water injection of any use?
Mr Jim
chamber? IIRC at least one version of the Harrier jet carried water intended
for injection into the engine. Though I can't remember the name of the book,
it explained the reason was to increase the reaction mass flowing through
the engine during vertical landings and takeoffs, thus increasing thrust
when the engine was at maximum power and fuel consumption couldn't be
increased any more.
J
Henry Spencer
Mr Jim <nospam...@cox.net> wrote:
>Would there be any value to injecting water into a rocket combustion
>chamber? IIRC at least one version of the Harrier jet carried water intended
Water injection was a traditional trick for getting a bit more out of jet
engines, for heavily-loaded takeoffs in particular. It gradually went out
of fashion as the engines got better and people got tired of having yet
another fluid that needed replenishing for each flight.
Would it make sense in a rocket? Well, kinda, maybe. Spreading the same
amount of combustion energy over a larger amount of reaction mass *does*
increase performance, because a given amount of energy translates into
more momentum when it's spread over more mass at lower velocity. The
problem is that it's not as good as adding more fuel/oxidizer mix, which
increases energy as well as mass. In general, doing that is better. You
might be able to get some benefit from water injection in unusual cases
where just boosting the fuel and oxidizer flow wasn't possible for some
reason.
Jets can benefit because their fuel flow is often limited by turbine-blade
temperature limits. Rockets don't really have any comparable structures
exposed to the exhaust.
Well, that is, their *main* exhaust. Many big rocket engines do use
turbines to drive pumps... and water injection *has* been used to cool the
turbine gas. The alternative is to run the turbine's gas generator very
fuel-rich or very oxidizer-rich to limit energy release, but those
approaches have their own problems. The original Ariane family, in
particular, added water to the output of a gas generator running at a
fairly balanced mixture ratio. (And then added some more water to cool
some of the gas further, and used it to pressurize the fuel tank. Yet
more water was used for cooling some of the turbine bearings.)
--
spsystems.net is temporarily off the air; | Henry Spencer
mail to henry at zoo.utoronto.ca instead. | he...@spsystems.net
Pat Flannery
Water injection was used on a lot of early jet engines to boost thrust.
On the early Harrier's Pegasus engine the water injection was mainly
used to cool the twin rear exhaust nozzles of the aircraft when it was
in hover, but did generate around a thousand pounds extra thrust as it
did so.
Remember that you don't want to get too much trust back there, as the
front nozzles are just shooting out hot compressed air from the fan
section of the engine without any combustion, and if you generate too
much thrust at the back, the aircraft is going to become unbalanced and
end up somersaulting nose end first before crashing on its top side.
It carried fifty imperial gallons of water for this purpose, and that,
not fuel usage, was what limited its hovering time. Once that water was
used up, the rear tailpipes could overheat with disastrous consequences
(they could detach from the back of the engine, letting the exhaust go
straight into the rear fuselage*). To prevent this, a temperature sensor
on the rear tailpipes automatically decreased fuel flow to the engine if
the temperatures started going too high. You could override this in an
emergency, but you don't want to be fiddling with controls while making
a rapid descent from around fifty feet in the air when the water runs out.
* A friend of mine served with the Marines when they were getting the
first AV-8 Harriers at their base; they told the pilots if they ever
heard a odd sound followed by the "fire" warning light coming on they
should eject immediately, as that's what just happened, and the whole
plane was going to start coming apart inside of two seconds.
Pat
zzbu...@netscape.net
Not really, rocket propuslion and jet propuslion
are two different idealized extremes of propulsion.
Rockets are optimized for linear acceleration,
are jets are optimized for rotory stability.
Water injection would only be useful
foe a kitchen sink in rockets.
>
> J
Mr Jim
"Henry Spencer" <he...@spsystems.net> wrote in message
news:JEoqD...@spsystems.net...
> In article <6NuIh.3540$Gr....@newsfe21.lga>,
> Mr Jim <nospam...@cox.net> wrote:
>>Would there be any value to injecting water into a rocket combustion
>>chamber? IIRC at least one version of the Harrier jet carried water
>>intended
>>for injection into the engine...
>
> Water injection was a traditional trick for getting a bit more out of jet
> engines, for heavily-loaded takeoffs in particular.
>
Thanks, Henry. I was wondering if this would be a workable poor man's way of
turning an LH2/LOX engine into a tripropellant engine during liftoff.
Alan Jones
wrote:
Probably not. It would certainly reduce ISP.
However, in another thread I suggested that the aluminum structure of
a SSTO could be sacrificed and used as fuel. (Carbon would work
better.) Burning aluminum produces lots of heat energy, but the
molecular weight is so high that poor exhaust velocities result.
Indeed some studies suggest that after burning and extracting the heat
energy, it should just be dumped, and not through the engine exhaust.
IF you had water available, and water is easy to store at a
hypothetical refueling station, or perhaps be obtained from
extraterrestrial sources, then water makes a decent storable
propulsive mass, with the heat energy coming from burning aluminum or
using another heat energy source.
