Jump to content

Fuel Types


Kerbalsaurus

Recommended Posts

2 hours ago, sevenperforce said:

You will enjoy this short story very much.

Ahh, this one. "Chemistry horror story" is an underrated genre.

2 hours ago, sevenperforce said:

A fuel-switching tripropellant engine has always been one of my favorite proposals for a 1.5-stage-to-orbit reusable launcher. 

1.5 stage refers to a rocket that gets you most of the way there and a lightweight kick stage to finish, correct?

Link to comment
Share on other sites

1 hour ago, AckSed said:

1.5 stage refers to a rocket that gets you most of the way there and a lightweight kick stage to finish, correct?

There are multiple definitions of "1.5 stage", but most typical is a booster/sustainer setup, like for example the Space Shuttle or the original Atlas rocket. A lower-thrust, long burning "sustainer" and high power "booster" are ignited together at launch. Then when the booster burns out, it is dropped. The "sustainer" continues burning all the way up to orbit.

(The Space Shuttle typically completed its orbital insertion using the OMS engines, so it could be argued that it was a 2.5 stage rocket rather than a 1.5 stage rocket.)

Typically the booster is high-thrust, low-ISP, and the sustainer is low-thrust, high-ISP. Hydrolox is the most common choice for sustainer stages, although the Atlas rockets (such as those that put the Mercury spacecraft into orbit) used kerolox for the whole thing.

Edited by mikegarrison
Link to comment
Share on other sites

8 hours ago, mikegarrison said:

There are multiple definitions of "1.5 stage", but most typical is a booster/sustainer setup, like for example the Space Shuttle or the original Atlas rocket. A lower-thrust, long burning "sustainer" and high power "booster" are ignited together at launch. Then when the booster burns out, it is dropped. The "sustainer" continues burning all the way up to orbit.

Got it. Now I know that, the designers of the TAN definitely had this on their mind. It's a neat way of combining the booster engine and sustainer into one rocket engine. Interestingly, they used the same injector and combustion chamber used for the LANTR testing. They may even have been thinking of the 300-bar RD-701, as they cite a lack of need for high combustion chamber pressures when you inject the propellant downstream of it.

Link to comment
Share on other sites

10 hours ago, AckSed said:
13 hours ago, sevenperforce said:

A fuel-switching tripropellant engine has always been one of my favorite proposals for a 1.5-stage-to-orbit reusable launcher. 

1.5 stage refers to a rocket that gets you most of the way there and a lightweight kick stage to finish, correct?

No, that would still be a TSTO. As @mikegarrison explained above, a 1.5 stage rocket (also referred to as a "stage and a half design") has a single stage which fires from the launch pad all the way up to orbital insertion, but is also supported by one or more boosters. Examples here are the original R-7 that put Sputnik 1 in orbit (core that went to orbit, boosters that didn't), the Shuttle (operationally it used the OMS engines to circularize, but it COULD have burned the RS-25s all the way to orbit), the Atlas LV-3B that put the Mercury capsules in orbit (technically the "booster" here was just a pair of engines but close enough), and the Long March 5B (four boosters plus a core that goes all the way to orbit and then tends to drop back uncontrollably).

A fuel-switching tripropellant engine is attractive because it can start with high thrust and lower specific impulse in a sea-level nozzle, then switch to lower thrust and higher specific impulse, using the same nozzle as a vacuum nozzle. Basically an altitude-compensating engine (as opposed to an altitude-compensating nozzle). If you wanted to build a reusable spaceplane, you could use this kind of engine and have it launch vertically with a couple of fairly modest strap-on boosters, and it would be able to have both high TWR at liftoff and high specific impulse at orbital insertion.

Link to comment
Share on other sites

23 hours ago, sevenperforce said:

A fuel-switching tripropellant engine is attractive because it can start with high thrust and lower specific impulse in a sea-level nozzle, then switch to lower thrust and higher specific impulse, using the same nozzle as a vacuum nozzle. Basically an altitude-compensating engine (as opposed to an altitude-compensating nozzle). If you wanted to build a reusable spaceplane, you could use this kind of engine and have it launch vertically with a couple of fairly modest strap-on boosters, and it would be able to have both high TWR at liftoff and high specific impulse at orbital insertion.

