but not sure if the data backs me up on this.  Of course, even if drag is a wash, there are advantages to Mk 2 -- they generate some lift, have better passenger capacity, cargo bays can hold a 1.25m stack, etc.

EDIT - did a very quick and dirty altitude test to compare.  Difficult because Mk 2 lift makes it hard to keep rocket pointing straight up.  Results were very close - with the same mass and thrust, the Mk 2 stack got to 2412 meters, whereas the 1.25m  bicoupler stack got to 2,360 meters.  

 

I would take neither,  the 1.25m bicoupler is very draggy.  Funnily enough the 2.5m bicoupler and tricoupler make far less drag.  

But, in a mk1 design i'd just attach type b nose cones or ncs adapters either side of the main stack if i only wanted 2 engines, and have a nose cone on the back of the main stack.

[MisterKerman]

2 hours ago, Aeroboi said:

Minimal amount of engine is best to avoid excess dead weight in space, that means as little rapiers as possible. To achieve this you want to minimize drag. Best ways to avoid drag is using only cockpit, fuel fuselages, intakes, engines, wings and landing gear that are to be exposed to air. That means all the other parts are best to be tugged into cargo bays.
(A) In your case I would put a deployable solar panel inside a cargo bay. Why all the parachutes? You want to go to Duna? They won't be needed on Kerbin and it doesn't seem capable of reaching Duna. I never use parachutes because you can land planes safely at 50-60 m/s and Maximum wheel brakes (set to 200 in the editor) should stop most craft. If it isn't capable of stopping using wheel brakes you can use add friction to the wheels, always make sure the friction settings are a little higher on the rear wheels. Assuming your front wheel is for steering its friction should be lowest so it doesn't steer across the surface like a crazy animal.

(B) Since your capable of breaking using wheels I also see no reason for airbrakes. Are they needed in your design case? I use to pitch over and pull up violently to arrest speed close to the runway. I assume you use airbrakes for the final descent? I would discard them unless you find it is cool.

Another way to minimize drag is to minimize the frontal cross section. If the spaceplane travels through the center of the prograde reticle the cross section is lowest as the fuselage will be dead center in the direction of travel.
This never happens if the wing chord line is perfectly horizontal parallel to the fuselage. This is how most people attach their wings, which is straight on.
Luckily you can put the wings on a incline (Wing incidence) so that the spaceplane can stay in level flight or even climb while staying on prograde to minimize all excess body drag.
This in turn means less required engine to go through the sound barrrier.

(C) Additionally always try to configure wing incline on the fwt and aft wings/control surfaces so that it stays level flight without key controls. If the incline fwt <> aft is mismatched the spaceplane will either tip over or pull up natively, this means any of the available control surfaces has to be extended which causes additional drag if this balance is mismatched.
This balance is further affected as you burn fuel and CoM shifts. I try to build my spaceplanes so wet and empty mass is at the same place. IME you have to be experienced to know where and how to organize your engine mass and fuel mass in relation to the CoM.

The general rule to further reduce body drag is that if you need more then one engine at the end of a 1.25m stack is that you'd better use a adapter like a bi-coupler rather then bolting 2 new stacks on the sides as the added cross section of a added stack will be greater then the extended surface of i.e. any engine adapter.
For that reason I would discard the MK0 liquid fuel tanks and use the larger 400LF MK1 size at the front or back at any of the available 1.25m stacks. Alternatively you can use the BiG-S strake that holds 100 LF. It also has the lightest dead mass in relation to it's wing lift so it's the best wing piece out there to be used if you want the maximum wing lift for the least amount of mass.

(D) MK 1 fuselages are the least draggy on the frontal cross section. MK2 may look prettier but it is draggier. Because it is draggier you are normally required to move your main wings further to the back to compensate for this in order to achieve a controlled re-entry. Using MK1 your are less draggier and you generally have your wings moved more to the front as you wont have to deal with the drag of a Mk 2 cockpit fuselage on the front.

