Buran-Energia style shuttle design discussion

[sturmhauke]

I've been working on the excellent Shuttle Challenge series in the link above. I thought I'd share some of my design tips on my Buran-Energia analog. I call mine Bustard-Phoenix. I'm working on a Phoenix II launch system, which hopefully will solve some of the thrust-induced torque issues I'll discuss below. My design uses the Near Future and Mark IV Spaceplane mods pretty heavily, along with Tweakscale and some other stuff. I also used Kerbal Engineer Redux, Editor Extensions Redux, and RCS Build Aid extensively during the design process.

These are screenshots of my shuttle in the VAB, as configured for the STS-2a mission for launching geostationary satellites. The Phoenix launch system is fitted with 4 KL-21 Odin main engines, using liquid hydrogen, and 16 R1-D Kite booster engines, using standard rocket fuel. The Bustard orbiter is fitted with an LV-601-4 OMS engine cluster, using monoprop, and 4 Whiplash engines for atmo crossrange capability.

I started by designing the orbiter and payload in the SPH, then copied them both to the VAB, added the payload to the cargo bay, then the launcher core, and finally the boosters. So the screenshots are in design order, which is the reverse of the staging order.

This is the orbiter both with and without payload. The CoM stays pretty close to the center of the cargo bay by design at all times, with the CoL (not shown) slightly aft. The OMS engines are angled a couple of degrees to minimize torque, and the gimbal takes care of the rest. That was the easy part.

Spoiler

Next is the orbiter with the launcher core's main engines still burning, post booster separation at ~40 km, until core separation post orbital insertion. At booster separation, the core has roughly 45% fuel remaining. To minimize torque from that point onward, I simulated it in the VAB by setting the core tanks to half full, and angled the main engines more towards the dry CoM (red marker). As you can see, the CoM is on the edge of the core tank facing the orbiter, and moves not-quite-vertically as fuel is burned. The main engines are set at ~30 degrees. I don't know the exact value; I used EEX to fine tune them while looking at the torque readouts.

Spoiler

Finally we come to the most challenging part of the design, which is from launch until booster separation. Here, the overall vehicle CoM starts near the center of the launcher core tanks, but moves quite a bit downward and towards the orbiter as fuel burns. Booster separation happens when the current CoM is somewhere between the average CoM (orange marker) and dry CoM. Therefore the booster engines are angled at roughly 15 degrees, to minimize torque in that range of CoM movement. That leaves high torque from launch up to a few thousand meters. All the engines have gimbal, but it's barely enough. The orbiter's control surfaces can make up the difference, but flight is a bit tricky right after takeoff.

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So to summarize, the Bustard orbiter is great, but the Phoenix launcher is pretty finicky. I found myself having to adjust the engine angles for each payload, which got real tedious real fast. I made a more kerbally launch system called Firetail, which is less realistic and kind of a staging headache, but also easier to set up and fly. That one's on my KerbalX page if you want to check it out: https://kerbalx.com/sturmhauke/Bustard-Firetail

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As I mentioned earlier, I'm also working on a Phoenix II launch system. It seems that my geometry was a bit off. The real Energia's main engines were offset towards the orbiter side, to bring the thrust vector more in line with the CoM movement and reduce the gimbal needed. The boosters were also attached closer to the orbiter for the same reason, although that makes separation more dangerous also due to increased risk of hitting the wings. Here's a prototype of my new main engine arrangement. The engines themselves are not angled, only the mounting structure to achieve the offset.

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I hope this was useful for your future shuttle design adventures. And if anyone has tips of their own, feel free to post them. See you in the shuttle thread!