couple spaceplane questions

[Ketatrypt]

After a few attempts, I finally managed to build a space plane that managed to make it to orbit and back! pretty happy about that! was one of the more difficult things I've built so far.

 

I was trying to make it with 1 and a half cargo bay things there, and in previous attempts I found that the 4 engine combo + wider wings works wonders. But then another issue imerged: wing strength. It maxes out at about 10g's before the wings snap. they have a lot of excess flex. Any ideas to stiffen up the wings?

And also, I am wondering if the swivel engines are the best for this job? should I be using something else?

 

thanks

[WhiteKnuckle]

Short answer: Struts!

A few struts going from the wing root to the part it's attached to will help strengthen things.

Long answer: Too many parts.

(from the center out) You've got a central body, then one nacelle, then another nacelle, then a wing and then another wing bolted to that. That's a lot of connections that can flex and snap.

I would say ditch one set of the side nacelles and the engines bolted to them, and ditch that complicated multipart wing for a Big-S wing witch has the added benefit of being able to carry fuel. This will make the craft stronger, less draggy, faster, lighter, ect.

I would ditch the whole RCS system unless you really need to dock this thing (and I don't see a docking port)

I would ditch the swivel engines. Two Rapiers and two turbojets should be enough, four Rapiers would be another way to go. Or precooled turbojets and nukes (but that would involve changing all your fuel tanks to LF only)

 

Spaceplanes/SSTO design can be complex, but it should always boil down to:

1. What do you want it to do?

2. How can you use the fewest parts/lowest mass/most efficient parts to accomplish that goal?

 

You've got a good looking plane there, and it makes it to orbit so that's most of the battle right there. Your goal now should be to put it on a diet and get rid of any excess fat that isn't really helping you.  

Edited February 6, 2016 by WhiteKnuckle

[Ketatrypt]

ahh thanks. I don't think I have any of those parts unlocked yet (I am playing on carreer, heard its a good way to learn, and ive been enjoying it) and I just unlocked the turbojet engines. So thought I would try to build a spaceplane

 

I havn't really a use for it yet other then experimentation, and maybe some contracts for launching sats/rescuing kerbals. I did throw on a jr docking port, as I may use it for refueling things, but, I don't really know yet. I've only tested it to orbit once so far. I am not even sure of the max payload yet, but I would be entirely happy with getting 15 or so tons to LKO. On my test run I went up with an empty bay and had about 35% rocket fuel remaining.

I will also have to try different combos of engines. 3 of each engine might make it, but with 2 engines I ran out of rocket fuel too soon. The other engines I have unlocked of that size would be the reliant which has more thrust but not as efficient, and also doesn't swivel, and the terrier which I doubt would have enough thrust.

So yea, thanks! im gonna go add some extra struts and see if that at least helps the wing flex

Edited February 6, 2016 by Ketatrypt

[Starhawk]

As WhiteKnuckle said, wing parts connected to other wing parts ends up giving you a lot of flex.  Especially when connected to the fuselage through several other parts.

A couple of things I do.  I always attach my wings directly to the core of the craft.  With creative use of the rotate and offset gizmos in combination, you can place them so that it looks like they're attached to the outer set of fuel tanks if you want that, but gives you the solidity of direct attachment..

I use larger wing parts, if available, to reduce part count.

For instance, I would try to use two pairs of delta wings on the craft you show, rather than building a large delta wing out of several pieces.  Wings work just as well if placed backwards, so that can give you some esthetic alternatives if you hate the look.  Another possible advantage to this approach is that your roll control surfaces can be set close to your CoM on the forward set of wings and your pitch control surfaces can be set far behind the CoM on the rear set of wings, both of which are advantageous to handling.

On the matter of struts.  I always try to absolutely minimize the number of struts I use on spaceplanes.  This matters much more with smaller planes than larger ones because of the square-cube law.  Struts and external fuel ducts are very draggy.  I used to build wider wings and strut them for strength pre-1.0, but with the new aero and the drag forces I find that method prohibitive now.

Hope this helps.

Happy landings!

[Snark]

+1 to "too many parts."

Instead of having fuselage -> nacelle -> nacelle -> wing, consider doing fuselage -> wing directly, and then mount the nacelles with 4x symmetry around the fuselage.  It'll be a lot stronger.

