Why does the shuttle throttle on ascent?

But the actual thrust abilities of the smes was increased over the life of
the Shuttle, I'm sure I read that.

Brian

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"Morgans" wrote in message
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"Richard Riley" wrote in message
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On Sat, 6 Jan 2007 20:36:52 -0600, "Danny Deger"
wrote:

Why does the shuttle throttle to 3 Gs on ascent?

Danny Deger


Heck, I still want to know how to go to 103% throttle.

Rather like the old prop jobs (DC-3 comes to mind as an example) that had
a wire across the throttle travel, which serves as a stop for full
throttle during normal operations. If it was needed for an emergency,
like an engine failure on takeoff, you can push through and past the wire
for extra emergency power. (100% plus power)

Of course, on some engines, that was grounds for grounding the aircraft to
inspect the engine, to see if it was damaged from exceeding 100% power.
--
Jim in NC

In article ,
Brian Gaff wrote:
But the actual thrust abilities of the smes was increased over the life of
the Shuttle, I'm sure I read that.

Correct -- originally 100% was to be tops (surprise, surprise), but later
the engines were qualified for 104%, and at the time of Challenger there
were plans to qualify them for routine operation at 109%, and possibly
more. Those plans got scaled back in the post-Challenger safety rethink.

There is nothing particularly unusual about this; most rocket engines grow
in thrust as experience builds up and small improvements are made. It
attracted attention on the shuttle only because of how it was expressed:
numbers above 100% sound vaguely alarming to the ignorant. The RS-27A
first-stage engine on modern Delta IIs runs at 153% of its original thrust
rating. The H-1 first-stage engines on the Saturn IBs that launched ASTP
were running at 124% of the thrust of the first H-1s, and even those were
110% upgrades of the S-3D Thor/Jupiter engines, which were themselves
substantially more powerful than still-earlier versions. Had there been a
second production batch of Saturn Vs, almost certainly the first-stage
engines would have been F-1As, running at 120% of the original F-1 thrust.

That said, the SSMEs are cranky, marginal engines, and taking *them* up to
120% (as was once intended) is much more iffy than doing the same for
robust engines like the H-1 or F-1.
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"Henry Spencer" wrote in message
...

interesting stuff snipped

That said, the SSMEs are cranky, marginal engines, and taking *them* up to
120% (as was once intended) is much more iffy than doing the same for
robust engines like the H-1 or F-1.
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spsystems.net is temporarily off the air; | Henry Spencer
mail to henry at zoo.utoronto.ca instead. |


Henry,

What makes the SS engines "cranky and marginal" vs the H-1 and/or F-1?

Thanks...

KB

In article ,
Kyle Boatright wrote:
That said, the SSMEs are cranky, marginal engines, and taking *them* up to
120% (as was once intended) is much more iffy than doing the same for
robust engines like the H-1 or F-1.

What makes the SS engines "cranky and marginal" vs the H-1 and/or F-1?

Mostly, they were too much of a leap into the technological unknown at the
time: NASA tried to pioneer bold new technology on what was supposed to
be a long-lived reusable engine, and unsurprisingly, this didn't work too
well. The H-1 and F-1 were much more conservative designs -- notably,
although the F-1 was a lot bigger than anything previously built, the
project tried hard to *avoid* pioneering in any other way -- and although
they did hit surprises and ended up breaking some new ground, they had
much more continuity with previous experience.

(For a while there was some feeling that the SSME's staged-combustion
cycle was just *inherently* troublesome, but that idea sort of collapsed
when it became clear that every major Russian rocket engine since about
1960 had been a staged-combustion design, typically including features
like oxidizer-rich preburners, which even NASA had deemed impractically
difficult...)

Three specific snags also aggravated this problem on the SSME:

(a) Most of the technology development on staged combustion had been done
by Pratt & Whitney, but oddly, the contract for the staged-combustion SSME
went to Rocketdyne instead. So the experienced people were shut out, and
the guys who were actually doing the work were having to come up to speed
on a technology that was new to them.

(b) The SSME program, like the shuttle in general, was starved for money
and opted to cut corners on subsystem testing in particular. The result
was an unusually long and painful development process, with subsystem
problems often not surfacing until whole-engine testing.

(c) Partly as a result of (a) and (b), it didn't become clear until too
late that the main LOX (I think it was) turbopump really needed one more
pump stage. Since a major redesign was politically and financially
impossible at that point, the result was a pump in which each stage was
pushed to the ragged edge of engineering practicality to meet a very
ambitious spec.

The combination of (b) and (c) was particularly nasty, because all too
often, a LOX-pump failure becomes a LOX-pump fire, which destroys the
evidence of what went wrong. Having this happen repeatedly to whole test
engines was just what an already-stressed development program didn't need.
--
spsystems.net is temporarily off the air; | Henry Spencer
mail to henry at zoo.utoronto.ca instead. |

"Henry Spencer" wrote in message
...
Three specific snags also aggravated this problem on the SSME:

(a) Most of the technology development on staged combustion had been done
by Pratt & Whitney, but oddly, the contract for the staged-combustion SSME
went to Rocketdyne instead. So the experienced people were shut out, and
the guys who were actually doing the work were having to come up to speed
on a technology that was new to them.

(b) The SSME program, like the shuttle in general, was starved for money
and opted to cut corners on subsystem testing in particular. The result
was an unusually long and painful development process, with subsystem
problems often not surfacing until whole-engine testing.

(c) Partly as a result of (a) and (b), it didn't become clear until too
late that the main LOX (I think it was) turbopump really needed one more
pump stage. Since a major redesign was politically and financially
impossible at that point, the result was a pump in which each stage was
pushed to the ragged edge of engineering practicality to meet a very
ambitious spec.

The combination of (b) and (c) was particularly nasty, because all too
often, a LOX-pump failure becomes a LOX-pump fire, which destroys the
evidence of what went wrong. Having this happen repeatedly to whole test
engines was just what an already-stressed development program didn't need.

To be fair, the engine has improved greatly since the first ones were built.

The current Block IIs appear to have incorporated several major changes
improving them, prolonging their cycles between tear-downs and over-all
making them far better than the originals.

(and much of the work was done by Pratt & Whitney.)

(of course I still think some people continue to hold old biases against the
SSME :-)

--
spsystems.net is temporarily off the air; | Henry Spencer
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Henry Spencer wrote:
Three specific snags also aggravated this problem on the SSME:

[...]

In his book "Advanced Engine Development at Pratt & Whitney," Dick
Mulready devoted a chapter to the competition to develop the space
shuttle engine. By the time of selection, P&W's XLR129 had over 251
seconds of operation, versus 0.461 for Rocketdyne's engine.

During a visit, Dick Bissell, a consultant for United Aircraft and
formerly of the CIA and progenitor of the U-2 and Blackbird, said,
"I am sorry, but you cannot win. It was already decided in advance.
The only reason for the competition was to transfer your technolody
to them."

Does anyone have any opinion on the relative merits of the P&W and
Rocketdyne designs? On the politics?

Thanks,

Paul