Regarding gimbal interstage steering:
The background for this thought came many years ago when I was a flight
instructor, and I would demonstrate to students that you could induce a
significant climb or descent in a Cessna merely by leaning forward or backwards
without touching the flight controls. In later years I found you would
definitely notice a flight attendant moving a heavy cart up and down the aisle
could put you ever so slightly out of trim.
More specifically
Aerodynamic steering occurs by changing the relationship between the Cg and Cp.
- Most aircraft do this by using (relatively small) flight controls to move the
Cp, with negligible changes to Cg.
- Weight shift aircraft do this by moving the Cg, with little changes to Cp.
The first has the option of scaling up as large as you care to go. The second
option quickly becomes impractical as you find yourself moving a larger and
larger mass over a larger and larger distance to maintain control authority.
However, it’s incredibly simple and at the bottom end of the size scale, that
wins out.
Gimballed thrust steering likewise occurs by changing the relationship between
the Cg and thrust line.
- Most rockets do this by moving (relatively small) chamber or nozzle to move
the thrust line, with negligible changes to Cg
- So why not, by analogy, move the Cg instead of the thrust line?
The same scaling concerns as above apply. Putting a joint in the middle of a
rocket big enough to have propellant slosh concerns is obviously impossible.
Once again at the bottom end I think the simplicity wins, especially if
considering COTS hobby hardware which is structurally overbuilt and the
relative strength of RC aircraft linear actuators.
That being said, I’m not about to channel my inner Lutz Keyzer, for while the
purchase of a small pile of HPR motors and actuators is within my modest means,
developing a way to cheaply determine attitude and location of a fast-moving in
space is both technically challenging and discouraged by the national security
apparatus.
