Is this a good SSTO design? and what would I do to make it better?

[SalehRam]

I was testing this on sandbox to move it to career, but I want to know if it is good or I need to modify anything in it...

I do "have" some issues taking off, it seems a little bit hard on the craft to take off, but I don't know what is exactly wrong with the design...

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Here is the craft file also

https://drive.google.com/file/d/0B4vxG7-rvmD2MHpKN3otQVFFak0/view?usp=sharing

[Van Disaster]

Pitch control surfaces are pretty close to CoL - easy fix, add some small canards somewhere by the nose.

[Mister Dilsby]

Couple of comments:

--You probably have more engines than you need for the mass of that craft. A good rule is one turbojet for every 8-12 tons.

--You may not have enough intakes. Some people like to "airhog", I think 1 ram air or shock cone +2 big radial intakes per engine, or the equivalent, is a good number.

--Your center of lift is way at the back which could be the reason you're having trouble on takeoff. Moving the wings forward should help: putting canards on the front may help even more. Be careful where your center of mass is relative to COL, of course, and be sure that your COM is still forward of COL when the tanks are empty (you can check this in the SPH by right-clicking and temporarily emptying the tanks)

There are some very good comprehensive guides out there, but you will learn faster and IMO have more satisfaction if you just fly the ship and pay attention to what goes wrong. Then with a little thinking and maybe some help here on the Forum you will get a real deep understanding.

Good luck!

[Kelderek]

At first glance you might be a little low on lifting surfaces which would impact the necessary takeoff speed. The CoL might need to be closer to the CoM so you don't turn it into a lawn dart. Also, if you move the rear wheels forward so that they are immediately behind the CoM then that should make takeoff easier too. Add more intakes if you want a more efficient ascent, right now your engines probably need switch to rocket mode at a relatively low altitude due to low intake air.

[DeMatt]

Just off the pictures, I think A) it's got too many engines, and it's got too little wing surface. As-is, I think it's got too much "rocket" and not enough "jet".

Consider swapping the Rapiers for 2x Turbojet and 2x LV-909. Half the engine weight, and the thrust should still be adequate.

Also consider adding a pair of canards to the nose - AV-R8's or the actual canards. When you try to lift off the runway, these will help by pulling the nose UP, as opposed to the control surfaces at the rear trying to push the tail DOWN.

One question... is this designed for stock aerodynamics, or NEAR (or FAR)?

[SalehRam]

One question... is this designed for stock aerodynamics, or NEAR (or FAR)?

Oh, I'm using FAR, sorry for forgetting to mention that...

I'm changing some stuff based on your comments guys, thanks a lot and I'm looking for more advise...

I take it the main issues now are:

1- Too many engines

2- Engines are the wrong type (or I could use better engines to suite this design)

3- More control surfaces (need on nose)

4- CoL should be fixed by changing the wings...

[Van Disaster]

If it's for FAR then you definitely need more pitch control - for that design I might sort CoL and pitch issues by using fixed canards up front & moving the pitch controls to there. For FAR you might even have too much engine, actually.

Edit: Make that definitely too much engine - have put 65t into orbit with two RAPIERS, although that wasn't any fun.

Edited April 5, 2015 by Van Disaster

[Empiro]

Like others have said: fewer engines. You'll want CoL closer to CoM or you'll never be able to take off. Canards help by giving you more pitch control and by moving CoL forwards.

Make sure you have close to 2000 m/s delta-V in your rocket stage. In the current FAR, you probably won't get more than 1300 m/s and there's a lot more drag in the upper atmosphere.

Edited April 6, 2015 by Empiro

[Pecan]

Take the wings off and launch it vertically ;-0

Follow other people's advice if you want to make it a spaceplane rather than a SSTO.

[Van Disaster]

Even two engines is a bit much I think -

11 mins to a circular orbit is rocket territory.

[celem]

I take it the main issues now are:

1- Too many engines

2- Engines are the wrong type (or I could use better engines to suite this design)

3- More control surfaces (need on nose)

4- CoL should be fixed by changing the wings...