Hyper
> On Sat, 10 Mar 2007 17:38:57 -0800, "Mr Jim" <nospam.mr...@cox.net>
> wrote:
>
>
>
>
>
>
>
> >"Henry Spencer" <h...@spsystems.net> wrote in message
> >news:JEoqD...@spsystems.net...
> >> In article <6NuIh.3540$Gr.1...@newsfe21.lga>,
> >> Mr Jim <nospam.mr...@cox.net> wrote:
> >>>Would there be any value to injecting water into a rocket combustion
> >>>chamber? IIRC at least one version of the Harrier jet carried water
> >>>intended
> >>>for injection into the engine...
>
> >> Water injection was a traditional trick for getting a bit more out of jet
> >> engines, for heavily-loaded takeoffs in particular.
>
> >> Would it make sense in a rocket? Well, kinda, maybe...
>
> >Thanks, Henry. I was wondering if this would be a workable poor man's way of
> >turning an LH2/LOX engine into a tripropellant engine during liftoff.
>
> Probably not. It would certainly reduce ISP.
>
> However, in another thread I suggested that the aluminum structure of
> a SSTO could be sacrificed and used as fuel. (Carbon would work
> better.) Burning aluminum produces lots of heat energy, but the
> molecular weight is so high that poor exhaust velocities result.
> Indeed some studies suggest that after burning and extracting the heat
> energy, it should just be dumped, and not through the engine exhaust.
That doesn't make much sense.
> IF you had water available, and water is easy to store at a
> hypothetical refueling station, or perhaps be obtained from
> extraterrestrial sources, then water makes a decent storable
> propulsive mass, with the heat energy coming from burning aluminum or
> using another heat energy source
So you have LOX, Al and water instead of LOX and some hydrocarbon.
AFAIK, water makes sense only in nuclear/solar-thermal propulsion.
Henry Spencer
Mr Jim <nospam...@cox.net> wrote:
>> Would it make sense in a rocket? Well, kinda, maybe...
>
>Thanks, Henry. I was wondering if this would be a workable poor man's way of
>turning an LH2/LOX engine into a tripropellant engine during liftoff.
Interesting idea. My gut feeling is that it wouldn't work well enough to
be appealing, but I'd have to do the numbers to be sure.
Pat Flannery
Mr Jim wrote:
> Thanks, Henry. I was wondering if this would be a workable poor man's way of
> turning an LH2/LOX engine into a tripropellant engine during liftoff.
This would be odd, as the exhaust of the LOX/LH2 engine is superheated
steam.
So you'd be upping the trust by injecting water...into superheated
water. :-\
Pat
Henry Spencer
Pat Flannery <fla...@daktel.com> wrote:
>> ...I was wondering if this would be a workable poor man's way of
>
>
>This would be odd, as the exhaust of the LOX/LH2 engine is superheated
>steam. So you'd be upping the trust by injecting water...into superheated
>water. :-\
Slightly odd though it sounds, it's not ridiculous. Neglecting losses,
power = 0.5 * thrust * exhaustvelocity. So for constant available power,
i.e. constant flow of LOX/LH2 mix, you can indeed raise thrust by lowering
exhaust velocity, e.g. by injecting inert mass. Of course, if you do that
for very long, it eats a lot of mass.
(Also, a quibble: the exhaust of a LOX/LH2 engine also contains a fair
bit of unburned H2. It's so light, and its nice gas properties are so
good for nozzle efficiency, that running quite fuel-rich is worthwhile.
In practice, this has to be tempered by consideration of how bulky it is,
which is one reason why modern LOX/LH2 systems don't run as fuel-rich as
the early ones did.)
Pat Flannery
Henry Spencer wrote:
> (Also, a quibble: the exhaust of a LOX/LH2 engine also contains a fair
> bit of unburned H2. It's so light, and its nice gas properties are so
> good for nozzle efficiency, that running quite fuel-rich is worthwhile.
> In practice, this has to be tempered by consideration of how bulky it is,
> which is one reason why modern LOX/LH2 systems don't run as fuel-rich as
> the early ones did.)
>
What causes the red color of the RS-68's exhaust? I thought that was due
to running fuel rich.
Pat
Alan Jones
Yes, this does not make much sense. It was posted with regard to the
senseless game of making a single use SSTO vehicle look better. The
new question is then is there any contrived scenario where water
injection might benefit a rocket engines performance. An aluminum
fueled engine has poor performance because of the high molecular mass
of aluminum, and it burns very hot. You are probably going to run
extra LOX through it. You have to ask where this LOX is coming from,
especially the "extra" parts, and just how storable is this LOX.
Consider replacing the "extra" LOX with water. So it is storable
water instead of "extra" LOX, not Al and water instead of hydrocarbon.
Nuclear would be even better, but if you're going to that expense, you
may want to go all the way and just use LH.
Henry Spencer
Pat Flannery <fla...@daktel.com> wrote:
>> (Also, a quibble: the exhaust of a LOX/LH2 engine also contains a fair
>> bit of unburned H2. It's so light, and its nice gas properties are so
>
>What causes the red color of the RS-68's exhaust? I thought that was due
>to running fuel rich.
