Newsgroups: sci.space.tech
From: henry@spsystems.net (Henry Spencer)
Subject: Re: Is Roton Dead?
Date: Mon, 8 Jan 2001 13:45:10 GMT

In article <Pine.OSX.4.31.0101061636160.11185-100000@kapu.net>,
Michael J Wise  <mjwise@kapu.net> wrote:
>> > build a non-nuclear SSTO vehicle?
>> Yeah. It was called the "J-2."
>
>Based on a few seconds of research, that was the LOX/LH engine used in
>the second and third stages of the "Saturn 5" stack, yes?

Correct.

>But... that would mean that it was optimized for vacuum, and an SSTO
>would require a different config for the initial boost, yes? Or at least
>that the ISP would be different at different portions of the flight.

You can't run a J-2 at sea level without doing *something* about this,
e.g. shortening the nozzle.  That does hurt performance, alas, unless you
add some unproven trick like a telescoping nozzle.

(Well, you can run a J-2 at sea level... if you don't care which way the
thrust points, because the flow tends to separate and flow along one wall
of the nozzle, unpredictably.  The side forces are large; sea-level J-2
testing mostly used a large brace to take the load off the gimbal
actuators, which were not up to it.  Gimbal testing was done using a
water-cooled collar inside the nozzle to force symmetric flow separation.)

>And if so, would that not demand SRBs, or another stage?

With the J-2, it's a bit iffy.  But the Titan II first-stage engines have
no problem running at sea level; in fact, the central problem with using
the Titan II first stage as an SSTO is that it has *too much* thrust to
fly an efficient trajectory.
--
When failure is not an option, success  |  Henry Spencer   henry@spsystems.net
can get expensive.   -- Peter Stibrany  |      (aka henry@zoo.toronto.edu)