My previous thread about the SSTO rockets I use to send kerbals up and down from orbit got lost in the Great Forum Wipe, but here is some information about an even smaller SSTO that I've been playing with today. I was testing some cut-down versions of my SSTO crew rocket to make a single-kerbal rescue vehicle for use on Laythe (so that if one the kerbals gets stranded on another island by wreaking his BirdDog rover/plane, there will be some way to retrieve him without using the bigger 3-kerbal SSTO to do so (which would be wasteful of fuel...and I'm quite limited on fuel out there). But I was a bit surprised and pleased when the cut-down version managed to reach a minimal Kerbin orbit during testing.
The rocket: SSTUBBY - Single Stage To Up Barely Beyond Yonder. So named because it's rather tubby, being designed to be more compact for easier transport to Laythe. It has 40 parts and masses 17.86 tons at liftoff. Here is, somewhat unexpectedly, in orbit at 74 km for the first time:
If there is enough fuel left onboard these things after they make orbit, I can let MechJeb land them on their rocket flames. But for this first build, I forgot to include MechJeb on the ship...so I had to bring it in manually. I was pleasently surprised (since I fear at times that I've become to reliant of MechJeb for these targeted landings) that I was able to land the ship back on KSC property (below). I was coming in a little long, so I had to burn up all the rest of the fuel to keep the ship over dry land, but that's OK since it has lots of parachutes (6) for landing. That's more parachutes than it needs for this landing...but when it gets used at Laythe, it will drop from orbit and land with nearly-full tanks (because it must carry down the fuel it needs to fly back to orbit), so the extra chutes are needed in that situation.
Below we see the SSTUBBY in the VAB. It has four side tanks, but they are not arranged in a cruciform symmetry because I wanted an easy route for the ladders that allow the kerbal to enter and leave the capsule. So the tanks are arranged in positions where some tanks would be located if 6-symmetry was used. Two of the side tanks have standard jet engines under them. All four tanks have ram air intakes on top (so I get two intakes feeding each jet engine). There are spherical RCS tanks under the other two side tanks, and there is a LV-T45 rocket engine under the center tank. Fuel lines run in from the outer tanks to the center tank...don't forget to add these! There is an OKTO2 probe boby under that big-ass docking port (no kerbals were put at risk during the testing of this rocket...since I really didn't expect it to quite reach orbit around Kerbin). There is an ASAS unit and six parachutes (two on the center tank, one on each side tank). The four heavy-duty landing gear and the four RCS quads are located on the side tanks a tick beyond 180 degrees from the tank-to-center-tank connection line so that they are located closer to the 45-degree lines from the center tank. The quads are positioned so that they produce minimal rotation when during translation firings.
The action groups are set up as follows: 1 - toggles the rocket engine. 2 - toggles the two jet engines. 3 - toggles the ladders. 4 - toggles the air intakes (I have the 3 key as a buffer between the 2 and 4 keys so it's less likely that I'll accidentally close the air intakes when toggling the jet engines). All of the engines and the two lunch clamps are in the first stage of the staging setup. To launch the ship, turn on ASAS, throttle up to 100%, press "2" to start the jet engines. Wait for the jets to spool up to full thrust (listening to the pitch of the whine...if the overheat warning shows up, you've waited a bit longer than necessary). The press the space bar...this activates the rocket engine and releases the launch clamps. We have liftoff.
Press "1" to cut off the rocket engine once the ship reaches 50-60 meters/sec. The jet engines will do the work alone for the next phase. As the ship slowly accelerates, the overheat warning will come on for the jets, but the ship will reach 80 m/s before the overheat reaches critical level, and after that is slowly decreases. At 7,000 meters, turn 10 degrees east. As the prograde marker moves to the right, keep your direction indicator just inside the left edge of the prograde marker. If you do this, the ship will be approaching a 40-degree-from-vertical angle by the time you reach 18,000 meters.
