JF Mezei wrote on Fri, 11 May 2018
16:50:59 -0400:

On 2018-05-11 01:22, Niklas Holsti wrote:

What/where are those "documents", please? From what you say, they seem
wrong. Commentary from several launches has said that Falcon 9 first
stage engines are throttled down around the Max-Q point, to reduce the
stresses, and the engines are also throttled down for the landing. And,
of course, re-ignited two times (entry burn, landng burn).

I know about throttling back at MAX-Q and as stack gets lighter. Which
is why the following commentary during yesterday's (and today) launch
got me confused:

(roughly 3 minutes prior to launch)

Block 4:
Constant chamber pressure which means thrust increases as atmpsphere
density went down. (they mentioned by 10 to 15,000lbps PER engine.


I'd bet what you're missing is that that's thrust increases FOR A GIVE
THROTTLE SETTING.


Block 5:
constant thrust of 190,000lbs till cutoff, which means we'll be slowly
decreasing chamber pressure over time to maintain constant thrust.


Which means it's essentially an 'auto-throttle'.


So I started to look and:

https://www.spaceflightnow.com/falcon9/001/f9guide.pdf

Table 2-1 , page 10, states Throttle capability of "NO" for Stage1 ,and
"yes" for stage 2. However, this appears to be an older document (not
that I looked more carefully) and while originally from SpaceX, is
hosted by spaceflightnow and is for Block 2.


I really have to question that, since I don't see how they could do
the Max Q throttle reduction without, well, being able to reduce
throttle. However, this document is for Falcon 9 v1.0 with Merlin 1C
engines.

The Payload User's Guide for Falcon v1.1 specifies that the first
stage is throttleable. See the chart on Page 11. This indicates,
though, that it's been throttleable for a long, long time (since back
in Block 2) and this is not something new for Block 5.

http://www.spacex.com/sites/spacex/f...de_rev_2.0.pdf


https://en.wikipedia.org/wiki/Merlin...ily)#Merlin_1D


Something wrong with your link. It doesn't go to the rocket engine.


(I assume the 1D is block 4). It does mention that the engine gained
variable thrust, initially from 100 down to 70% and later down to 40%.


If the Wiki page is right (always something of a crap shoot) the
Merlin 1C actually existed in two different variants and they say that
the Sea Level version is NOT throttleable. This still leaves me
puzzled about how they get through Max Q, although that may not have
been a problem because 1C has much lower thrust than 1D.

The 1D first flew on Falcon 9 1.1, which corresponds to Block 2 (as I
make it). Full Thrust v1.1 is Block 3. Then we have Block 4 and Block
5. So I make it that the Merlin 1D engine was used for Block 2-5.
There was an upgrade going to the 'Full Thrust' vehicle, but they
still call it the 1D (upgrade) engine.

There's a chart here that shows which engines go with what.

https://en.wikipedia.org/wiki/Falcon_9#Capabilities


If block 4 had variable thrust, but constant chamber pressure, how did
they maintained pressure if they reduced fuel flow?


They didn't. As I said, I suspect the 'constant chamber pressure' is
at any specific throttle setting, not across throttle settings.


Does the chamnber (I assume combustion chamber?) have an outflow
valve/iris that allows it to maintain pressure with variable flow of
fuel/oxidizer going in ?


Almost certainly not. What would be the point?


--
"The reasonable man adapts himself to the world; the unreasonable
man persists in trying to adapt the world to himself. Therefore,
all progress depends on the unreasonable man."
--George Bernard Shaw