On 2020-03-07 1:22 PM, Jeff Findley wrote:
> This was the first large LH2/LOX upper stage. So I believe that it was
> pretty much an engineering pathfinder for the IV-B. It provided
> experience with designing, building, and flying a large LH2/LOX upper
> stage. I'm sure the lessons learned went into the IV-B.

It's hard to have a discussion when we agree on almost every point. ha.

So let's expand the envelop a bit. The J2 used in the Saturn 5 third
stage had to have a restart capability to do LEO and TLI insertions. Was
the ignition system used for that significantly different than that used
for the second stage engines? I suspect not.

I seem to remember the bell housing on the third stage was longer than
that used on the second stage even though same J2 engine. Vacuum
optimization?

Are there ANY applications these days for the new and improved J2X?
I should do a Google search and see. Opinions welcome.

Dave

In article <r431q6$ajf$1@dont-email.me>, nospam@127.0.0.1 says...
>
> On 2020-03-07 1:22 PM, Jeff Findley wrote:
> > This was the first large LH2/LOX upper stage. So I believe that it was
> > pretty much an engineering pathfinder for the IV-B. It provided
> > experience with designing, building, and flying a large LH2/LOX upper
> > stage. I'm sure the lessons learned went into the IV-B.
>
> It's hard to have a discussion when we agree on almost every point. ha.

True.

> So let's expand the envelop a bit. The J2 used in the Saturn 5 third
> stage had to have a restart capability to do LEO and TLI insertions. Was
> the ignition system used for that significantly different than that used
> for the second stage engines? I suspect not.

I'm guessing not as well. Here's a web page detailing the J-2's spark
igniter (the venerable RL-10 also uses a spark igniter).

http://heroicrelics.org/info/j-2/augmented-spark-igniter.html

One of the (few) advantages of LH2/LOX is that it's relatively easy to
ignite. So you can use a spark igniter to create an engine design that
can ignite as many times as necessary. Unlike kerosene/LOX which often
relies on the chemicals TEA/TEB to insure that ignition takes place
(Merlin does that).

> I seem to remember the bell housing on the third stage was longer than
> that used on the second stage even though same J2 engine. Vacuum
> optimization?

I can't find any evidence of that. This Wikipedia entry lists one
expansion ratio for the J-2 (27.5:1 expansion area ratio).

https://en.wikipedia.org/wiki/Rocketdyne_J-2

Astronautix.com says essentially the same thing:

http://www.astronautix.com/j/j-2.html

The J-2X had a different expansion ratio (80:1), which increased
efficiency compared to the original J-2:

http://www.astronautix.com/j/j-2x.html

Still, if you don't need the higher thrust of the J-2X, the later
versions of the RL-10 are better in almost every way.

> Are there ANY applications these days for the new and improved J2X?
> I should do a Google search and see. Opinions welcome.

I doubt it. As the designs for the Exploration Upper Stage show, you
can cluster modern RL-10 engines to better effect. Modern RL-10 engines
have more thrust, have larger bells, are generally more efficient than
the early RL-10 used on the original Saturn IV stage.

The fact is, the J-2X was only really needed for Ares-I due to the fact
that they needed very high thrust for the second stage and due to the
small diameter of the five segment SRB first stage, there simply was no
room for a cluster of more efficient RL-10 engines.

Even the Northrop Grumman Omega launch vehicle will use dual RL-10
engines in its upper stage. They are able to do this because underneath
isn't a single solid stage, but two large solid stages (same diameter as
Ares I's five segment SRB, but shorter) *plus* additional solid strap-
ons!

https://www.spacelaunchreport.com/ngl.html

At any rate, here is a very cool web page which details the evolution of
the RL-10 engine:

https://www.b14643.de/Spacerockets/Specials/P&W_RL10_engine/index.htm

One of the great innovations on the RL-10 was the addition of an in
flight extensible nozzle. This allows for much higher expansion ratios
while not requiring a longer interstage in the launch vehicle's design,
saving mass on the launch vehicle.

According to Wikipedia, the "EUS will use the new RL10C-3 version, the
biggest and most powerful of the RL10C-X engines".

Jeff
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