The down-side of needing a combustion chamber that can handle properly mixing and expelling both combinations seems highly problematic.

Even with a normal engine, you have space issues putting all the injectors you want where you want them, so having two sets would be very difficult(not to mention the space for piping), then you need the throat to be properly tuned to both sets of combustion, which seems unlikely to be an easy thing to manage, as well as putting substantial constraints on the relative fuel flow between the two reaction types(volume, temperature and pressure capabilities of the combustion chamber and throat will not change between fuels after all, and this may reduce isp to the point of making it worthless for many/most combinations)

Link to comment
Share on other sites

1 hour ago, Terwin said:

The down-side of needing a combustion chamber that can handle properly mixing and expelling both combinations seems highly problematic.

This is only if you assume that multiple injectors are needed. The Merlin 1D uses a single large pintle injector and has gone on to be a successful kerelox engine. Tuning that for two different fuels would indeed be a pain, but it depends upon the state of what's being fed into it. A gas-liquid or gas-gas staged-combustion would increase the stability.

Anyway, going by the RD-701 and its single-chamber brother, the 704, they found injectors that would work, with 50 firings and a "smooth transition" between modes. So it has been done. Had to reach chamber pressures of 300 bar for the boost mode, but it was done.

Side note, I was wrong earlier: it wasn't switching between fuels, it was a true tri-propellant engine, burning hydrogen AND kerosene. In fact it used more H2 in the boost stage until the kerosene ran out than in the H2-only sustain mode. Perhaps for cooling?

Edit: on further thought, nozzle and combustion chamber cooling is a good argument for keeping the LH2 flowing all the way through the climb.

Edited by AckSed
Link to comment
Share on other sites

42 minutes ago, AckSed said:

it was a true tri-propellant engine, burning hydrogen AND kerosene. In fact it used more H2 in the boost stage until the kerosene ran out than in the H2-only sustain mode. Perhaps for cooling?

Exactly for cooling.
A hydrogen engine uses it to make the coolng system lighter, and thus must use the hydrogen cooling non-stop while it works.
Then it utilizes it as a fuel.

The kerosene is added at the very beginning to increase the thrust, then it's switched off, so the engine becomes hydrolox.

Link to comment
Share on other sites

3 hours ago, AckSed said:

Side note, I was wrong earlier: it wasn't switching between fuels, it was a true tri-propellant engine, burning hydrogen AND kerosene. In fact it used more H2 in the boost stage until the kerosene ran out than in the H2-only sustain mode. Perhaps for cooling?

Here's a tweet showing how it was mounted and what the power cycle diagram looked like:

 

Detail view of the power cycle:

D48m6smXkAEeXkK?format=png&name=900x900

The engine used three separate-shaft boost pumps, two separate kerolox oxidizer-rich preburners, a separate-shaft high-pressure hydrogen pump operated off of one preburner, and common-shaft high-pressure oxygen and kerosene turbopumps operated off of the other preburner. The high-pressure hydrogen flow was sent through a cooling loop before being injected into the combustion chamber and also sent a little of the hot hydrogen back upstream to operate the hydrogen boost pump in an expander cycle (the expander turbine downstream still had a higher pressure than the boost pump downstream, so it could flow back into the boost pump downstream readily). The kerosene boost pump did the same, except that it didn't also pick up extra heat. Curiously, the LOX boost pump was operated not by high-pressure LOX tap-off, but from the actual preburner exhaust tapoff, meaning that hot oxygen-rich exhaust was being mixed back into the cryogenic liquid oxygen flow upstream of the turbopumps.

Initially, both preburners operated at full throttle. However, as the kerosene tanks started to run low, one preburner (the one on the right in the diagram above) was throttled down significantly while the other stayed approximately constant. As a result, the flow of hydrogen to the engine was reduced by 7% but the LOX flow was reduced by 60% and the kerosene flow into the chamber was extinguished. However, since the entire power cycle was provided by the oxygen-rich kerolox preburners, both preburners stayed "on" for the full flight; the "throttled-down" turbopump needed to at least provide enough power to continue pumping the LOX for the "full thrust" turbopump as well as pump the kerosene it continued to use. This at least meant that the oxidizer flow coming into the engine was always an oxygen-rich gas, which helped maintain consistent combustion even though the chamber pressure was cut almost in half.