(E) Why two Mk2 drone core? isn't 1 sufficient? It isn't like one of them breaks randomly lol

(F) Why 4 reaction wheels? Isn't 1 enough? In a spaceplane you are constantly under aerodynamic stress. This stress is so great that the reaction wheel torque is completely futile. It would fly as good as it would without any of them. Furthermore, according to the size of your spaceplane 1 reaction wheel is enough to tumble freely in space, so why even 2, let alone 4?
The only reason I can see is if you have problem during landing or takeoff. During landing and takeoff the airspeed is very low so then the reaction wheels will start to have effect. As you increase speed the torque generated by control surfaces will become greater. For this reason a aircraft becomes more maneuverable as it increases in speed which is why you are capable of controlling your vessel using your control surface only past a certain speed while it wouldn't be able to takeoff or land if it hadn't the available reaction wheel torque.

(G) I would always go rapier unless you have very draggy cargo that is enormously large. In such a case it might be difficult to breach the sound barrier. Whiplashes have more thrust at transonic speeds so using them will get you past 400m/s. Usually a combination of whiplashes and rapiers is used. (1) so that you have closed cycle mode on the rapiers so that you wont need additional engines, another tip is to amount only the least amount of whiplashes in relation to rapiers to achieve the maximum air breathing speed. Another reason might be to use whiplashes if your aircraft cant take off past the runway. But IMO that is a design mistake as any sized spaceplane should be able to lift off the runway using any engine layout by properly placing landing gear.

P.S. : 1 rapier per 20 Ton is to be expected at the top of the table. Most people struggle doing 10-15 Ton as it is crucial to learn how to reduce drag. I expect 25Ton is the utmost using a proper balance of all the drag inducing parts and a proper climb<>dive<>climb.

 

It's a bit of a mess. I'll be using your post as a bible as well as the other suggestions made by other members, but yours in particular will be useful for explaining myself and giving you guys an idea of how clueless I am with designing SSTOs.

I won't be defending myself, these aren't arguments, just misguided reasons!

A) Safety. Just so I know the thing will stop right away if deployed should I overshoot the runway. I use them like a drag racer at the end of his quarter mile. I'm actually a really decent pilot and prefer IVA and instruments generally, so I'll probably remove them.

B) The air-brakes are for descent... also for looks. But if you don't think I need them (as they overheat too quickly anyways) I won't be using them in the future.

C) That ought to help lots.

D) Will do.

E) I... wanted their reaction wheels and batteries... Also fly by wire is nice. Not sure if the MK2 cockpit has that built in, but it was mostly to hold electric charge anyways if that's the case.

F) They're for snappy reorientation in space, and in the past I've had some tumbling issues during certain flight conditions which I no longer remember the details of. Looking at it now it's sort of ridiculous but I felt like they were appropriate at the time.

G) Rapier is my engine of choice. It's just a matter of getting the right fuel mix so you aren't hauling around too much of an unused fuel, but it sounds like that's only a slight issue compared to some of the solutions you guys have already tossed my way. I'll probably continue to use them.

@Aegolius13

Awesome suggestions. Thank you very much. Also it's my personnel shuttle and 1.25m cargo delivery vehicle when using cargo bays instead of passenger seating, so she docks with an in-line MK2 located to the rear of the drone cores, whether Kerbals or assets.

@bewing @Starhawk

Thanks you guys!

I'll leave this open for more user's input and update it with a new design when I can!

Edited January 29, 2019 by MisterKerman

[AeroGav]

2 hours ago, Aeroboi said:

P.S. : 1 rapier per 20 Ton is to be expected at the top of the table. Most people struggle doing 10-15 Ton as it is crucial to learn how to reduce drag. I expect 25Ton is the utmost using a proper balance of all the drag inducing parts and a proper climb<>dive<>climb.

Best airbreathing combo in my opinion is one rapier and one panther per 50 tons of launch mass.       Erring to the low end (say, 35 tons)  will allow a bad design or ascent profile to get supersonic, but increases the risk of running out of fuel.