And yes, struts can help... but they're very draggy.  A couple of struts are fine, if they're well-placed and help a lot, but if you find yourself putting half a dozen or more, it's probably better to rethink the design rather than spamming struts.

Bigger parts are better.  Once you get a little deeper into the tech tree, you'll unlock the "Big-S" airplane parts.  A single Big-S delta will be about as big as your whole wing that you currently have made up of lots of little parts, while being lots stronger (and having some fuel-storage capacity, too).

A suggestion, move the canards that you currently have on the nacelles farther forward-- e.g. up at the front of the fuselage.  They'll have a longer lever arm and give you a lot more pitch authority.

One question-- why the SRBs?  Those are almost always a poor choice for spaceplanes.  I love SRBs, but they belong in one place and one place only, and that's on the launchpad, never on an upper stage.  They have crappy Isp, which means you really, really don't want to be lifting them with other engines; too little benefit for too much mass.  The only place for SRBs is as a 1st stage, and spaceplanes generally don't need that because they have air-breathing engines to take off with, which have stupendously better Isp.

So... if you're using them as an initial boost down the runway because your jet engines don't have enough power to get you airborne before the end of the runway, and/or you just like taking off with a roaring column of flame streaming behind you, then fine. SRBs are a fairly cheap amusement, that way.    However, if you're using the SRBs as an upper-stage engine-- for example, as a booster stage to help get to orbit once the airbreathing engines conk out-- then you'd be better served by replacing them with liquid-fueled engines.

[Ketatrypt]

So I took some of the advice yas gave, and refined the ship to this:

Stripped out a lot of the wing section, added another pair of the air breathing engines (was having trouble at the mach barrier), and extended the cargo bay (yay!!)

it works much better then the first one I did, was able to lift a 8.5 ton payload to a 80km LKO, and still have about 25% fuel remaining.

And about those air-breathing engines.. when I initially put this together, I figured each engine would need an air intake, but, then I made this, and even with 6 engines sharing 4 air intakes the engines didn't have any problems. What sort of ratio is ideal for those intakes to engine quantity? would just 2 intakes be able to feed all 6 engines?

I have the SRB's on there for rocket assisted take off. They look cool. no real reason. they are jettisoned at the end of the runway (that's as far as they last) and they parachute down for recovery. I also now have a pair of drop tanks as well, which get me to 2km altitude before I start into the main tanks.

 

Thanks very much! this thing really is coming together

Edited February 6, 2016 by Ketatrypt

[WhiteKnuckle]

8.5t to LKO with a Mk2 spaceplane is nothing to scoff at. Sounds like you're on a roll.

 

[Ketatrypt]

so after delivering the cargo into orbit, I came back in. Re-entry itself went quite nicely, even though I came in a bit steep, nothing overheated too badly. very hot none the less.

But then the troubles started. As I entered into the lower atmosphere at a sedate  650 m/s I suddenly entered a flatspin. Even with all my fuel as far forward as possible I could not regain control. Ended up in an inverted flat-spin and fell like that for the last 10km or so. popped the single chute I had at the rear, and even that wouldn't pull me outta the spin until it fully deployed.. so I have some serious stability issues with empty-ish tanks thankfully the craft is still in its unmanned testing phase.

[Vegetal]

I see you are getting some good cargo to orbit with that. Just some optimization advice: If you are using 4 swivels, I think it's time to consider the skipper. It's way more weight efficient. Try using a mk2 to 2.5m adapter and then the skipper.

Also, 6 jets seem a bit too much for that cargo size. I think your plane would benefit a lot from streamlining: Positive incidence angle on the wings, and ditching those bi-adapters for the jets.

But the best advice: Define a mission. Experiment until you can accomplish that mission with the smallest and most efficient craft possible.

Edit: Ah I almost forgot, you could use the aerospike too instead of swivels. They are quite a bit more fuel efficient.

Edited February 6, 2016 by Vegetal

[Snark]

1 hour ago, Ketatrypt said:

But then the troubles started. As I entered into the lower atmosphere at a sedate  650 m/s I suddenly entered a flatspin. Even with all my fuel as far forward as possible I could not regain control. Ended up in an inverted flat-spin and fell like that for the last 10km or so. popped the single chute I had at the rear, and even that wouldn't pull me outta the spin until it fully deployed.. so I have some serious stability issues with empty-ish tanks thankfully the craft is still in its unmanned testing phase.