Note that in the diagram above, it is difficult to see that the liquid hydrogen (Fuel2) is actually flowing into the combustion chamber, but it is.

Officially, the dual-chamber engine had four preburners, seven turbines, and nine turbopumps (two of which were common-shaft), because while there was only one set of boost pumps downstream of the tank outlets, the two chambers had separate preburners, turbines, and pumps. Despite Due to this complexity, the whole engine assembly came in at just 1.9 tonnes a hefty 3.7 tonnes and boasted a whopping 3.2 MN of sea level thrust, for an incredible a reasonably serviceable TWR of 170:1 88.6:1 in Mode 1 with a sea level specific impulse the same as Raptor 1 and slightly better than Raptor 2. And in Mode 2 it had better vacuum specific impulse than the RS-25s despite requiring a trickle of kerosene the entire time.

Truly a marvel of design and engineering. Too bad it never flew.

3 hours ago, AckSed said:

Edit: on further thought, nozzle and combustion chamber cooling is a good argument for keeping the LH2 flowing all the way through the climb.

Yes, the heat capacity of LH2 is really quite impressive. I wonder if there would be a way to do an expander-cycle tripropellant engine to take more advantage of hydrogen's heat capacity.

EDIT: Fixed the dry weight and corresponding calculations, which Wikipedia had wildly, wildly wrong. 

Edited by sevenperforce
Correction to large whopper
Link to comment
Share on other sites

10 minutes ago, sevenperforce said:

Yes, the heat capacity of LH2 is really quite impressive. I wonder if there would be a way to do an expander-cycle tripropellant engine to take more advantage of hydrogen's heat capacity.

Most of the time it's simpler to just pick one fuel, but... I have an idle thought about chilled propane/oxygen/hydrogen. It's never been tried as far as I know, but what with propane being both dense as kerosene at LOX temps and able to cool cryogenically, mixing in hydrogen for extra specific impulse, then using the expanded H2 in an expander bleed cycle... there's something there.

Link to comment
Share on other sites

2 hours ago, sevenperforce said:

Truly a marvel of design and engineering. Too bad it never flew.

The RD-701 was planned to be used on the MAKS spaceplane, which (being air-launched) would have had lower TWR requirements (around 1.2 instead of the 1.4-1.5 that is preferable for ground-launched kerolox rockets).  It was supposed to be able to separate from the top of the double-tailed carrier aircraft with a separation mass of 275 tonnes, deliver an 8-tonne payload to LEO, drop its expendable fuel tank, and return for a winged landing.

Using the Launch Vehicle Performance Calculator from Silverbird and imagining a ground-launched expendable SSTO, I get a modest 2.8 tonnes to LEO from the Cape. That's if you stack a shorter version of an Atlas V core on top of a shorter version of a Delta IV core and bolt a single RD-701 to the aft end with a TWR of 1.4 or thereabouts.

2 hours ago, AckSed said:

Most of the time it's simpler to just pick one fuel, but... I have an idle thought about chilled propane/oxygen/hydrogen. It's never been tried as far as I know, but what with propane being both dense as kerosene at LOX temps and able to cool cryogenically, mixing in hydrogen for extra specific impulse, then using the expanded H2 in an expander bleed cycle... there's something there.

Hydrogen is such a great working fluid. Using it for the power cycle is either a free lunch or a terrible idea, and I'm not sure which.

Link to comment
Share on other sites

Digging in to solid rockets now, and really, the history of solid rockets is intimately linked with explosives research because the chemistry is similar: fuels with oxidisers mixed in had been used for hundreds of years in guns and cannons to create gas that pushed out a projectile, only trying to get it to burn without exploding. Astronautix's history of solid propellants is comprehensive, and there is a forest of additives, fuels, binders and oxidisers. I'll just be listing particularly interesting or hair-raising mixtures.