Dont forget Kelderek's excellent point about the rear wheels. I suspect this is much of the cause of the difficulties you have in getting the craft to rotate off the runway. Those back wheels need to be as close to the CoM as they can get while remaining behind it (so that the craft doesnt pop a wheelie when loaded). Otherwise, if you think about physics, specifically levers and moments, then it requires a lot of force to nose the aircraft up since its pivoting around the rear wheels. Obviously lift is doing part of the work, but if its sluggish to rotate, this may well be the culprit. (actually your CoL is behind the back wheels, so lift is actually punishing you by lifting the back-end and preventing rotation and the craft will theoretically lift easier at low speeds, very odd)

When it comes to tweaking the CoM and CoL. I know people say the CoM should be ahead of CoL, and this is true, but it should never be that far apart. I usually like the two indicators to be slightly overlapping. Since much of the front of your craft is drymass (the tanks are at the back) then the CoM is going to shift very little during flight and this issue is going to come back and bite you every step of the road to orbit.

Finally, the amount of pitch authority you have is partly (largely) dependant on the distance from the CoM to the pitch control surfaces. With your CoM so far back you lose effective pitch from these. This is physics levers again. Id want to shunt the CoM forward a chunk, and the CoL forward significantly (canards up-front will accomplish a lot of this). That way you have a nice long lever for the pitch control surfaces to work with. If you look at historical real world designs you might spot some odd-looking things with really long tails, this is specifically to lengthen that lever.

Since its FAR there are some more fun issues for you down the road such as mach tuck (CoL moves backward as you go supersonic), this means you dont want to nestle the two centres too closely together, but still, much closer than they are now.

Edited April 7, 2015 by celem
fix

[capi3101]

FAR eh? Well, dammit. If it'd been a stock aero craft I could've told you what was wrong with it - I'd have pointed you to DocMoriarty's KSP Space Plane Construction and Operation Guide (which would've confirmed everyone else's diagnosis of too much engine and not enough wing). As it is, I'm still learning the rules for FAR planes my own self.

From what I've learned so far, you've definitely got more engine than you need; in general a TWR of between 0.55 and 0.75 at takeoff is plenty. Your pitch authority is also minimal; I might suggest a set of canards for this design, placed high on the cockpit. Take that advice with a grain of salt as I'm still learning; generally the pitching surfaces will have less effectiveness if they're placed in-line with the main wing. The rear wheels could stand to be moved forward a tad; finding the exact right spot for those things can be a pain (even in stock aero).

Start with that and see where it gets you. Myself, I think I'll clone your design and see what I can learn from it...

EDIT: Have you thought about installing RCS Build Aid? I'd highly recommend it for any spaceplane designer; it'll let you know how much (and where) your CoM will shift in flight. Also tells you how much torque you'll get from your engine placement and (as you might expect) greatly assists with the placement of RCS blocks. One other thing I've learned - and this in the stock game - is that when it comes to spaceplanes the little single-spurt RCS ports tend to work a lot better than the general RCS blocks. Just a suggestion there.

Edited April 6, 2015 by capi3101

[SalehRam]

Thank you all for your valuable comments <3

Right now, I have came up with this based on your feedback guys:

Well, here are the images on my Google drive https://drive.google.com/folderview?id=0B4vxG7-rvmD2ZmF4bTJVNGNJcmc&usp=sharing

One question though, controlling the thing now is a bit more sensitive, and I guess it is a normal thing, however, on high altitudes, is it normal for the craft to be "more" out of control?

I say that because I stall every time I reach about 20,000m high...

Also, when taking off, I guess I need to do it slowly not to hit the engines with the runway ground... so it is normal as well?

- - - Updated - - -

I will check also RCS Build Aid as well now

Edited April 6, 2015 by SalehRam

[capi3101]

I can't see your craft at the moment; I think imgur is having a bad hair day, because it's not just you...

