GoSlash27

haha that sounds like a challenge RAPIERs were definitely inferior in .90 and earlier. I used to do some pretty outlandish things with turbojets and 48-7Ss back then and I didn't even need to airhog to do it. But from what I've seen so far, those days are gone. My RAPIER designs are soundly trouncing my TJ designs. I'll see if I can come up with an all-fuel spaceplane for this challenge. Best, -Slashy

Rune, I'm not so sure about that. 800m/sec at 20km takes a whole lot more DV to overcome than 1200m/sec at 23km. Plus you've got a tonne more engine mass and 2 nodes that the RAPIER doesn't need, which means it's draggier. My experience so far has been that by the time you load down a TJ to where it's barely clearing Mach 1, it's top end is completely trashed to the point that it can't even hit Mach 2 at 18km and it's payload fraction drops precipitously. What kind of payload fraction is this one getting? Best, -Slashy

They rarely are Best, -Slashy

Well, you live and you learn You will always have deviations. Even in real life, there's something out there to toss you around a bit. Whether it's a rocket or airplane, you want it to naturally seek a stable attitude when disturbed. Rockets with the drag in the back naturally tend to point where they're going. Rockets with the drag in the front naturally want to fly backwards. -Slashy

Tarmenius, They were always the least effective intakes (excepting the fuselage intakes). You haven't been using them incorrectly. Best, -Slashy

All well and good. You can kick off the Jumbos with a couple sepratrons and proceed. The point is that you can't have all the drag in the nose of the rocket and expect it to not flip out. so long as you have more drag in the back than you do in the front, it'll fly fine. Also, those Mainsails have about twice the thrust needed for this job. You could add a whole lot of DV just by increasing the fuel. -Slashy

You don't have to add wings. You don't even have to add control fins. Just put the drag in the back like I said. -Slashy

*Snort* +rep -Slashy

Nikky, At this point, how could you be "pretty sure" about anything in this area? You're the one with the rapid disassembly problem! JoCRaM's advice is sound.

I'd just send a rescue tanker with a klaw. Clamp onto the ship, refuel it, send it home. Best, -Slashy

Connocclar, Actually, they are built backwards from the staging. First item of business is to build the spaceplane itself. You need to get it all balanced out so it'll fly okay. Then you attach the tank, check the CoM location both full and empty, and point the main engine(s) so it's aimed at the center of that line. Then you attach the SRBs. They need to be aligned with the CoM when the tank is full, which is toward the shuttle's side of the tank's CoM. This should get you in the ballpark. After that, it's a lot of trial-and-error tweaking, rebalancing, etc. Good luck! -Slashy

NikkyD, Observe... Scale model of your launch vehicle. It wants to fly anywhere but straight. But if you build it like I say... It flies just fine. Draggy stuff needs to be in the back. Best, -Slashy

Nikky, Chicken or the egg argument. It's "wobbling" (oscillating) because it's unstable and your SAS is trying to correct it. This leads to bigger and bigger deviations until it finally disassembles. If you want a working booster, I suggest trying my advice. Spaceplane mounted as far down the stack as you can get it and fins in the back of your boosters instead of the front. You'll see the entire problem disappear. Best, -Slashy

georgTF, The difference is primarily the amount of intake area. If your intake area is insufficient, it's like an olympic sprinter trying to run while breathing through a straw. You need "enough" intake area, but not necessarily "more". A single shock cone, or a pair of XM-G50s, or about 6 of the structural scoops is fine to feed a single turbojet. Any more than that is wasted mass and added drag. There's also the drag coefficient of the forward-most part. A docking port is draggier than a nose cone, which is draggier than a tail cone. Best, -Slashy

NikkyD, There are a lot of designs that will fly just fine so long as there is no deviation from the course. They are what we call "dynamically unstable". The problem isn't that you're getting small deviations, it's that your rocket is unstable. If you build it so that the drag is behind the CoM instead of in front of it, your rocket can handle deviations without going 'splodey. Best, -Slashy

Oh, just saw your design. I don't know why it didn't come up the first time. Yeah, unfortunately that's not gonna work that way. You could try moving the control fins on the boosters from the top to the bottom. That would help some. Perhaps sandwich the plane in between the boosters further down. You really can't have all that drag in the nose of the rocket like that. It's like trying to shoot an arrow with the fins on the front. Best, -Slashy

NikkyD, Oh, it's possible. Just a major pain. http://s52.photobucket.com/user/GoSlash27/slideshow/KSP/shuttle%20konstitution/Kontender The trick is to align the thrust with the center of mass so that the thrust isn't trying to rotate the launch stack. Notice here that the main engines are pointed at the center of the CoM's shift and the SRBs are offset slightly toward the shuttle. It helps to have the tank as far forward as you can manage. You don't necessarily want more control surfaces to add stability, either. I placed Vernor engines around the launch vehicle and launched with RCS engaged. Good luck! -Slashy