Black powder is the first true rocket propellant. What's surprising is how long it was the rocket propellant, with the Chinese having rockets in the 13th century, all the way through to Congreve in the early 19th and on to Goddard in the early 20th. With trial and error and military applications hammering them in to shape, they reached their apogee in the first years of World War II before being dropped like a bad habit.

Potassium Perchlorate/Asphalt was the first real castable solid rocket motor, only made possible by the availability of high-purity perchlorates. (Lower-purity perchlorates tend to explode.) It had been used before, but this became the basis of JATO units until the end of WWII. The asphalt served as combined fuel and binder. Binders are important, as they hold the inside of the rocket motor together both before and during firing. If it can act as fuel for the rocket, so much the better.

Rubber/Ammonium perchlorate/Aluminium - since asphalt has the undesirable trait of softening on hot days, the militaries of the world and especially the USA tried nearly everything they could think of as alternate binder/thickener/fuel (napalm was created as one of them!) until Thiokol (polysulphide rubber) was tried and then popularised by the Thiokol corporation. Ammonium perchlorate was used as it didn't create so much white smoke in the form of potassium chloride. The rubber could be set, undergo a greater range of temperatures without deforming and withstand the pressures of a core-burning rocket. Other, more available rubber and plastic materials (polybutadiene, polyureathane, polyamide) would follow. Soon, it was found that adding aluminium powder to it as a secondary fuel increased the temperature and the thrust, and it's this that the Shuttle and Artemis SRBs are filled with.

Nitrocellulose/Nitroglycerin - told you. This is a "double-base" propellant, with two highly energetic oxygen-containing compounds mixed together at a molecular level and allowed to set. It was the prototype propellant for the neutron-bomb-tipped Sprint anti-warhead missile, which also used zirconium staples as both fuel and for conduction of heat into the propellant. This increased performance and burn rate, pushing it to Mach 10 in seconds inside the atmosphere. Unfortunate explosions forced a change to a safer but unknown nitrocellulose/ammonium perchlorate/solvent mix with a taste of aluminium.

Aluminium/water ice sounds like it shouldn't work - it'd melt - but a small rocket was actually fired with this. If the particle size is small enough, the layer of aluminium oxide around each particle is thin enough that it can burn vigorously enough to crack hydrogen and oxygen out of the water. Another attractive ISRU propellant for the Moon.

Calcium carbide/binder/50% hydrogen peroxide - hybrid rockets have always been attractive for their higher performance and ability to be turned off. This was one slightly alarming proposal that leveraged the water in the lower-concentration peroxide to create acetylene and oxygen, which would have a fairly high theoretical performance.

 

Link to comment
Share on other sites

And it's not like you have included Mentos.

Oops/ Mentos is a hybride fuel. Sorry.

But the matches aren't. A sulfic microfuel.

Spoiler

 

 

And salt. Also, it's a kind of the electrocketry.

Spoiler

 

 

Edited by kerbiloid
Link to comment
Share on other sites

2 minutes ago, AckSed said:

they reached their apogee in the first years of World War II before being dropped like a bad habit

I see what you did there.

7 minutes ago, AckSed said:

Nitrocellulose/Nitroglycerin - told you. This is a "double-base" propellant, with two highly energetic oxygen-containing compounds mixed together at a molecular level and allowed to set. It was the prototype propellant for the neutron-bomb-tipped Sprint anti-warhead missile, which also used zirconium staples as both fuel and for conduction of heat into the propellant.

There's something just delightful about the problem being a lack of sufficiently-aggressive metallic chemistry and the solution being "staple it!"

8 minutes ago, AckSed said:

Calcium carbide/binder/50% hydrogen peroxide - hybrid rockets have always been attractive for their higher performance and ability to be turned off. This was one slightly alarming proposal that leveraged the water in the lower-concentration peroxide to create acetylene and oxygen, which would have a fairly high theoretical performance.

Hmm, I don't think I've heard of this. Link?

On 2/17/2023 at 2:38 PM, AckSed said:

I have an idle thought about chilled propane/oxygen/hydrogen. It's never been tried as far as I know, but what with propane being both dense as kerosene at LOX temps and able to cool cryogenically, mixing in hydrogen for extra specific impulse, then using the expanded H2 in an expander bleed cycle...