At 18,000 meters, press "1" to toggle the rocket engine back on. Tip over a bit more to 45 degrees, and by then your ship should be at 20,000 meters and about to flame out (but with the extra speed from the rocket engine push, the intakes bring in enough air to keep the jets going a little past 20K). Press "2" to toggle off the jet engines (I do this by 20,300 meters), and press "4" to close the air intakes (to reduce drag). The rocket engine takes you the rest of the way to orbit. I tend to be at around 40-degrees from horizontal at 30,000 meters, and 30 degrees from horizontal at 40,000 meters, and 10 degrees from horizontal at 50,000 meters (approximate values). When the apoapsis hits 72 km, I cut the engine. You will still be traveling through the upper part of the atmosphere for a while, so the apoapsis may drop somewhat...but as long as it stays above 70 km all is well. At apoapsis, burn prograde to circularize the orbit. You have now managed to throw yourself at the ground and miss, which is the secret to orbiting.
Below, we see SSTUBBY is Spaaaaace! The amount of fuel you have left will depend on your launch trajectory. I had 97.2 liters of fuel left when I reached a 72-km orbit on this particular flight (and an excess of oxidizer, since the jet engines used a little fuel and no oxidizer during their part of the boost...but they use a surprisingly small amount of fuel, which is why these jet/rocket SSTOs work. And the ship has 80 units of RCS fuel, which can do rather a lot of orbital change as well.
Time to return to the KSC! I had MechJeb installed this time, so I figured I'd let it try to land the ship on its rocket flame (I wasn't sure if 97.2 liters would be enough for this). Below we see the retro firing. It took 37.2 liters of fuel to do the retro firing and targeting, leaving 60 liters for landing.
Landing on the engine flame. The fuel ran out about a meter above the surface, and the ship dropped to the ground safe and sound.
Actually, it barely missed hitting the VAB on its way down. MechJeb is such a showoff.
One of my concerns with these SSTO ships is that they might become useless once reentry heat damage is implemented. I'm hoping somebody will supply me with a toroidal inflatable heat shield (see http://forum.kerbalspaceprogram.com/showthread.php/24942), but in case such a useful thing never materializes, I have been considering ways around the problem. I presume we will get inflatable heat shields of a more conventional nature (that spread out into a shallow cone shape), but fitting those to my reusable rockets will require that there be open space in the center of the base to mount such a shield. So I experimented with a modified SSTUBBY design that moves all the engines out to the radial tanks.
Below is the SSTUBBY 2. I has two LV-T45 engines instead of one, mounted under two of the side tanks. The RCS balls were moved to the bottom of the center tank for now, but could be radially mounted later so that an inflatable heat should canister could be mounted on the bottom of the center tank. Hopefully the size of an FL-T200 tank...but probably more realistically the size of a FL-T400 tank.
The launch profile of the SSTUBBY 2 is the same as the SSTUBBY 1. Note that the fuel lines must be arranged differently! The fuel lines run from the side tanks with the jet engines mounted on them into the center tank; then fuel lines run from the center tank to the side tanks with the LV-T45 rocket engines mounted on them. The two jet engines can still lift and accelerate the ship fine with the extra 1.5 ton engine attached.
At 18,000 meters, we fire the rocket engines again. 20,000 meters, we switch off the jet engines (below). The two LV-T45 engines are more power than this ship needs... but when I tried LV-909 engines, they weren't powerful enough. What I really need for this design is an intermediate engine (half the power of the LV-T45, at hopefully half the weight).
But even though the two LV-T5s are not optimal for this ship, it can reach a 72 km orbit with a small amount of fuel left over. Depending on how heavy the inflatable parachute canister would be, some extra fuel will almost certainly be needed. And then we may get to the limit where the two jet engines aren't strong enough to lift the ship...especially if I can't have the desired half-size version of the LV-T45.
Anyway...when it came time to try to land this ship back at the KSC, it ran out of fuel a little ways into the retro burn. It was enough for reentry, but it was going to end up several hundred kilometers beyond the KSC in the ocean. But I quickly fired the RCS engines to slow the ship up more...and was barely able to get it to come down on dry land (but I used all of the RCS fuel in the process).
I also popped out the eight parachutes as soon after the reentry flames abated as I could (well, there were still a few flames licking the craft) to reduce the overshoot problem.
But the ship landed at KSC over near the shore, but on kerba firm and not in the water. Success...barely.