So, a couple of things.

First, make sure that your vertical stabilizers are behaving as expected.  I've heard that designs with twin vertical stabilizers can run into some sort of control issues, depending on how they're set up.  If you can manage, a single big vertical stabilizer may be preferable.  For example, instead of having two steerable stabilizers the way you have now, consider having just one (mounted in the middle), and then add some fixed (non-steerable) vertical fins at the back to help with yaw stability.

Second, make sure the vertical stabilizer is mounted as far aft as you can get away with.  The farther behind your CoM it is, the more effective it will be.

Third, disable roll authority on your vertical stabilizer.  You can do this by right-clicking in the VAB or SPH.  Vertical stabilizers have one purpose and one purpose only, and that's to help with yaw; so they should be enabled only for yaw.  Here's why it's important:  Because the vertical stabilizer is "unpaired" (i.e. is just on top, without a counterpart underneath), then it unavoidably couples roll and yaw.  Every time it deflects to try to induce yaw, it will cause some roll-- it's unavoidable.  And the roll will be in the opposite direction as the yaw, so that if the stabilizer has roll authority turned on, it will end up fighting itself.  Also, if roll authority is turned on, then SAS will try to use the stabilizer to roll the plane, whcih induces yaw, and hilarity ensues.

The moral of the story is that vertical stabilizers become far more effective when they have roll authority turned off.

[Ketatrypt]

So think I finally got this polished off pretty nicely fixed the spinning issue by adding a couple basic fixed fins, and overall, redone the rear wing system. and also added the skipper engine which does add a bit of efficiency. Not quite as much thrust, but it is half the weight of those 4 swivels 

 

Now I just wish I could get some contracts where I can put this excrementstle to use

 

As for the air breathing engines - I can't seem to get away with less then 6 for this. When I try 4 engines I will get stuck on the mach barrier, and without breaking the mach barrier I can't get the thrust needed to climb to higher altitudes. Where as with 6 engines I can pretty much keep a fairly steady 20 degree climb up to altitude. although I am still getting used to 'flying' to orbit, so I will prolly learn how to gain altitude more efficiently with time.

[Starhawk]

10 minutes ago, Ketatrypt said:

As for the air breathing engines - I can't seem to get away with less then 6 for this. When I try 4 engines I will get stuck on the mach barrier, and without breaking the mach barrier I can't get the thrust needed to climb to higher altitudes. Where as with 6 engines I can pretty much keep a fairly steady 20 degree climb up to altitude. although I am still getting used to 'flying' to orbit, so I will prolly learn how to gain altitude more efficiently with time.

If you're happy with the way it's performing then I think you should go with the six engine setup.  Ease of use (i.e. play time) can trump efficiency quite easily depending on the circumstances.

Anyway, congratulations!  It's looking good.

Happy landings!

[ForScience6686]

20 degrees seems high for your initial climb, you could always nose down a bit to get over the mach barrier.  Or accelerate over the barrier at lower altitude before making your climb.

[Ketatrypt]

2 hours ago, ForScience6686 said:

20 degrees seems high for your initial climb, you could always nose down a bit to get over the mach barrier.  Or accelerate over the barrier at lower altitude before making your climb.

what is an optimal climb? I just did a mission with my current setup, and made a note to time it, and it took me aprox 11 mins from runway to 75km LKO.. and all that time the engines are doing their thing at full blast (I start up the rocket engine once my velocity starts dropping from the ram jets, at ~20 kms)

 

also, is there a way to secure objects in the cargohold when in space so I can capture something and re-enter with it? I tried using a docking port, but it flexed like crazy and didn't work good. And also would be nice to take things down without them needing a docking port. Are the grabber things strong enough for tht?

 

thx

Edited February 7, 2016 by Ketatrypt

[ForScience6686]

Sorry what I meant was, it seems high if you're having troubles at the mach barrier.  On low Twr, I will climb slowly until I break that barrier then increase the climb rate.  You have more thrust available at lower altitudes, but you may struggle when in the thinner air.  