Did a little digging on this. You are correct that propane does have the significant advantage of having kerosene-level density (when subcooled) with a theoretical specific impulse only about 2% lower than methane. A further advantage is that it can share an uninsulated common bulkhead with LOX because its subcooled temperature is comparable to ordinary LOX.

However, there's a problem: it cannot readily be used for cryogenic cooling.

Properly understood, regenerative cooling in rocket engines is a supercritical open-loop organic rankine cycle. In an ordinary rankine cycle, an incompressible liquid coolant is pressurized by a pump (requiring very little work because it starts in its liquid phase), forced into a boiler that turns it into a "saturated vapor" (a fluid at equilibrium below the critical point), allowed to expand almost isobarically through a turbine to perform work, and then condensed at constant pressure to return to its liquid state and make the loop again:

800px-Rankine_cycle_Ts.png

The total work done by the system is the work extracted from the turbine minus the work done by the pump.

The rankine cycle has advantages over other thermodynamic cycles because the turbine operates entirely in the "dry vapor" phase while the pump operates entirely in the "incompressible liquid" phase, limiting the overall change in pressure. The phase change maximizes the amount of heat that the fluid is able to accept. An open-loop rankine cycle uses a working fluid that is exhausted after the expansion and thus is never recondensed.

In a supercritical rankine cycle, however, the fluid is allowed to be heated beyond its critical point to become a supercritical fluid that is neither liquid nor vapor. This dramatically increases the amount of work that the system can do:

9C-4-TS-supercrit-Rank.png

In a supercritical rankine cycle, there is no bubble formation: the transition from liquid to supercritical fluid is smooth. The main disadvantage, of course, is extremely high pressurizes and thus high material stresses.

 

So why can't propane be used in a supercritical open-loop rankine cycle? Unfortunately, the three-carbon chain of propane will coke when forced into its supercritical phase, even at temperatures as low as 500°F. Trying to operate a regeneratively-cooled propane rocket would require keeping the fuel below 500°F which seriously limits the amount of work that can be extracted from an expander cycle.

Due to the coking issues, propane is similarly ill-suited for fuel-rich staged combustion.

However, the addition of hydrogen to the mix could prove interesting. Propane works beautifully for oxidizer-rich staged combustion, after all. Its ideal mixture ratio with LOX is a whopping 4.5:1, significantly higher than methane's 3.8:1, further increasing the overall bulk density of the propellant mix to offset the addition of (extremely fluffy) hydrogen.

I'm imagining an engine with a single oxidizer-rich propalox preburner, two staged-combustion turbopumps, and two expander-cycle turbopumps: one high-temp, one low-temp. The propane-oxygen preburner exhausts into two separate turbines, one to pump the LOX for the entire engine and one to pump the propane. The combustion chamber and nozzle are cooled entirely by liquid hydrogen operating in a high-temperature expander cycle, while the preburner is cooled by a low-temperature propane split expander cycle that operates the boost pumps, allowing the propane to enter the combustion chamber in a supercritical state and thus have improved combustion. Because all of the hydrogen expander cycle's energy goes to pumping liquid hydrogen, it can operate closed at a power level comparable to a split bleed (open) expander cycle. You get the high chamber pressure of staged combustion plus the high specific impulse of hydrogen plus the maximum regenerative cooling power uptake.

 

Because the propalox bulk density is so high, you could theoretically get an overall bulk density similar to methane but at significantly higher specific impulse due to the hydrogen you're adding to the system.

Now to come up with a flow diagram...

 

Link to comment
Share on other sites

1 hour ago, kerbiloid said:

And salt. Also, it's a kind of the electrocketry.

*snip video*

Despite not knowing Russian, he helpfully listed the reaction. I think I know what he's getting at: collecting sodium chlorate (NaClO3) from Martian soil and electrolysing it to make sodium perchlorate (NaClO4), or collecting it directly, to act as the oxidiser in a solid rocket. The silicone bathroom caulk he uses as fuel and binder would take a bit of chemistry to make, but it or another polysiloxane would be capable of being made in-situ. Performance would be... average, but the cheapness and the thrust would cover for that. The use of a hot wire and a match to make a electrically-activated pyrophoric starter is a nice touch.

