You could use a variation of the above double stack idea by having the LOX ET on the second stage sitting on top the first stage’s methane tank, and vice versa on the other side. That way the vehicle is balanced at liftoff (no yawing needed), grows gradually more unbalanced toward first stage ET separation, gradually shifts (due to first stage core fuel consumption), then gradually and completely rebalances as the second stage ET’s propellant is consumed (since both empty tanks are the same size and weight).

]]>Compact, simple, strong, and mostly common tooling in all tanks. ]]>

I’m mostly just exploring the design space of separating the fuel tanks from the oxidizer tanks to see if there’s other local optimums where you can get some other serendipitous benefits.

As an example, last night I ran some simplified numbers (based on cylinders) to look at a single RP-1 tank of diameter d and length L, offset by a LOX tank of length 1.8 L (similar to your example drawing), keeping the external tank diameters the same for tooling commonality. Then I went with a core first stage of diameter 1.66d and length L, to match the length of the RP-1 ET. It’s kind of odd looking but lets the first stage ET’s and the first stage core have about the same fuel mass.

Then, on top of the first stage LOX ET of length 1.8 L you have a stubby second stage LOX ET of length 0.47 L. On top of the 1st stage RP-1 ET of length L you have a second stage LH2 tank of length 1.27 L, so the stacked ET’s are the same length at launch (though not nearly the same weight). That took a little algebra on relative fuel/oxidizer densities so the ET stacks came out the same length with proper O/F ratios.

That configuration gives you a second stage with the LH2 ET the same length as the second stage core, with an stubby LOX ET attached on the other side near its top. The relative propellant masses work out to give you about 100 X of RP-1/LOX propellant in the first stage ET’s, 100 X in the first stage core, 22 X of LH2/LOX in the second stage ET’s, and 44 X in the second stage core, so the second stage total fuel mass is about a third of the first stage fuel mass.

It *looks* symmetric on the pad (most of the mass on the LOX side), becomes very asymmetric after the first stage ETs drop off (one is long and one is short), stays asymmetric after the first stage core separates (a long core and LH2 tank and a stubby LOX tank), and stays that way until the second stage ETs drop off. But the relative stage weights look pretty good for an RP-1 first stage and a cryogenic upper stage, so maybe it could be some serendipity.

If size and performance are issues, perhaps a surface effect ship (SES), a hybrid between a catamaran and a hovercraft? There’s a japanese ship called the TSL Ogasawara that might be going to the ship breakers sometime in the future due to the scrap value of the aluminum. It’s a SES with waterjet propulsion, originally intended as a high speed long distance ferry, powered by somewhat fuel hungry gas turbines.

]]>Or you could use a big tank and a little tank, possibly extending the concept into the second stage, where the long second stage tank was above the short first stage tank and vice-versa. That idea is inspired by the Space Shuttle carrying a 66,000 lb tank almost all the way to orbit, instead of splitting the fuel between two tanks, a big and a little one, allowing it to dump more dead mass on the way up.

]]>While my concept may not work, yours has added four failure modes that I can see offhand. Long fall, uncontrolled tip over may not hit the net, various pulley issues that must be just right, and adding a sideways solid to the stage.

Ken,

Agree with your points.

Born1930,

It is fun to consider. I prefer simple systems with as few failure modes as possible. The slanted flight into the wind was well demonstrated five years ago by Masten Space Systems. The pivot boom is a variation on the roll off dumpster systems that are used thousands of times a day. Two understood systems on a new platform.

Another problem with the triangle is that it could more easily interfere with normal pad access to the payload. Inconvenience can get expensive in the aerospace field.

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