For your second question I won't be much help.  I send probes up if I need to bring an item down.  Only use the ssto for delivering or rescues, occasionally a lko tourist.  The claw may be better but I cannot confirm this.

 

[Vegetal]

From what I see, you have too little wing area, and because of that you are probably working with too high Angle of Attack (AoA). Mk2 fuselages do generate lift, but are not very efficient at that.

The ideal flight profile is pointing straight at prograde, so as to neutralize most of the fuselage drag. The way to do this is to give your wings some positive angle of incidence (the wings point up) by just a little bit, 5 degrees or less. But your plane could definitely benefit from a larger wing area (and less wing loading). If you try using some angle on your wings and something goes wrong, try at least increasing wing area, you will not regret it.

If you succeed in streamlining your plane this way, i bet you could ditch 2 of those jets easily. Because of your high AoA flight, you surely have a lot of unwanted drag that's making it hard to push through the mach barrier.

As for the cargo bay issue: From my experience, once you have nothing docked, the next thing you dock *won't* clip through the bay. That means you could go back and land even with a non-strutted payload, I already did this a couple of times with Mk3 spaceplanes. If your payload really needs strutting, then I can only recommend the KAS mod. I usually don't do mods but this one is top quality.

EDIT: Just tested your design, with some modifications by me . 4 jets, increased wing area and slight incidence adjustment. Around 1:10 after takeoff, mach barrier is broken and plane is accelerating fast. 80km orbit reached and KER indicates more than 1600 dV. Yeah, the plane has no cargo, but that's enough to orbit the Mun and come back. Give it a try!
 

Edited February 7, 2016 by Vegetal

[Temstar]

22 hours ago, Ketatrypt said:

what is an optimal climb?

For large cargo SSTO with medium TWR I climb at 5 degrees up angle pretty much all the way.

For low TWR you still climb the same way, but around the sound barrier you need to do a shallow dive in order for the engines to gather enough ramjet power to break mach 1.

Edited February 8, 2016 by Temstar

[Ketatrypt]

3 hours ago, Vegetal said:

EDIT: Just tested your design, with some modifications by me . 4 jets, increased wing area and slight incidence adjustment. Around 1:10 after takeoff, mach barrier is broken and plane is accelerating fast. 80km orbit reached and KER indicates more than 1600 dV. Yeah, the plane has no cargo, but that's enough to orbit the Mun and come back. Give it a try!

 

Oh wow! Can you post a screenshot so I can see what exactly you did? I've been trying to get rid of those 2 extra engines now for a bit... I am really wondering where you put in the extra wings. the center of mass on these kerbal planes is always so far back that its hard to get good wing area without making extra wide wings (which are extra wobbly)

 

edit - Woo finally did it! the lack of lift was exactly what was causing the issue.. I still feel like I should be adding a bit more lift, as I still don't fully straighten out into the air stream until about 550 m/s. The wings have the smallest amount of angle of incidence that I could give them, because any more then the smallest amount the fuselage generates negative lift at high speed which I doubt is a good thing.

but yea, I felt like a boat trying to plane out on the water - before, I just powered my way up onto plane - now this is definitely better I think I still might need another small wing, as I feel I am still stuck in the hole a bit - I needed to do a shallow dive to get up over the barrier, but it worked, and I do have about 7.5t in the cargo bay.

here is what it is now with the extra wings: (I still think 1 small wing piece wouldn't hurt, and I gotta get rid of those front canards. I don't like them interfering with intake air, even though I realize it isn't modeled in the game)

Edited February 8, 2016 by Ketatrypt

[Vegetal]

Well I was about to post a pic of what I did, but it looks like I don't need to do that anymore, because you basically did what I did, with slight differences.

I feel your urge to ditch the canards, but you are going to have less pitch authority if you do that. A lot less.

[Ketatrypt]

yea thanks for the tips! this thing  is flying great now. im glad to finally ditch those 2 extra engines, that saves like 4 tons of weight

pretty fun tho, brings an entire new dynamic to rocket building

[Edax]

On 2/6/2016 at 5:58 AM, Ketatrypt said:

 

But then another issue imerged: wing strength. It maxes out at about 10g's before the wings snap. they have a lot of excess flex. Any ideas to stiffen up the wings?