 

54 minutes ago, sevenperforce said:

Hmm, I don't think I've heard of this. Link?

It wasn't serious, just an idle Usenet post, but it was novel. Henry Spencer is more known as a computer nerd, but he was a founding member of the Canadian Space Society, so if he thinks it's interesting, I take note. Speaking of which...

 

54 minutes ago, sevenperforce said:

So why can't propane be used in a supercritical open-loop rankine cycle? Unfortunately, the three-carbon chain of propane will coke when forced into its supercritical phase, even at temperatures as low as 500°F. Trying to operate a regeneratively-cooled propane rocket would require keeping the fuel below 500°F which seriously limits the amount of work that can be extracted from an expander cycle.

Due to the coking issues, propane is similarly ill-suited for fuel-rich staged combustion.

I don't really know much, I'm more of a librarian than any rocket scientist, but another of Henry Spencer's collected posts on propane says otherwise: that coking (in cooling channels) is a) down to sulphur impurities in the gas b) virtually eliminated once propane is subchilled, suggesting the impurities condense out. He cites:

"Deposit Formation and Heat-Transfer Characteristics of Hydrocarbon Rocket Fuels", Journal of Spacecraft & Rockets v. 22, 1985

Quote

I finally dug this up and read it, and it contains a couple of interesting things that Jeff didn't mention.  Much the most interesting result, for propane enthusiasts anyway :-), is that the coking tendencies of propane drop spectacularly -- to vanishingly small levels -- with even slight pre-chilling of the propane.  Their tentative conclusion is that the coking is not a problem of the propane itself, but is probably caused by sulfur-containing impurities which are being frozen out by the chilling.

The paper also mentions that at higher temperatures nickel cooling channels reduces any carbon coking by a factor of 4 compared to copper, and no surface degradation to the tube walls.

"Compatibility of Hydrocarbon Fuels with Booster Engine Combustion Chamber Liners" Nov-Dec 1992 Journal of Propulsion and Power

Quote

The one-sentence summary of the paper is that neither methane nor propane shows any coking in the traditional sense of the term, but even tiny sulfur impurities in either will give you copper-sulfide deposits which have much the same effect.

Do you have a more modern reference that says propane cokes with carbon when you reach really high pressures? I'll gladly read it.

54 minutes ago, sevenperforce said:

I'm imagining an engine with a single oxidizer-rich propalox preburner, two staged-combustion turbopumps, and two expander-cycle turbopumps: one high-temp, one low-temp. The propane-oxygen preburner exhausts into two separate turbines, one to pump the LOX for the entire engine and one to pump the propane. The combustion chamber and nozzle are cooled entirely by liquid hydrogen operating in a high-temperature expander cycle, while the preburner is cooled by a low-temperature propane split expander cycle that operates the boost pumps, allowing the propane to enter the combustion chamber in a supercritical state and thus have improved combustion. Because all of the hydrogen expander cycle's energy goes to pumping liquid hydrogen, it can operate closed at a power level comparable to a split bleed (open) expander cycle. You get the high chamber pressure of staged combustion plus the high specific impulse of hydrogen plus the maximum regenerative cooling power uptake.

Thank you for taking my half-baked musing and (hah) expanding on it.

Edited by AckSed
Citations
Link to comment
Share on other sites

1 hour ago, AckSed said:

Do you have a more modern reference that says propane cokes with carbon when you reach really high pressures? I'll gladly read it.

This is not a modern reference; it's from 1983. But it's by Aerojet under a NASA contract so it should be pretty reliable for that era. In the results summary on page 4, it says that coking started at under 500°F and that higher propane purity reduced the coking rate but did not reduce the coking threshold temperature, suggesting that it is an issue with propane itself. I haven't had time to read the rest of the paper but I'm assuming it talks a lot more about propane as a fuel type.

