Improved Specific Impulse Rocket Engines

Looking only at the vacuum specific impulse of plain hydrogen-oxygen
rockets, is it possible to advance their specific impulse to 500, or
even 520?

For example, I recall Mr. Spencer recently mentioning that an ideal
oxidizer:fuel ratio for hydrogen & oxygen was 4:1, but 6:1 was used
because of tankage mass penalties. Would a 4:1 ratio provide an
increase in specific impulse? If so, approximately how much?

Mike Miller, Materials Engineer

(Mike Miller) wrote in
om:

Looking only at the vacuum specific impulse of plain hydrogen-oxygen
rockets, is it possible to advance their specific impulse to 500, or
even 520?

For example, I recall Mr. Spencer recently mentioning that an ideal
oxidizer:fuel ratio for hydrogen & oxygen was 4:1, but 6:1 was used
because of tankage mass penalties. Would a 4:1 ratio provide an
increase in specific impulse? If so, approximately how much?

The highest Isp I could find was 480 sec for the Advanced Expander
Cycle engine, a testbed; I recall some sort of development effort in
the 80's that was talking 490 sec and was having problems with the
turbopumps having to spin very, very fast to generate enough power
and pressure.

http://www.astronautix.com/engines/aec.htm (aka RS-44, Rocketdyne)

I just found a discussion by one of the RL-10 developers that goes into
a lot of detail of the history of that engine, including tradeoffs
with mixture ratios and how it affected total stage mass. It's a
bit lengthy, but very, very interesting:

http://fac14.cmps.subr.edu/Foustall.htm

My guess is that Isp in this case is limited by total available energy;
run too rich and there's not enough heat/energy to drive the
turbopumps to create the necessary high pressure. The excess hydrogen
can't get any hotter, so exhaust velocity hits a wall. Too hot and one
runs into practical materials problems and disassociation of the
combustion product which absorbs heat and you once again hit that
wall.

So far as I can tell from my limited understanding is that a simple
LH/LOX chemestry just doesn't have enough energy get even to 500 sec
with known materials. I'm sure Henry will have more perspective on this
subject.

--Damon

(Mike Miller) wrote in
om:

Looking only at the vacuum specific impulse of plain hydrogen-oxygen
rockets, is it possible to advance their specific impulse to 500, or
even 520?

For example, I recall Mr. Spencer recently mentioning that an ideal
oxidizer:fuel ratio for hydrogen & oxygen was 4:1, but 6:1 was used
because of tankage mass penalties. Would a 4:1 ratio provide an
increase in specific impulse? If so, approximately how much?

The highest Isp I could find was 480 sec for the Advanced Expander
Cycle engine, a testbed; I recall some sort of development effort in
the 80's that was talking 490 sec and was having problems with the
turbopumps having to spin very, very fast to generate enough power
and pressure.

http://www.astronautix.com/engines/aec.htm (aka RS-44, Rocketdyne)

I just found a discussion by one of the RL-10 developers that goes into
a lot of detail of the history of that engine, including tradeoffs
with mixture ratios and how it affected total stage mass. It's a
bit lengthy, but very, very interesting:

http://fac14.cmps.subr.edu/Foustall.htm

My guess is that Isp in this case is limited by total available energy;
run too rich and there's not enough heat/energy to drive the
turbopumps to create the necessary high pressure. The excess hydrogen
can't get any hotter, so exhaust velocity hits a wall. Too hot and one
runs into practical materials problems and disassociation of the
combustion product which absorbs heat and you once again hit that
wall.

So far as I can tell from my limited understanding is that a simple
LH/LOX chemestry just doesn't have enough energy get even to 500 sec
with known materials. I'm sure Henry will have more perspective on this
subject.

--Damon

Looking only at the vacuum specific impulse of plain hydrogen-oxygen
rockets, is it possible to advance their specific impulse to 500, or
even 520?

For example, I recall Mr. Spencer recently mentioning that an ideal
oxidizer:fuel ratio for hydrogen & oxygen was 4:1, but 6:1 was used
because of tankage mass penalties. Would a 4:1 ratio provide an
increase in specific impulse? If so, approximately how much?

The highest Isp I could find was 480 sec for the Advanced Expander
Cycle engine, a testbed; I recall some sort of development effort in
the 80's that was talking 490 sec and was having problems with the
turbopumps having to spin very, very fast to generate enough power
and pressure.

snip

The highest chemical engine Isp I've seen is about 520-530... it was done
using a three-propellant engine (of which one propellant was fluorine).
Now, there are many problems inherent in using fluorine... extreme toxicity
and reactivity the chief ones (fuel that wants to eat up the tank it's in
isn't all that great). In short, a nice experiment, but impractical.

I did see a proposal at the JPC last summer for a TSTO using such an
engine... combined, though, the 2 stages used 6 different propellants (not
counting on-orbit RCS/OMS fuel). Not good from an operability standpoint,
and still the fluorine problems (which he claimed to have solved).

