Falcon-9 and kerosene engines in a vacuum

[ka9q]

I assume at least some of you saw the recent successful launch of the Falcon 9, right?

This launcher is highly unusual in burning kerosene/LOX in an upper stage. Outside of some Russian designs, from which I have yet to see an onboard launch video, I can't think of another launcher that does. They all burn either LH2/LOX or hypergols, which we know produce nearly invisible plumes.

What surprised me about the Falcon 9 video was that, in a vacuum, even its kerosene-fueled upper stage had an invisible plume. This confirms what we've thought all along: atmospheric oxygen burning residual carbon is what makes the plume of a kerosene-burning rocket glow brightly.

In a rational world this would put yet another nail into the silly Apollo denier claim that plumes should be seen from the LM ascending from the moon.

Aside from the brief flashes we see at ignition, is there any visual record of any kind of liquid rocket engine producing a steady, bright plume while burning in a vacuum?

I leave out solid propellants because they produce solid combustion products (e.g., aluminum oxide) that catch the sun. They make for the most spectacular dawn and dusk rocket launches.

[echnaton]

Jul 12, 2010 8:36:58 GMT -4 ka9q said:

I leave out solid propellants because they produce solid combustion products (e.g., aluminum oxide) that catch the sun. They make for the most spectacular dawn and dusk rocket launches.

Certainly correct me if I am wrong, but doesn't burning RP2 also leave solids in the form of soot? That black stuff left hanging in the air following the path of the rocket.

[Bob B.]

Jul 12, 2010 8:36:58 GMT -4 ka9q said:

This launcher is highly unusual in burning kerosene/LOX in an upper stage. Outside of some Russian designs, from which I have yet to see an onboard launch video, I can't think of another launcher that does. They all burn either LH2/LOX or hypergols, which we know produce nearly invisible plumes.

As far as I know, yes, the Russians are the only ones that use kerosene fueled upper stages. There are, however, a couple American examples that burn their core stages for a much longer time then you would normally expect for a first stage.

Although it has an hypergolic second stage, the Delta II burns its LOX/RP-1 first stage for about 4:20. It essentially serves the same function as the sustainer engine on the old Atlas, continuing to burn after the solid rocket motors drop away. The Atlas V also burns it RP-1 fueled core booster for about 4 minutes. By the time both of these stages get to the ends of their burns, the altitude is such that I think you could consider it a vacuum. I don't know if there is good launch video of either one.

The only non-Russian rocket I can think of that was all LOX/RP-1 fueled was the old 1.5-stage Atlas, though technically it did not have a second stage.

Jul 12, 2010 8:36:58 GMT -4 ka9q said:

Aside from the brief flashes we see at ignition, is there any visual record of any kind of liquid rocket engine producing a steady, bright plume while burning in a vacuum?

Not that I've ever seen.

[ka9q]

I've seen numerous rocketcam videos of Delta-II launches showing how the RP-1 fueled first stage engine plume behaves as the launcher rises through the atmosphere. (Obviously until SRB staging you don't see much of the RP-1 plume alone.) It dims and blossoms out until all you see are irregular radial black streaks (which must be a perspective illusion).

But the camera on Falcon 9 showed no visible plume during second stage flight at all. I attribute this difference to the difference in atmospheric pressure between where the two stages operate.

Most launchers drop their payload fairings shortly after staging, typically corresponding somewhat arbitrarily to the point where aerodynamic heating on the payloads is equal to or less than solar heating (1.3 kW/m²). So at least by this definition of "atmosphere", Delta-II's first stage never burns outside it while the Falcon 9's second stage certainly did.

(BTW, when the fairing is dropped, aerodynamic pressure is already nil. Peak heating and peak pressure occur at different times in both ascent and descent because of that crucial difference in the exponent on velocity in the equations for momentum - mv - and kinetic energy - 1/2 mv².)

[Bob B.]

Jul 13, 2010 6:40:09 GMT -4 ka9q said:

I've seen numerous rocketcam videos of Delta-II launches showing how the RP-1 fueled first stage engine plume behaves as the launcher rises through the atmosphere. (Obviously until SRB staging you don't see much of the RP-1 plume alone.) It dims and blossoms out until all you see are irregular radial black streaks (which must be a perspective illusion).