Link to comment
Share on other sites

3 hours ago, AckSed said:
4 hours ago, sevenperforce said:

I'm imagining an engine with a single oxidizer-rich propalox preburner, two staged-combustion turbopumps, and two expander-cycle turbopumps: one high-temp, one low-temp. The propane-oxygen preburner exhausts into two separate turbines, one to pump the LOX for the entire engine and one to pump the propane. The combustion chamber and nozzle are cooled entirely by liquid hydrogen operating in a high-temperature expander cycle, while the preburner is cooled by a low-temperature propane split expander cycle that operates the boost pumps, allowing the propane to enter the combustion chamber in a supercritical state and thus have improved combustion. Because all of the hydrogen expander cycle's energy goes to pumping liquid hydrogen, it can operate closed at a power level comparable to a split bleed (open) expander cycle. You get the high chamber pressure of staged combustion plus the high specific impulse of hydrogen plus the maximum regenerative cooling power uptake.

Expand  

Thank you for taking my half-baked musing and (hah) expanding on it.

2164113.jpg

Here you go, a power cycle diagram.

I ended up putting the LOX and propane pumps on a single shaft attached to the ORSC preburner turbine, rather than shoving two separate turbines into the mix.

You can eliminate a tricky fuel-ox shaft seal on the boost pumps by removing the LOX boost pump entirely and running a separate LOX boost pump off a simple tap-off downstream of the LOX turbopump. You could go all the way and move the propane turbopump off the preburner turbine and onto the hydrogen expander turbine, but that would tend to limit chamber pressure because the hydrogen is having to do considerably more work than before, and it will have a tough time competing with the high pressure capabilities of the ORSC preburner. However, if you do a low-performing version of the ORSC preburner and a high-performing version of the much simpler expander cycle, this could work.

Link to comment
Share on other sites

1 hour ago, sevenperforce said:

You can eliminate a tricky fuel-ox shaft seal on the boost pumps by removing the LOX boost pump entirely and running a separate LOX boost pump off a simple tap-off downstream of the LOX turbopump. You could go all the way and move the propane turbopump off the preburner turbine and onto the hydrogen expander turbine, but that would tend to limit chamber pressure because the hydrogen is having to do considerably more work than before, and it will have a tough time competing with the high pressure capabilities of the ORSC preburner.

Example:

2164114.jpg

This is almost a staged-expander staged combustion cycle, because the expander cycles run in series. 

Link to comment
Share on other sites

On 2/21/2023 at 12:11 PM, AckSed said:

Thank you for taking my half-baked musing and (hah) expanding on it.

Just for some added fun...

Someone on NSF saw this idea and remarked that tripropellant designs can also be good for a Thrust-Augmented Nozzle, where additional propellant and oxidizer are injected into the nozzle extension at launch in order to fully fill the nozzle (preventing overexpansion) and add thrust. Once you get high enough and specific impulse becomes more important than thrust, you shut off the additional injection; at altitude you can fill up the whole nozzle without overexpansion.

I proposed this configuration:

2164228.jpg

Here, the ORSC preburner+turbine pump only the LOX, while the closed hydrogen expander pumps the hydrogen along with the small amount of propane needed to operate the preburner. Using propane for the preburner is better than using hydrogen because propane is more dense and so the preburner will have more power this way.

In the low-thrust, high-efficiency mode, both of the valves are closed, and so all of the preburner gases flow through the first turbine stage and directly into the combustion chamber. However, in high-thrust, augmented injection mode, the valves are open and so a portion of the preburner gases flow through a second turbine stage. This increases the power output of the LOX turbopump, and so you still have the same amount of gases flowing into the combustion chamber, but you also have extra gases that are coming out at lower pressure. Those lower-pressure gases can be injected into the nozzle. The boost pumps (not shown) will give enough pressure to inject the corresponding amount of propane as well, since the pressure down there is pretty low.

Link to comment
Share on other sites

On 2/21/2023 at 5:41 PM, sevenperforce said:

2164114.jpg

Interestingly, it looks like Aerojet already had a similar idea:

US20040177603A1-20040916-D00000.png

This is a hydrolox expander-cycle engine that uses a fuel-rich preburner as a "first stage" of the combustion chamber, not to operate a turbine, but simply to provide an additional source of heat for the hydrogen to use in the expander:

US20040177603A1-20040916-D00002.png

Very cool concept. Provides some of the advantages of staged combustion without the moving turbine in superheated gases, which is the hardest part.

