wrote:



Heavy but skinny, so the second stage has to be skinny
too -- a problem for a stage that needs lots of bulky LH2, and also it
limits your engine options because there's no *room* for a multi-engine
cluster in such a narrow stage.



Ahem: aerospike or plug cluster. Would fit great.



Ignition altitude is pretty high for that; at the low ambient air
pressure where the second stage will light, you might just as well use a
conventional bell nozzle.

pat