I've found a couple of sources (see below) that indicate the RS-27 engine of Delta II uses film cooling, which could likely be the source of the black streaks.

I don't know exactly what method the RS-27 uses, but commonly the turbine exhaust is ducted to a manifold around the periphery of the nozzle and is injected through orifices into the nozzle where it forms a thin film between the hot engine exhaust gases and the nozzle wall. The turbines run at a very fuel-rich mixture to keep the temperature down, so this exhaust is very dark and sooty in comparison to the that from the engine.

From all accounts I've read, the Merlin engine dumps its turbine exhaust through a separate exhaust assembly rather than injecting it into the main nozzle.

Bases on the above, I suspect this accounts for the appearances of the exhaust in the Delta II and Falcon 9 videos. The Delta's RS-27 is going to have more unburnt fuel and soot around the periphery of its exhaust stream, while the Falcon's Merlin engine exhaust should appear cleaner. (Of course, I don't rule out the possibility that higher atmospheric pressure at the cutoff point of the RS-27 might also play some part in the appearance of the exhaust.)

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2fwww.pwrengineering.com/articles/heart.htm - "Film cooling provides protection from excessive heat by introducing a thin film of coolant or propellant through orifices around the injector periphery or through manifolded orifices in the chamber wall near the injector or chamber throat region. This method is typically used in high heat flux regions and in combination with regenerative cooling. The F-1, J-2, Atlas, RS-27 and SSME have all used this technique."

ftp.rta.nato.int/public//PubFullText/RTO/EN/RTO-EN-AVT-150///EN-AVT-150-06.pdf - Page 6-23 states "Sample engines where film cooling is applied are the SSME, F-1, J-2, RS-27, Vulcain 2, and the RD-171 and RD-180 with the latter two being the only ones where an additional cooling film is generated near the throat."

[JayUtah]

Film cooling does not necessarily use turbine exhaust as the cooling film. This was certainly true in the Rocketdyne F-1 which ducted the turbine exhaust into the nozzle skirt. But film cooling can begin at the injector and use unreacted propellant.

The point of film cooling is to create a layer of gas between the mainstream flow in the thrust chamber and the chamber wall. The chemical composition of the film (typically fuel-rich and reasonably opaque at NIR wavelengths) reduces radiative heat transfer. The fluid dynamics of the film layer, which typically does not mix with mainstream flow, prevent convective heat transfer to the chamber wall.

Open-loop engines (i.e., those that do not recover the turbine exhaust) can still employ film cooling, typically by injecting fuel only -- without oxidizer -- around the rim of the injector. This is how the V-2 team cooled their thrust chamber although their injection regime differed radically from modern engines.

Mainstream flow in LOX/RP-1 engines contains sooty particles that are susceptible to afterburning, but the principal component of that plume radiation in the visible spectrum is not afterburn but simply incandescence in the particles. At high altitude the plume disperses rapidly. The rapid expansion of gas and the more favorable radiation environment causes the plume to lose heat rapidly; hence it does not retain enough heat to allow the plume to incandesce effectively in the visible wavelengths.

Nearly any imperfection in the chamber and nozzle walls will produce visible effects in the exhaust plume. This includes intentional imperfections such as film injection port patterns and unintended imperfects arising from manufacturing irregularities.

[Bob B.]

Jul 13, 2010 11:21:16 GMT -4 JayUtah said:

Film cooling does not necessarily use turbine exhaust as the cooling film. This was certainly true in the Rocketdyne F-1 which ducted the turbine exhaust into the nozzle skirt. But film cooling can begin at the injector and use unreacted propellant.

That's why I said I wasn't sure what method the RS-27 uses - I was unable to find a clear description. Since my last post, however, I found the following:

RS-27A Propulsion System

That cover photo clearly shows the turbine exhaust being ducted away without being injected into the nozzle skirt. If that engine uses film cooling, it must be by another method, such as injecting propellant. Accordingly, I retract some of my prior statements.

[ka9q]

Thanks, Jay, for the writeup on film cooling. Very informative.