Panthers are light and have the strongest thrust subsonic and transonic.   After mach 2.5 they quickly lose power, but by that point the Rapier will be pushing very hard.

But surely it is better to have an all RAPIER setup ?  Well, the thing is you'll need more engine mass to get supersonic,   and your top speed in airbreathing flight, so long as you're not climbing,  won't be that different.   As you get above mach 5,  RAPIER thrust falls away rapidly

What's the best launch profile ?  In my opinion, if you're looking for max delta v/payload fraction,   it's 4 stage process

1.  Initial climb at subsonic speed to get out of the soupy lower atmosphere.   Speed below 240,  nose not more than 5 degrees above prograde.

2.  Level off to go supersonic, above 440 m/s, start to climb again

3.  Level off again at 16-21km for the speedrun.   You're looking to get at least 1350 m/s airbreathing

4.  Once the rockets are lit,  pitch up so the nose is 5 degrees above prograde marker,  this gives the best lift drag ratio and the lowest overall drag losses

Alternatively,  

  will help you find the best altitude/airspeed relationship for your aircraft.

 

Finally, something that's not been mentioned yet - I'd loose the pointy cockpit.    They tend to overheat when flying the most efficient launch profile.   Switch to an inline cockpit for less stressful thermals.

[MisterKerman]

Oh I forgot. @capi3101 I'm stock.

[Aegolius13]

18 minutes ago, MisterKerman said:

hank you very much. Also it's my personnel shuttle and 1.25m cargo delivery vehicle when using cargo bays instead of passenger seating, so she docks with an in-line MK2 located to the rear of the drone cores, whether Kerbals or assets.

Ah yes... did not see the inline cargo bay.  So you might need RCS if docking with a station.  You might want to look at using the two of the one-way thrusters (on top/bottom and sides) instead of two four-way blocks.  I think they're less draggy overall that way , but not positive(can be checked via console aero menu).  You can potentially get by without forward/rear RCS by using engines.  

[AeroGav]

14 minutes ago, MisterKerman said:

G) Rapier is my engine of choice. It's just a matter of getting the right fuel mix so you aren't hauling around too much of an unused fuel, but it sounds like that's only a slight issue compared to some of the solutions you guys have already tossed my way. I'll probably continue to use them.

Generally all the fuel you need for airbreathing parts of the flight can go in wing parts and intakes -  remember you can turn big S strakes vertical and use them as  vertical stabilizers,  because they will stick out far behind your CoM and have a good bit of area,  they are very effective at surpressing sideslip.

OTOH,  I prefer to solve the problem by just going liquid fuel only.    That requires a heavy emphasis on high lift, low drag,  and on getting as much fuel into wing parts instead of fuselage ones as possible (wing parts produce less drag than liquid fuel fuselage tanks of same capacity and of course the lift is a bonus),   as well as making sure the craft is stable and controllable enough to fly an accurate AoA (drag doubles with every 2 degrees you move off prograde).

https://kerbalx.com/AeroGav/MK1-Griffon-Deep-Space-Crew-Shuttle

40 ton mk1 inline cabin with 11 seats, 2 nerv 1 rapier 1 pantherslightly larger liquid only shuttle -

Spoiler

 

Its not so easy to work into larger ships because the size adapters between mk3 and 2.5m only come in rocket fuel format

[MisterKerman]

10 minutes ago, Aegolius13 said:

Ah yes... did not see the inline cargo bay.  So you might need RCS if docking with a station.  You might want to look at using the two of the one-way thrusters (on top/bottom and sides) instead of two four-way blocks.  I think they're less draggy overall that way , but not positive(can be checked via console aero menu).  You can potentially get by without forward/rear RCS by using engines.  

I generally use a 1-way plus a 4-way to make a 5-way, and then put those 5-ways on either side of the craft (a total of two 5-ways) where most appropriate for docking on a spot inbetween my wet and dry CoM to compromise for versatility's sake.