In my experience, "swept wings" are the among the sturdiest.  I would try and avoid attaching any wing part to another wing part.  A solution I have found to increase lift using "swept wings" is to stack them on top of each other to create a bi-plane of sorts.

 

5 hours ago, Ketatrypt said:

 

Oh wow! Can you post a screenshot so I can see what exactly you did? I've been trying to get rid of those 2 extra engines now for a bit... I am really wondering where you put in the extra wings. the center of mass on these kerbal planes is always so far back that its hard to get good wing area without making extra wide wings (which are extra wobbly)

 

edit - Woo finally did it! the lack of lift was exactly what was causing the issue.. I still feel like I should be adding a bit more lift, as I still don't fully straighten out into the air stream until about 550 m/s. The wings have the smallest amount of angle of incidence that I could give them, because any more then the smallest amount the fuselage generates negative lift at high speed which I doubt is a good thing.

but yea, I felt like a boat trying to plane out on the water - before, I just powered my way up onto plane - now this is definitely better I think I still might need another small wing, as I feel I am still stuck in the hole a bit - I needed to do a shallow dive to get up over the barrier, but it worked, and I do have about 7.5t in the cargo bay.

here is what it is now with the extra wings: (I still think 1 small wing piece wouldn't hurt, and I gotta get rid of those front canards. I don't like them interfering with intake air, even though I realize it isn't modeled in the game)

I'm questioning the amount of wings pieces you've got there anyway, I think you have more then enough lift there.  The closest analog I've got is my orbital crew shuttle, and it's only using the one wing, and it has enough lift to safely land on the island runway fully loaded.  I think your problem is the lack of control surfaces that makes control difficult [notice I'm using double winglets on the back of my ship for extra pitch control] (you don't need much lift to take off into space anyway, it's landing where lift become extremely important.)  When looking at your ship, I'm not seeing any tailplane at all, just control surfaces at the edges of the main wing (which best control roll, just 2 of those small control surfaces are not enough to control pitch on such a large plane in my opinion)

Edited February 8, 2016 by Edax

[RizzoTheRat]

If vegetal's getting a modified version to orbit with plenty of fuel left, you might want to try swapping out the twin nacelle/tank setup for Mk2 fuselage tanks with bi-adaptors both ends for intakes and engines.  This will give you the same number of engine and intake mounts, less weight, and additional lift, but at the cost of less fuel

[Vegetal]

I don't know Edax, i just threw together quickly a crew shuttle with the same tech level, and used half the engines you did. 2 jets and one spike. I think you have too much gear on that plane for a 6 crew shuttle...

[Edax]

6 hours ago, Vegetal said:

I don't know Edax, i just threw together quickly a crew shuttle with the same tech level, and used half the engines you did. 2 jets and one spike. I think you have too much gear on that plane for a 6 crew shuttle...

I'll admit, I designed this thing much earlier in the tech tree and in a different incarnation of KSP [1.0.2] (as if having 4 shockcones for 4 turbojets wasn't a dead giveaway).  I've only slightly modernized it, but it still flies well, looks cool, lands easily, and still does it's job of getting a crew to my station and landing back at KSC, so I've never saw the point of giving it an overhaul to increase efficiency.  (Plus, my space program only has 8 Kerbals in it, so a 6-seater is already plenty).  I only dug this thing out because it look similar to the OP's ship to do a comparison.

 

And anyway, if Mad Max has taught us anything, MOAR ENGINES is never a sin.

Edited February 9, 2016 by Edax

[Ketatrypt]

I think I got this quite nice. Did a few trips hauling some equipment to orbit, and it seems really nice. I still need to swap a rocket fuel tank to a jet fuel tank, but that's not a huge issue. either way I have tremendous amounts of fuel. its great to haul up to my fuel station. soo cheap compared to lifting a disposable tank with a disposable rocket.

It doesn't have the awesome cool arrow look it had, but function over form I suppose.

 

Lining up to dock just as the mun eclipses the sun

 

and clamps engaged. Just in time for a great view... too bad the windscreen is facing the wrong way for the pilot

Edited February 9, 2016 by Ketatrypt