In article ,
John Pelchat wrote:
Thanks for a great reply, one follow-one: Part of your reply spoke of
dumping exhaust overboard. In photos of some launch vehicles, you see
a rather non-propulsive flame to one side of the engine exhaust,
especially the older Atlas vehicles . . . is that the same exhaust?

No, the side flames on the old Atlases (there are two, 180deg apart) are
its vernier engines, used for roll control after booster-engine jettison,
and for final velocity trim after sustainer-engine cutoff. They fired
continuously for the same reason that Atlas used its 1.5-stage trick,
jettisoning two booster engines midway up: because it was designed at a
time when igniting rocket engines at high altitude was somewhat of an
unknown, so having all engines running before takeoff was a design goal.

The vernier engines were always less fuel-efficient than the main engines,
and as part of the design cleanup that produced the commercial Atlas II,
they were deleted. Since the modern Atlases always fly with an upper
stage, there is no need for the Atlas proper to do velocity trim, and roll
control after booster-engine jettison could be done by a small thruster
package on the interstage ring. (Even that became superfluous when
re-engining produced Atlas III, because now it has two engines(*) and no
jettison events, and can do its own roll control throughout.)

(* The RD-108 is best considered a two-engine cluster. Counting engines
by chambers rather than by pump sets introduces fewer contradictions.)
--
MOST launched 30 June; first light, 29 July; 5arcsec | Henry Spencer
pointing, 10 Sept; first science, early Oct; all well. |