 

Also @AeroGav

Sounds a bit fancy for me. If I carry just a little unburnt oxidizer with me it's not the worst thing in the world. I've never had much of an issue balancing fuels, but then again I've never had much of a successful cargo SSTO either...

Edited January 29, 2019 by MisterKerman

[Aeroboi]

52 minutes ago, MisterKerman said:

I generally use a 1-way plus a 4-way to make a 5-way, and then put those 5-ways on either side of the craft (a total of two 5-ways) where most appropriate for docking on a spot inbetween my wet and dry CoM to compromise for versatility's sake.

 

Also @AeroGav

Sounds a bit fancy for me. If I carry just a little unburnt oxidizer with me it's not the worst thing in the world. I've never had much of an issue balancing fuels, but then again I've never had much of a successful cargo SSTO either...

My way of doing things.

For small craft I put 2 x 4 one way rcs ports clustered together on both sides inside the 1.25m service bay. This way you can reduce drag of the entire rcs system. The only issue is the angle inducing cosine losses. I generally use very little fuel when docking and only use rcs for translation so I personally don't care. 8 parts may be a bit heavy but it's the lightest weight option and reduces the drag entirely. Additionally you can put the other stuff into the same service bay.

This is what I mean...

 

[Laie]

47 minutes ago, MisterKerman said:

Sounds a bit fancy for me. If I carry just a little unburnt oxidizer with me it's not the worst thing in the world. I've never had much of an issue balancing fuels, but then again I've never had much of a successful cargo SSTO either...

That's a good mindset.

While I heartily recommend Aerogav's tutorial posts (link is currently the top-rated answer in this thread), you should also keep in mind that you don't have to optimize everything. This is KSP, after all -- even with a lot of drag, you can still brute-force your way to space (and within a large margin, it won't look brute or even forced).

Still it helps to know what's best and why, if only so you can make a good guess as to what a certain tradeoff (or sheer laziness) will cost you.

 

 

[MisterKerman]

@Aeroboi

I'll stick with my 2×5-ways for now. I prefer right angles in regards to back/forth/lateral/rotational movements. It satisfies my OCD. I might steal the cargo bay idea but I sort of headcanon things too which is sort of silly. It would stress me out knowing the jets are technically pushing against the craft which IRL would push the craft in that direction fighting the force they're providing... Like I said I might have mild OCD lol.

@Laie

Yeah definitely. KSP leaves room for style as well as "moar boosterz", but it's also a game of precision and slim margins for error at other times. You can't brute force everything so it's best to know how to do things right should it come to that. I have a few unhealthy habits but for the most part I'm focusing on learning to do things the right way. No more burning toward the horizon at Munrise for me...

Edited January 29, 2019 by MisterKerman

[capi3101]

2 hours ago, MisterKerman said:

Oh I forgot. @capi3101 I'm stock.

Fair enough. Still, about the only thing that I said that wouldn't necessarily be applicable was my advice with the wings. Even in stock, 1500 m/s is still a good rocket delta-V target for a well-built spaceplane, assume your plane's takeoff mass is going be your payload mass divided by 0.15, use a 1.6 wet-to-dry mass ratio for that theoretical mass when guessing how much fuel you're going to need, and have it about 45% ox, 55% liquid fuel. And then go with however many engines for that theoretical takeoff mass that you'll think you'll need. Include one airbrake per engine.

For example, let's say you build your payload and it comes out at 4.5 tonnes (not unreasonable). You divide that by 0.15; your theoretical takeoff mass is 30 tonnes. Twenty tonnes per RAPIER has been suggested; you've got thirty tonnes, so use two RAPIERS. You still assume the 1.6 ratio for forty tonnes since you're using two engines - that comes out to 25 tonnes of fuel, of which 11.25 tonnes will be oxidizer and 13.75 tonnes will be liquid fuel. At 200 units per tonne, that comes out to 2250 oxidizer and 2750 liquid fuel. Two engines, so two airbrakes. Already sets a lot of the parameters you're going to want for your final design right there.

(Yes, I know that those figures are going to shoot over thirty tonnes - the "theoretical mass" is really little more than a back of the envelope estimate, but my experience shows it to be a good one).