Pappus, Sorry I missed this earlier. Mach 2 is around 650 m/sec, so you're still over Mach 2 there. Turbojet spaceplanes have a tighter margin than RAPIERs, so the balance point where your spaceplane will work is also tighter. This means that a TJ design is harder to pull off than a RAPIER design. What I've found for TJ designs is that they don't respond well to overloading. A RAPIER can be loaded to where it needs to dive to break Mach 1 and still get plenty of speed and altitude for the closed cycle portion of the ascent. Turbojets are different. They will lose top end speed and altitude long before they have any issues around Mach 1. This is crippling for turbojets because they run out of steam at a lower speed and altitude and it takes a lot of fuel to muscle out of the hole on rockets. 100 m/sec or 1 km altitude is a lot to make up in the region where turbojets crap out. But OTOH you *have* to have enough fuel on board and engine thrust to get you out of the hole and into orbit. If you don't have that, it doesn't matter how high and fast you're going when you kick in the rockets. You ain't making orbit, so there's no such thing as "better" or "worse". So try this: Start over on your spaceplane. Keep it around 30 tonnes total mass. 300 units of fuel for the jets, the rest LF&O for the Poodle. And here's the important part: Make the whole darn thing fuel tanks. Get that into orbit and see how much LF&O you had left over. The leftover fuel and oxidizer (minus the reserve for maneuvers in orbit) is how much of your design can be dedicated to payload instead of fuel. This is where I always start when I'm designing an SSTO spaceplane, whatever the type. Good luck! -Slashy

Pappus, Q1: the turbojets will take fuel from anywhere on the craft, while the Rockets will only drain fuel that is connected to them. You can use plumbing to feed fuel from unconnected tanks. Q2: you could use less than optimal engines, but their lower Isp means they will require more fuel to make orbit. More fuel means more structure and wings, which compounds the problem. I really recommend going with the poodle for this job. Best, -Slashy

Pappus, It looks to me like you're about 10-15 tonnes short on fuel & O2, and you're underpowered on rocket thrust. Your turbojets are easily strong enough to push a 30-35 tonne spaceplane up to 20km at Mach 2. Your rockets can take over from there, but if you're under .5 t/w ratio, you will lose too much to drag and may not be able to escape. Finally, don't expect to exceed 14% of your total vehicle mass as payload if you're running a turbojet hybrid. You need to be pretty efficient to hit that mark. Here's an example of a working 2 jet SSTO: The wings are half- filled with fuel. All other tanks are full. It makes 72x72 LKO with over 350 m/sec DV and still some reserve fuel for powered landing. Even at this size, you can see how little payload fraction a turbojet hybrid is capable of. You can probably get 2 tonnes or so out of it if you omit the passenger compartment and docking port. You can push that up to 4 if you go with remote guidance instead of a flight crew. Good luck! -Slashy

^ that. Best, -Slashy

Tarmenius, Thanks for hosting this challenge. It really got me thinking about turbojet SSTOs and what it takes to make them work efficiently. First thing that I noticed is that they shouldn't be balanced to just barely get supersonic in level flight like you would do with RAPIERs. By the time you hit that wall, you've already wrecked your top speed and altitude, and this is far more important for the TJ. Probably a good idea for efficiency to keep the mass around 13-15 tonnes per engine. I also built a twin engine testbed to compare it to my twin RAPIER design. The RAPIER got 22% payload fraction while the turbojet managed 14%, which really ain't too shabby. There are disposables that can exceed 14% payload fraction, but I could easily see saving money by doing small jobs with the turbojet SSTO. Thanks again, -Slashy

You kiddin' me?? If I had a Jeb plushie, I'd hug it every night! /too old for petty concerns like "shame" -Slashy

All true, but again... it misses my point. Yes, the math is daunting and it's certainly not for everyone. It's easier (for the most part) to use a mod to make the info more readily accessible and I don't suggest that anyone else should make their lives harder on my account or that they're somehow less worthy because they choose not to saddle themselves with the math. How they play their game is their business and whatever gives them the most enjoyment is what's "right". All I'm saying is that it's fallacious to assume that people who play stock don't have access to all the info that these mods provide and are therefore resorting to guesswork or flying blind. There's a lot of us out here who run stock and are doing all the same things as the people who use info mods. We have all the same info, it's just not in a popup window in-game. Best, -Slashy

^ This. Kerbal rescues and satellite contracts are also good experience and they pay well. Just be careful about which ones you accept. Best, -Slashy