Altitude compensation with gas injection

Suppose you have a high-altitude rocket nozzle: It expands the flow to
1 psi at exit. Now suppose you want to light this thing off on the
ground. You have an overexpanded nozzle, and you may get unpredictable
flow seperation and you will get Isp losses from the flow to ambient
pressure differential across the outer parts of the nozzle.

Suppose that for the low-altitude portion of the flight, and for ground
testing, you inject some significant amount of gas into the nozzle. The
idea is to at least get predictable flow seperation, to reduce the flow
to ambient pressure differential around the outer parts of the nozzle
and to perhaps reduce the Isp loss. This last bit is doubtful given
that you are chucking a cold subsonic massive stream of gas into the
supersonic portion of the nozzle.

Is this a known technique? Does it work?

In article ,
Iain McClatchie wrote:
Suppose that for the low-altitude portion of the flight, and for ground
testing, you inject some significant amount of gas into the nozzle. The
idea is to at least get predictable flow seperation, to reduce the flow
to ambient pressure differential around the outer parts of the nozzle
and to perhaps reduce the Isp loss...
Is this a known technique? Does it work?

Gas injection at a well-chosen point for controlled flow separation does
work; it's been tested. Unfortunately, it tends to need a lot of gas, and
this limits benefits.
--
"Think outside the box -- the box isn't our friend." | Henry Spencer
-- George Herbert |

Henry Gas injection at a well-chosen point for controlled flow separation
Henry does work; it's been tested.

Excellent.

Henry Unfortunately, it tends to need a lot of gas, and this limits benefits.

References?

If you want to do parallel staging, you need a way to make your upper stage
nozzle (a) survive the first 40 seconds of flight and (b) not throw your
thrust vector around randomly. If you add gas from the boosters' tanks,
you might be able to dump most of the weight of the system with the
boosters once you don't need it anymore. The downside is that you will
be carrying it for twice as long as you actually need it.

Has anyone done solid nozzle inserts that get ejected at altitude?

"Iain McClatchie" wrote in message
om...
Henry Gas injection at a well-chosen point for controlled flow separation
Henry does work; it's been tested.

Excellent.

Henry Unfortunately, it tends to need a lot of gas, and this limits
benefits.

References?

If you want to do parallel staging, you need a way to make your upper
stage
nozzle (a) survive the first 40 seconds of flight and (b) not throw your
thrust vector around randomly. If you add gas from the boosters' tanks,
you might be able to dump most of the weight of the system with the
boosters once you don't need it anymore. The downside is that you will
be carrying it for twice as long as you actually need it.

Has anyone done solid nozzle inserts that get ejected at altitude?

I have wondered about multiple thrust chambers using one expansion
nozzle. 7 chambers with 7 throats into 1 expansion nozzle.
Expansion ratio of 10 at sea level, shutting down chambers until
the last one has a 70 ratio. Plumbing similar to the Russian multi-
chamber engines.

In article ,
Iain McClatchie wrote:
Henry Unfortunately, it tends to need a lot of gas, and this limits benefits.

References?

Check the Hagemann et al survey paper on advanced nozzles in the Sept/Oct
1998 issue of Journal of Propulsion & Power -- he mentioned Aerojet tests
of the idea, and he may have supplied a reference.

Has anyone done solid nozzle inserts that get ejected at altitude?

Nobody's flown it, but lots of people have thought about it. The J-2
ground testing used a water-cooled fixed insert for some tests, and an
RD-0120 has been fired with a nozzle insert that provided a complete
secondary nozzle (not sure whether they tested ejection, although that
was the long-term intent). Main concerns are the release process:
reliability, mechanical and thermal shock loads, symmetry, collision risk.
There is also -- as with many other altitude-compensation schemes -- some
small performance penalty at sea level, due to suboptimal nozzle contours
and aspiration drag. (Ref: same paper.)
--
"Think outside the box -- the box isn't our friend." | Henry Spencer
-- George Herbert |

Iain McClatchie wrote:
If you want to do parallel staging, you need a way to make your upper stage
nozzle (a) survive the first 40 seconds of flight and (b) not throw your
thrust vector around randomly. If you add gas from the boosters' tanks,
you might be able to dump most of the weight of the system with the
boosters once you don't need it anymore. The downside is that you will
be carrying it for twice as long as you actually need it.

Allegedly, another way to do this is to use the ambient air- holes
around the nozzle at the appropriate place/angle can apparently trigger
separation.

As the rocket climbs, the ambient pressure reduces and the nozzle fills.

Of course you don't want the hot gas to go backwards through the holes,
but pretty much, with careful design, it won't- it's very difficult for
hypersonic gas to suddenly reverse direction.

Has anyone done solid nozzle inserts that get ejected at altitude?

That one's described in Sutton.

(Henry Spencer) wrote in message ...
In article ,
Ian Woollard wrote:
Of course you don't want the hot gas to go backwards through the holes,
but pretty much, with careful design, it won't- it's very difficult for
hypersonic gas to suddenly reverse direction.

Alas, the gas right at the wall isn't hypersonic.

The gas right at the wall is in the boundary layer; however this
boundary layer is very thin, so I think that the amount of gas going
'the wrong way' is going to be very small.

(Henry Spencer) wrote in message ...
In article ,
Ian Woollard wrote:
Allegedly, another way to do this is to use the ambient air- holes
around the nozzle at the appropriate place/angle can apparently trigger
separation.

It's been tried, notably in an RL10. It's not as well-behaved or as
dependable as gas injection, though.

Of course you don't want the hot gas to go backwards through the holes,
but pretty much, with careful design, it won't- it's very difficult for
hypersonic gas to suddenly reverse direction.

Alas, the gas right at the wall isn't hypersonic.

True, the boundary layer is subsonic.

But the boundary layer is exceedingly thin; and furthermore often much
colder than the bulk gas.

It seems to be a mostly negligable amount of gas.

One group successfully used a sliding nozzle section with no seal, and
noted no significant backflow during lab testing.