Looking only at the vacuum specific impulse of plain hydrogen-oxygen
rockets, is it possible to advance their specific impulse to 500, or
even 520?

For example, I recall Mr. Spencer recently mentioning that an ideal
oxidizer:fuel ratio for hydrogen & oxygen was 4:1, but 6:1 was used
because of tankage mass penalties. Would a 4:1 ratio provide an
increase in specific impulse? If so, approximately how much?

The highest Isp I could find was 480 sec for the Advanced Expander
Cycle engine, a testbed; I recall some sort of development effort in
the 80's that was talking 490 sec and was having problems with the
turbopumps having to spin very, very fast to generate enough power
and pressure.

snip

The highest chemical engine Isp I've seen is about 520-530... it was done
using a three-propellant engine (of which one propellant was fluorine).
Now, there are many problems inherent in using fluorine... extreme toxicity
and reactivity the chief ones (fuel that wants to eat up the tank it's in
isn't all that great). In short, a nice experiment, but impractical.

I did see a proposal at the JPC last summer for a TSTO using such an
engine... combined, though, the 2 stages used 6 different propellants (not
counting on-orbit RCS/OMS fuel). Not good from an operability standpoint,
and still the fluorine problems (which he claimed to have solved).

"Bob Martin" wrote in message ...

The highest chemical engine Isp I've seen is about 520-530... it was done
using a three-propellant engine (of which one propellant was fluorine).

I'm familiar with the high Isp tri-propellant engines (and their
problems), but I was particularly curious about how far current,
favored fuels (specifically hydrogen-oxygen) could be pushed.

Mike Miller, Materials Engineer

"Bob Martin" wrote in message ...

The highest chemical engine Isp I've seen is about 520-530... it was done
using a three-propellant engine (of which one propellant was fluorine).

I'm familiar with the high Isp tri-propellant engines (and their
problems), but I was particularly curious about how far current,
favored fuels (specifically hydrogen-oxygen) could be pushed.

Mike Miller, Materials Engineer

In article ,
Mike Miller wrote:
Looking only at the vacuum specific impulse of plain hydrogen-oxygen
rockets, is it possible to advance their specific impulse to 500, or
even 520?

To about 500, maybe, if you work really hard... 470-480 is the best I
recall anyone actually proposing to achieve with LOX/LH2, using
extremely-high-expansion nozzles (long enough that the limiting factor
on expansion is condensation of water in the exhaust...).

For example, I recall Mr. Spencer recently mentioning that an ideal
oxidizer:fuel ratio for hydrogen & oxygen was 4:1, but 6:1 was used
because of tankage mass penalties. Would a 4:1 ratio provide an
increase in specific impulse? If so, approximately how much?

It helps, but not hugely. Numbers depend on details, but the Saturn V's
upper-stage engines did change mixture ratios, for several reasons, and
going from 5.5 to 4.5 raised their Isp by only about 7 seconds.
--
MOST launched 30 June; first light, 29 July; 5arcsec | Henry Spencer
pointing, 10 Sept; first science, early Oct; all well. |

In article ,
Mike Miller wrote:
Looking only at the vacuum specific impulse of plain hydrogen-oxygen
rockets, is it possible to advance their specific impulse to 500, or
even 520?

To about 500, maybe, if you work really hard... 470-480 is the best I
recall anyone actually proposing to achieve with LOX/LH2, using
extremely-high-expansion nozzles (long enough that the limiting factor
on expansion is condensation of water in the exhaust...).

For example, I recall Mr. Spencer recently mentioning that an ideal
oxidizer:fuel ratio for hydrogen & oxygen was 4:1, but 6:1 was used
because of tankage mass penalties. Would a 4:1 ratio provide an
increase in specific impulse? If so, approximately how much?

It helps, but not hugely. Numbers depend on details, but the Saturn V's
upper-stage engines did change mixture ratios, for several reasons, and
going from 5.5 to 4.5 raised their Isp by only about 7 seconds.
--
MOST launched 30 June; first light, 29 July; 5arcsec | Henry Spencer
pointing, 10 Sept; first science, early Oct; all well. |

In article ,
Bob Martin wrote:
The highest chemical engine Isp I've seen is about 520-530... it was done
using a three-propellant engine (of which one propellant was fluorine).

Clark says 541s was demonstrated (with a high-expansion nozzle) using
fluorine-lithium-hydrogen, with the Li and F stoichiometric and about 25%
hydrogen (which is just reaction mass, it doesn't get involved in the
chemistry) by weight.

Jeff Greason once did some numbers on launchers using that combination,
and concluded that even if you ignore all the practical problems, the
*vehicle* performance is always inferior to straight LOX/LH2. The vehicle
performance penalties of using LH2, e.g. the mass of the huge hydrogen
tanks, completely overwhelm the Isp advantage.
--
MOST launched 30 June; first light, 29 July; 5arcsec | Henry Spencer
pointing, 10 Sept; first science, early Oct; all well. |