Edited January 29, 2019 by capi3101

[bewing]

5 hours ago, Aegolius13 said:

Interesting general question - if you need exactly two engines, are you better off drag-wise with a Mk 2 fuselage and Mk 2 bicoupler, or a 1.25m fuselage and bicoupler?  I tend to avoid the latter like the plague, but not sure if the data backs me up on this.  Of course, even if drag is a wash, there are advantages to Mk 2 -- they generate some lift, have better passenger capacity, cargo bays can hold a 1.25m stack, etc.  

 

I would take the 1.25m fuselage and bicoupler.

Passive stability requires low drag in the front, and high drag in the back -- and that's exactly what this pattern gives you. The MK2 version gives you high drag in the front and low in the back, so you're killing yourself and you need a lot of extra control authority to counter the instability, which adds secondary mass and drag.

[MisterKerman]

That's a good point. I'd give a thumbs up but apparently there's a daily quota. I'll probably use a bicoupler as well if I ever make another 1.25m fuselage SSTO.

The next SSTO goal I want to reach is a full big orange I guess. If I can lift one of those, I ought to be able to lift just about anything I want to take into space. It's a huge jump from 1.25m parts though...

[AeroGav]

1 hour ago, bewing said:

I would take the 1.25m fuselage and bicoupler.

Passive stability requires low drag in the front, and high drag in the back -- and that's exactly what this pattern gives you. The MK2 version gives you high drag in the front and low in the back, so you're killing yourself and you need a lot of extra control authority to counter the instability, which adds secondary mass and drag.

I think both options would be towards the rear of the craft overall, being just in front of the engines which are the last part on the main stack.

https://imgur.com/Kllc9ED

Whilst it is true that drag at the rear of a craft can stabilize it,  it is much better to use lift.       For example,   the big S strakes i turned into vertical stabilizers here,   have zero angle of attack and generate little drag when the craft is following prograde.   But if any sideslip develops, this gives these parts and angle of attack and they produce sideways lift that torques the tail back into line.  I have found that drag chutes are helpful when landing on Duna (stops the plane spinning out and flipping on rollout)  but you don't want to add drag when flying to orbit !

Edited January 30, 2019 by AeroGav

[Aegolius13]

As often happens, this got me interested in building one of my own to try some stuff.   It's not perfect by any means (e.g., no place for good roll control out on the wings), but it's a simple example of some of the tips listed above.  It can make it to orbit with a few hundred m/s of rocket delta-v and some LF left in reserve.  Wings have 270/300 LF, other thanks are full.  Plenty of thrust with 2 Rapiers to do a straight climb.  (I played around with 1 Rapier and it was possible with the RCS ports removed, but it ended up using more fuel to get to orbit).

[TheFlyingKerman]

Another trick not mentioned above: flying with prograde lock. Fuselage parts (even MK2 ones) have absolutely minimal drag when flying perfectly prograde. Besides prograde lock is easy and repeatable.

As an example this SSTO manages to deliver 24t to LKO with 2 rapiers and 68.5t MTOW

and can do 1600m/s at 21000m in jet mode

In practice I'd just design the wings so that I taking off and then setting prograde lock after reaching some speed/altitude gives a good ascend profile.

Edited January 30, 2019 by TheFlyingKerman

[MisterKerman]

Okay so I just whipped up something which is probably terrible and won't be worth legitimately biting into, but I didn't spend long or think too hard about it. This was the result of like 20 minutes to make the concept for a shape for a plane that at the very least looks okay? (Empty cargo bay. I just wanted a basic shape.)

What are your guys' opinions on the shielded docking port for a nosecone? (Also that's seven Rapier engines on an engine plate in the rear. No calculation, just seeing how it fit. I was hoping to get away with no engines on the wings. I've placed small nosecones on the nodes and recessed them. From what I understand that used to reduce drag on them but I honestly have no idea if that's true any more...)

Edited January 30, 2019 by MisterKerman