Minimize drag for SSTOs

[quasarrgames]

I've heard that drag is everything with 1.0.4 sstos. So, aside from the normal stuff, like shock cones on the front, small nosecones on rapiers, and making the plane as narrow as possible, are there any other ways to reduce drag?

[legoclone09]

You can use B9 procedural wings to make very thin wings and those have very little drag. And you can fill them with fuel! I don't think lift stays the same, it gets worse but you don't need much, about 1 lift unit per 10 tons. Here is the link: http://forum.kerbalspaceprogram.com/threads/104966-1-0-2-B9-Aerospace-Procedural-Parts-0-40-Updated-09-06-15

Edited August 10, 2015 by legoclone09
added link

[xtoro]

Use as small of wings as you can to get it to fly. If you're not against installing mods, FAR and MechJeb have tools that show how much drag you're fighting.

- - - Updated - - -

Ninja'd!

Also wanted to add a trick for long heavy planes: Tilt your wings 5 degrees. This allows the rest of the plane to stay straighter in the airstream and you won't have air hitting the bottom of the plane as much. Less drag.

[Pecan]

You can build rocket SSTOs more or less the same way you've always done, although taller and thinner is better and some fins or other draggy stuff on the bottom will pay off.

[ajburges]

Radial parts kill drag. If it's not wings, gear, or airbrakes, it belongs in a cargo bay. Jokingly: you could probably do without those items to. A two RAPIER flying sausage can lift a surprising amount of payload.

Set CoL balance for minimal craft stability. Stable craft need pitch authority to maintain AoA. Control surfaces induce drag. Also less stable craft need fewer surfaces for good attitude authority.

Fly a faster ascent. Lift is proportional to v². You can use less lift off you fly faster. RAPIERs also prefer a fast ascent anyway.

[Red Iron Crown]

Radial parts kill drag.

This is important. Particularly with fuel lines and struts, minimize their use as much as possible (and you never need fuel lines with Rapiers).

[cephalo]

Radial parts kill drag. If it's not wings, gear, or airbrakes, it belongs in a cargo bay. Jokingly: you could probably do without those items to. A two RAPIER flying sausage can lift a surprising amount of payload.

Someone around here said that the cargo bays don't actually eliminate the drag of the stuff inside them. Correct me if that is wrong.

This is important. Particularly with fuel lines and struts, minimize their use as much as possible (and you never need fuel lines with Rapiers).

Do you really not need the fuel lines with Rapiers in rocket mode? I thought that was just with airbreathing engines.

[Fearless Son]

Someone around here said that the cargo bays don't actually eliminate the drag of the stuff inside them. Correct me if that is wrong.

My understanding is that drag counts on those parts if the cargo bay is opened or if the center of mass of the individual parts clip outside the cargo bay. As long as everything is packed neatly and the bay is closed, it should only count the drag of the closed bay itself.

I would not recommend trying to "cheat" the drag system by letting parts clip just a little outside the bays though. That has a habit of the game physics trying to rapidly disassemble the parts from the bay.

[Red Iron Crown]

Do you really not need the fuel lines with Rapiers in rocket mode? I thought that was just with airbreathing engines.

Really.

[DancesWithSquirrels]

...and you never need fuel lines with Rapiers...

Yeah, I got surprised by that more than once, with the Rapier pulling fuel out of what should have been drop tanks for another stage.

[GoSlash27]

quazarrgames,

Low drag is certainly part of it, but it's not "everything" in SSTO spaceplanes.

Equally (or more) important considerations:

*use the proper proportions. No more or less than necessary of any one part of the combination of parts required to make a functional spaceplane. Everything affects everything else, so don't deviate from the recipe.

*Efficiency, efficiency, efficiency! There's no room on a spaceplane for bloat. Ruthlessly eliminate everything that isn't absolutely necessary. Efficiency feeds on itself. So does inefficiency.

* You gotta fly a precise profile or it ain't makin' orbit. Learn what your spaceplane likes and fly it that way.

Good luck!

-Slashy

Edited August 10, 2015 by GoSlash27

[problemecium]

I'll just second the earlier mention of struts. They contribute a surprisingly detrimental amount of drag and weight, even if they're very short, so as much as possible try and design the plane to not need any struts.

[Edax]

Use as small of wings as you can to get it to fly. If you're not against installing mods, FAR and MechJeb have tools that show how much drag you're fighting.

- - - Updated - - -

Ninja'd!

Also wanted to add a trick for long heavy planes: Tilt your wings 5 degrees. This allows the rest of the plane to stay straighter in the airstream and you won't have air hitting the bottom of the plane as much. Less drag.

