Major Launches this Week: - TUE AUS Eris-1 / Maiden Flight (didn't go far) • TUE FL Falcon9 / Starlink (Stg1's 26th mission) • THU CA Falcon9 / Starlink (classic Vandenfog SFB launch) (new graphic shows ballistic trajectory in blue approaching target in white as Stg2 burns) ▶︎ FRI FL Falcon9 / Crew Dragon RTLS landing ______ Updated: 2 AUG / 1200 (all times US Eastern) Last week Docking with ISS:
Watch Dragon and the Crew-11 astronauts dock with the @Space_Station → https://t.co/DIuBob7vs6 https://t.co/k5kYCy2Sdu
— SpaceX (@SpaceX) August 2, 2025
Engineering Fun:
Ever notice that Stg1 rocket plumes grow visibly wider and wider between maxQ and staging?
Sure, it’s pretty, but all that exhaust inertia that isn’t flung off directly aft is just inefficiency.
Nozzle exit pressure is a function of the bell’s expansion ratio,
so it’s essentially constant for a given engine;
but atmospheric pressure drops with altitude.
Stg1 engine bells are optimized for 1-atm exit pressure for maximum thrust at liftoff,
so they lose efficiency as they climb, due to more exhaust going sideways.
Stg2 vacuum engines, however, have larger engine bells for the highest practical expansion ratio
to get the exit pressure near zero in a vacuum, so there’s less sideways loss and more efficiency.
(I’ll stop there. Anything more requires a sliderule.)
thanks for not making me get out my slide rule because you’d have to teach me how to use it again, I also remember there was something called logarithms, that you’d also need to teach me.
but I remember that the slide rule and logarithms took us to the moon and back!
…those, and computers that had less horsepower than our phones. (CM and LM for sure!)
“Nozzle exit pressure is a function of the bell’s expansion ratio…etc.”
Wow! I never thought of that, but it makes so much sense – exhaust expanding sideways outside the bell is wasted energy.
Thanks for the epiphany!
could they make a bell with overlapping plates to change the aperture as it ascends?
(probably all wrong terminology, I’m no rocket surgeon)
The math works, but it’s an engineering nightmare.
Look into the aerospike engine idea.
Aren’t rocket engines designed in a very specific manner because they tend to explode if the vibration frequency changes? (vague memory of a problem with early iterations of the Saturn F1 engines)
Change the exhaust, change the vibration and we get an engine RUD, I would expect.
Yup, I think you’re talking about resonance,
way outside my wheelhouse.
Watch the Stg2 vacuum engine when it shuts down —
the bell’s exit wobbles as its pressurized stiffness drops, and the flow goes unsymmetrical and turbulent.
See: POGO
Gotcha.
Combustion instability: interaction between exit, combustion chamber, and feed lines.
That dangerous resonance in Apollo (I think) was minimized by adding damping within a propellant feed line.
Never encountered it in anything I worked with, hence my arm-waving.
Get as technical as you like, I *am* a Rocket Scientist. Heh.