I would guess that's mostly from the ablative liner of its nozzle --
unlike most LOX/LH2 engines, its nozzle isn't regeneratively cooled.
A pure hydrogen flame is colorless. Although, mind you, sodium in
particular can impart some color even in *very* small quantities, and I
gather that a trace of sodium isn't uncommon in LH2.
DC-X's exhaust was almost invisible in stable hover, but showed a hint
of color during throttle transients and the like.
Pat Flannery
Henry Spencer wrote:
>
> (Also, a quibble: the exhaust of a LOX/LH2 engine also contains a fair
> bit of unburned H2. It's so light, and its nice gas properties are so
> good for nozzle efficiency, that running quite fuel-rich is worthwhile.
> In practice, this has to be tempered by consideration of how bulky it is,
> which is one reason why modern LOX/LH2 systems don't run as fuel-rich as
> the early ones did.)
>
Here's an off-the-wall concept: LH2/H2O2
Automatic water injection! ;-)
Pat
Hyper
> Here's an off-the-wall concept: LH2/H2O2
> Automatic water injection! ;-)
>
> Pat
LOL
btw isp-384 vac
Pat Flannery
384? That's not bad, really.
Pat
Hyper
Yeah, but I still like FLOX/salami better though :-))
Henry Spencer
>
>384? That's not bad, really.
It's not an entirely ridiculous idea; it's been suggested. The high
density of the peroxide goes some way toward making up for the low density
of the hydrogen, especially since the mixture ratio is *very* skewed
toward the peroxide.
Of course, you still have all of hydrogen's ultra-cryogenic problems, so
the room-temperature oxidizer doesn't help as much as it usually does...
Jorge R. Frank
news:JEtC8...@spsystems.net:
> In article <12vb76s...@corp.supernews.com>,
> Pat Flannery <fla...@daktel.com> wrote:
>>>> Here's an off-the-wall concept: LH2/H2O2
>>> btw isp-384 vac
>>
>>384? That's not bad, really.
>
> It's not an entirely ridiculous idea; it's been suggested. The high
> density of the peroxide goes some way toward making up for the low
> density of the hydrogen, especially since the mixture ratio is *very*
> skewed toward the peroxide.
>
> Of course, you still have all of hydrogen's ultra-cryogenic problems,
> so the room-temperature oxidizer doesn't help as much as it usually
> does...
Exactly. If you're looking for a place to improve LH2/LOX, you can find a
lot more room for improvement in the fuel than the oxidizer...
--
JRF
Reply-to address spam-proofed - to reply by E-mail,
check "Organization" (I am not assimilated) and
think one step ahead of IBM.
Pat Flannery
Henry Spencer wrote:
> Of course, you still have all of hydrogen's ultra-cryogenic problems, so
> the room-temperature oxidizer doesn't help as much as it usually does...
>
How about running the turbopumps off some of the H2O2 via catalytic
decomposition?
Pat
Pat Flannery
Jorge R. Frank wrote:
> Exactly. If you're looking for a place to improve LH2/LOX, you can find a
> lot more room for improvement in the fuel than the oxidizer...
>
Whatever became of the wonder-of-wonders, slush hydrogen?
Pat
Henry Spencer
The same thing that happened to it in all previous projects proposing it:
nothing, because the project died. (People have been proposing slush
hydrogen since the 60s -- Phil Bono's S-IVB-derived "SASSTO" SSTO
demonstrator was to use it, for example.) The improvement isn't immense
and there are a bunch of annoying practical problems -- for example,
simply measuring how much you've got in a tank is difficult when the
density varies considerably depending on the solid percentage. (Cryo
liquids have a milder case of the same problem, since their density varies
with temperature, but with them you can at least *measure* the temperature
to calibrate this out. Slush hydrogen's temperature is fixed at the
melting point of solid hydrogen, regardless of solid percentage.)
Henry Spencer
If you've got peroxide around that's a pretty obvious thing to do, but
it's a convenience rather than a huge improvement. (Getting rid of the
LH2 cryogenic hassles *is* a huge improvement.)
Orval Fairbairn
he...@spsystems.net (Henry Spencer) wrote:
> In article <12vcu0n...@corp.supernews.com>,
> Pat Flannery <fla...@daktel.com> wrote:
> >> Of course, you still have all of hydrogen's ultra-cryogenic problems, so
> >> the room-temperature oxidizer doesn't help as much as it usually does...
> >
> >How about running the turbopumps off some of the H2O2 via catalytic
> >decomposition?
>
> If you've got peroxide around that's a pretty obvious thing to do, but
> it's a convenience rather than a huge improvement. (Getting rid of the
> LH2 cryogenic hassles *is* a huge improvement.)
No, it isn't H2O2 has an Isp of about 180, vs 425+ for H2/O2.
Also, the water injection idea causes a loss of performance, as it
greatly raises the molecular weight of the exhaust gas, thus lowering
Isp. This is why H2/O2 engines run very fuel rich.
A standard rule of rocket design is to make all of the propulsion
expendables do the most work practical for you.