Paradoxically drag is a good thing when it comes to spaceplanes, as this radically increases your ability to aerobrake and slow down during reentry. The key is to minimize drag when moving forward, and maximize wing surface area when pointing radially to maximize utility.

[Pecan]

Someone around here said that the cargo bays don't actually eliminate the drag of the stuff inside them. Correct me if that is wrong...

They are meant to eliminate the drag and heat buildup from parts inside when they are closed. They don't work too much of the time.

I'm having trouble with imgur at the moment but I'm trying to upload a picture of my cargo bay contents - battery, SAS and solar panels. No clipping, no fancy placement, lots of heat and drag. :-(

[ajburges]

Paradoxically drag is a good thing when it comes to spaceplanes, as this radically increases your ability to aerobrake and slow down during reentry. The key is to minimize drag when moving forward, and maximize wing surface area when pointing radially to maximize utility.

Most space planes have plenty of drag for safe reentry. At angels 7 terminal velocity of a good rocket stack is over Mach 3; it's about Mach 1 for a space plane. Most issues with reentry can be traced back to bad decent trajectory. (sometimes entering heavy can also contribute)

On struts. Those things are like parachutes. I only use them to stabilize landing gear, SRBs, and payloads (occluded from drag) these days. If the gear needs more than one each or my wings flex, I revisit the design first.

[Edax]

Most space planes have plenty of drag for safe reentry. At angels 7 terminal velocity of a good rocket stack is over Mach 3; it's about Mach 1 for a space plane. Most issues with reentry can be traced back to bad decent trajectory. (sometimes entering heavy can also contribute)

On struts. Those things are like parachutes. I only use them to stabilize landing gear, SRBs, and payloads (occluded from drag) these days. If the gear needs more than one each or my wings flex, I revisit the design first.

It's not just safe reentry I'm talking about, aerobraking is what I'm referring too. More drag means fewer aerobraking laps and better chance at an aerocapture. Plus, better braking power means more options on where to land on other worlds, when the terrain isn't more or less known.

Edited August 11, 2015 by Edax

[Nothalogh]

I'll just second the earlier mention of struts. They contribute a surprisingly detrimental amount of drag and weight, even if they're very short, so as much as possible try and design the plane to not need any struts.

KJR is your friend

[Feradose]

NonFAR: If your plane has open rocket nozzles do this

1-Put the blue tipped, blunt 1.25m aero cone on nozzle.

2-Clip it inside by offsetting until it doesn't block thrust.

Hacky, yes. But it works.

[KerikBalm]

Yeah, I got surprised by that more than once, with the Rapier pulling fuel out of what should have been drop tanks for another stage.

Yea, its actually pretty annoying, I wish we had a way to make fuel from a tank only flow downthe stack, without its liquid fuel being drained by jets (and Oxidizer as well if you've got rapiers on).

I've had problems with SSTOs having no fuel available to run LV-Ns, because the rapiers pulled everything out, and the remaining liquid fuel is sitting in a place the LV-Ns can't access.

That said... drag isn't so super important that you can't use a few struts or fuel lines, or a few radially attached items for big SSTOs.

If your SSTO masses 400 tons, the drag from several struts isn't going to affect it that much

When you get to high altitude, and thrust starts to drop off, it may mean you reach 1,250 m/s instead of 1,300 m/s... but it shouldn't kill your SSTO.

IIRC, drag in 1.04 is less than it was in 1.02. Jet thrust is just less. Overall, this should work to make draggy things a bit better than before once in rocket mode.

FWIW, I often don't bother fully enclosing my payloads in fairings, and oftenleave radial bits hanging out... I don't think the wider fairing (particularly as I can't have an oval cross section) is worth doing a full enclosure.

Sometimes this means adding a little dead weight to my otherwise vacuum optimied craft.

Like the LV-N tug sent to orbit with this fuel depot... its got some nosecones on it... but that weight shouldn't matter for a tug carrying even bigger payloads (I didn't even put a rear nose cone on it)

I didn't take a picture with the fairing still in place... it ended before the 3.75m tank, and the radial docking ports were exposed.

I can't be bothered to play find the pixel to make interstage fairings, so I make the fairing, and then put the payload on clipping through the fairing (this can make it pretty annoying to get the fairing debris to go away though)... I'm not even 100% sure it works well. I've seen from other threads that parts fully enclosed by the fairing are shielded... but I don't know how the rest of the stack fairs... does the large tank function as if it was occluded/placed behind another part, or does it function as if the stuff shielded by the fairing did not exist?

I don't know.