Don't know if it's a "normal thing" or not. What is the FAR stability analyses telling you? Check these benchmark values: 0k/Mach 0.35, 5k/Mach 0.85, 10k/Mach 1, 15k/Mach 2, 20k/Mach 3, 30k/Mach 4. Any red numbers pop up there? If so, you'll need to do simulation studies to determine if it's something that needs to be fixed or if it's just going to be annoying. Might also check to see what the level-flight AoAs are doing at each of those benchmark levels.

Get RCS Build Aid too - it's possible that your fuel load is making your CoM shift aft of your CoL at about the point you go into your stall. Wouldn't be able to say one way or another without being able to see the plane...

[SalehRam]

Just updated the reply with link to photos.

[capi3101]

Yeah - my guess is the CoM shift. I'm also wondering if I really should've suggested canards; as a rule they're destabilizing surfaces. Might try a smaller part there - like an AV-R8 winglet, perhaps - something that won't throw the CoL quite so far forward.

[UnusualAttitude]

SalehRam, you seem to have taken things to the opposite extreme in terms of rocketry... two LV-909s will not get your 24-tonne spaceplane to orbit.

I suggest you keep the outer Rapiers instead of those turbojets: you'll still have more than enough jet power. Use the Rapiers in rocket mode to boost out of the atmosphere and use the more efficient 909s for circularization and orbital maneuvering.

Yes, controlling an aircraft generally does get harder the higher you get. Looking at your craft I see it lacks reaction wheels (one is enough, but the torque provided by the cockpit alone may not be enough), and might not have enough fin area to keep it stable at high altitude.

I would suggest adding a small Mk2 cargo bay behind the docking port to lengthen the craft and pull the CoM forward (then adjust the wing accordingly). It will improve the effectiveness of your tail fin and give you somewhere to put accessories such as reaction wheels, batteries, solar panels, maybe even a small payload or a science package. Remember to readjust your landing gear if you do this...

Edited April 7, 2015 by UnusualAttitude
typo

[capi3101]

Went ahead and built a clone of your original craft this evening. There's some pretty hellacious Q right there at take-off; more than once I pulled up too hard and ripped the plane apart from the dynamic forces...and the plane has a very high takeoff speed, around 150 m/s. Level flight AoA for takeoff was 12 degrees. That's assuming I built the clone correctly. I did get it off the ground a few times - I just had to nurse it a bit to keep the Q survivable.

Stability analyses showed a number in the red - M_w - at 20k / Mach 3, and the simulation studies showed the system to be unstable at that point. M_w in the red means pitch-up behavior, and it's been established the plane has poor pitch authority. That's why you were going into the stall.

[Van Disaster]

SalehRam: Rapiers are not the most efficient engines of themselves, but when you allow for the mass of carrying seperate rocket engines around then they become more attractive. You can get a craft that size to orbit ok with just one Rapier - if you look at the bottom of the first page of the thread that was my version of your initial craft, with canards, without the extra two engines, and that was in orbit stupidly fast.

Rear wheels are under the vertical fins, pulls off the runway fine - that's another advantage of canards.

Irrespective of how many engines you end up with, there's a couple of guidelines to follow:

* Keep CoM and CoL in reasonable proximity - CoL is basically a force pushing up, and CoM is a force pushing down, so the further apart they are the bigger the leverage you have to counter with pitch controls. CoL moves rearwards when you go supersonic - how much depends on the chord length of your wings, it's quite a bit for deltas. You can move CoM in flight once you've burned some fuel by moving what's left of the fuel around.

* The further either are from your control surfaces the better, in general - this is forces & levers again. Force is the area of the surface in this case; that's a bit simplistic because it depends on the current Q figure as well, but in broad terms that will do.

* Unless they're close together I'd use partially fixed canards ( or a partially fixed tail ) - this is just for stability & it's quite easy to stall fully moving canards trying to pull up, and you're in trouble if they stall when you're struggling to fly level.

* TWR in jet mode of >0.35 is enough. Slow, but enough. Actually > 0.28ish is enough but not if you value your life

[Hodo]

This was a design I made a while ago for a stock (FAR) "fighter" it worked quite well as an SSTO and was quite easy to fly even at low speeds.