Ordinarily, you can't use the heat of a staged combustion preburner directly, because it is the heat which drives the expansion and powers the preburner. However, it might be possible to extract some heat energy from a standard ORSC preburner-turbine combo to lower the temperature and thus allow a lighter, simpler turbine while still operating at reasonably high pressures, and use that extra heat energy to make the expander cycle more powerful and thus able to match the high pressures of the preburner-turbine combo.

Link to comment
Share on other sites

After a little more poking around into this idea, I have what I think it is a pretty inventive way of doing a tripropellant power cycle. The trick is to use multiple turbines on the same shaft and to use a variable mixture ratio not in the chamber, but in the preburner.

The attached image has the propane-LOX power cycle on the left, a blended power cycle in the center, and a hydrolox power cycle on the right. However, for the sake of explanation, we'll start at the right and move to the left.

index.php?action=dlattach;topic=58340.0;

One of the limitations on an expander cycle is a lack of heat. If you can get a little more heat, you can have more power to operate the expander turbopumps and thus improve your pump power and total thrust. So I have an oxygen-rich preburner that burns a little hydrogen with all of the oxygen, not to operate a turbine, but to provide extra thermal energy to an oxygen-loop heat exchanger that operates the oxygen turbopump. On the right, you can see that LOX comes out of the pump, through a heat exchanger inside the preburner, through the turbine to operate the pump, and then into the preburner and subsequently into the engine. The hydrogen side operates like an ordinary expander cycle, pulling heat off the nozzle and chamber and splitting off just a little bit of hydrogen to operate the preburner while pushing the rest into the chamber. 

That's going to have really high efficiency (you really only need to put a tiny bit of hydrogen into the preburner, just enough to give you the heat you need to operate the oxygen turbopump), but it's not going to have great thrust. So let's switch over to the left-hand side, where you have the liftoff configuration: a propane-LOX engine running on a hydrolox gas generator cycle.

Let's unpack this. On the LOX side, you've used the three-way valve to run significantly less LOX through the heat exchanger and into the preburner; instead, you're pushing most of the LOX directly into the combustion chamber. On the fuel side, you're no longer pushing any hydrogen into the combustion chamber; you're sending it all into the preburner. You're pumping propane into the combustion chamber in its place. Because the preburner is no longer getting as much LOX and is getting significantly more hydrogen, it's now mildly fuel-rich. But more importantly, it's not exhausting into the combustion chamber anymore; instead, it's going into a turbine that exhausts into the nozzle extension. Because this exhaust is extremely low-pressure, the available power from that turbine goes through the roof, and so there's substantially more LOX flowing through the entire system at higher pressure. Similarly, because all of the liquid hydrogen is going into the preburner which exhausts to the nozzle extension, you've gone from a closed expander to an open expander, and so the power on the fuel side also skyrockets, allowing you to pump the same amount of hydrogen plus all of the propane at significantly higher pressures. The result is a high-pressure propane-LOX reaction in the combustion chamber with a hydrolox gas generator.

The center version demonstrates the transition from high-thrust, low-isp to low-thrust, high-isp: the preburner is now slightly oxygen-rich and is partially exhausting into the chamber and partially exhausting through the turbine, while the hydrogen is partly going into the chamber along with a small amount of propane. With three different valves in operation, you get a smooth transition between the two power cycles. And since you're pushing a little more fuel into the chamber than you have oxidizer, this reheats in an afterburn with the slightly oxidizer-rich preburner exhaust in the nozzle.

I feel like this design simultaneously maximizes the advantages of gas generator cycles, closed expander cycles, open expander cycles, oxidizer-rich staged combustion, and nozzle-injection afterburning. Seems promising. Also, because the propane is never in a fuel-rich precombustion state and isn't used for cooling, you can utilize any fuel here: methane, RP-1, even more hydrogen if you want.

Link to comment
Share on other sites

This thread is quite old. Please consider starting a new thread rather than reviving this one.

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

×
×
  • Create New...