I do know that this thing was 400 tons almost exactly on launch, it had 24 tons of fuel left after payload seperation*, and the payload was 124.97 tons

So (125+24)/400 = 37.25% payload fraction... and yes it used struts, fuel lines, and radially exposed parts. No, it did not use shock cones, nor did it use nosecones on the back of the rapier (I thought about putting those on, but i dont want to increase part count any further, nor do I want to deal with the action groups for intakes again, or sorting issues of making sure there is adequate intake air)

I know you can get over 40% with SSTOs now... putting very dense ore cans in a Mk3 bay... but this SSTO gets things to orbit that won't fit in a mk3 bay

Granted... this SSTO did use the LV-Ns of the payload to help get into orbit, but they didn't contribute much relative ot the rapiers... I'm sure it would do better with just a sleek line of ore tanks.

*Well, the payload had more space to take fuel, but on the SSTO I filled up 8 of the bicouplers, and 4 of themk2 short LF tanks, to give it ~350 m/s of dV to adjust its orbit/deorbit, and then some LF to fly around a bit from a bad deorbit - I had half a mind to launch the payload empty anyway, and supply it from Mun/inmus ISRU operations)

Edited August 11, 2015 by KerikBalm

[Nothalogh]

Yea, its actually pretty annoying, I wish we had a way to make fuel from a tank only flow downthe stack, without its liquid fuel being drained by jets (and Oxidizer as well if you've got rapiers on).

I've had problems with SSTOs having no fuel available to run LV-Ns, because the rapiers pulled everything out, and the remaining liquid fuel is sitting in a place the LV-Ns can't access.

That said... drag isn't so super important that you can't use a few struts or fuel lines, or a few radially attached items for big SSTOs.

If your SSTO masses 400 tons, the drag from several struts isn't going to affect it that much

When you get to high altitude, and thrust starts to drop off, it may mean you reach 1,250 m/s instead of 1,300 m/s... but it shouldn't kill your SSTO.

IIRC, drag in 1.04 is less than it was in 1.02. Jet thrust is just less. Overall, this should work to make draggy things a bit better than before once in rocket mode.

FWIW, I often don't bother fully enclosing my payloads in fairings, and oftenleave radial bits hanging out... I don't think the wider fairing (particularly as I can't have an oval cross section) is worth doing a full enclosure.

Sometimes this means adding a little dead weight to my otherwise vacuum optimied craft.

Like the LV-N tug sent to orbit with this fuel depot... its got some nosecones on it... but that weight shouldn't matter for a tug carrying even bigger payloads (I didn't even put a rear nose cone on it)

https://scontent-mxp1-1.xx.fbcdn.net/hphotos-xaf1/v/t1.0-9/s720x720/11822430_10103801336797413_628457289837148052_n.jpg?oh=e03d901905c4e6cc6d1015dc9181bea3&oe=563A9279

I didn't take a picture with the fairing still in place... it ended before the 3.75m tank, and the radial docking ports were exposed.

I can't be bothered to play find the pixel to make interstage fairings, so I make the fairing, and then put the payload on clipping through the fairing (this can make it pretty annoying to get the fairing debris to go away though)... I'm not even 100% sure it works well. I've seen from other threads that parts fully enclosed by the fairing are shielded... but I don't know how the rest of the stack fairs... does the large tank function as if it was occluded/placed behind another part, or does it function as if the stuff shielded by the fairing did not exist?

I don't know.

I do know that this thing was 400 tons almost exactly on launch, it had 24 tons of fuel left after payload seperation*, and the payload was 124.97 tons

https://scontent-mxp1-1.xx.fbcdn.net/hphotos-xfa1/v/t1.0-9/s720x720/11880444_10103801336752503_7982726344018221251_n.jpg?oh=50185d1de72cf0b8ed4c7de3d0792cb6&oe=5643AC7F

So (125+24)/400 = 37.25% payload fraction... and yes it used struts, fuel lines, and radially exposed parts. No, it did not use shock cones, nor did it use nosecones on the back of the rapier (I thought about putting those on, but i dont want to increase part count any further, nor do I want to deal with the action groups for intakes again, or sorting issues of making sure there is adequate intake air)

I know you can get over 40% with SSTOs now... putting very dense ore cans in a Mk3 bay... but this SSTO gets things to orbit that won't fit in a mk3 bay

Granted... this SSTO did use the LV-Ns of the payload to help get into orbit, but they didn't contribute much relative ot the rapiers... I'm sure it would do better with just a sleek line of ore tanks.

*Well, the payload had more space to take fuel, but on the SSTO I filled up 8 of the bicouplers, and 4 of themk2 short LF tanks, to give it ~350 m/s of dV to adjust its orbit/deorbit, and then some LF to fly around a bit from a bad deorbit - I had half a mind to launch the payload empty anyway, and supply it from Mun/inmus ISRU operations)

That thing gives me the heebie jeebies just looking at it