Aviation Week & Space Technology, 09/24/2007, page 10
Dale L. Jensen, Lawndale, Calif.
Regarding J2X rocket engines for Ares launch vehicles and your In Orbit item "Billion-Dollar Baby" (AW&ST July 23, p. 15), it is an unfortunate result of the rush to return to the Moon and venture to Mars that the U.S. will spend $1.2 billion for eight engines, or $150 million each. This is an exorbitant amount for powerplants that have been developed and successfully flown, but which are inefficient and polluting.
We should develop advanced-performance rocket engines before we return to the Moon or venture to Mars. If NASA had accomplished its research mission, we would have such engines ready to use for flights to the Moon and Mars.
The J2 engines were the best we could do 40 years ago, but we can do much better now. One difficulty was the inability to withstand combustion chamber temperatures at large mixture ratios. Typically, the Saturn SII stage carried 82,000 lb. of excess hydrogen, which was dumped overboard unused. If the hydrogen is unused, no energy is obtained from it. In addition, it pollutes because superheated hydrogen can combine with the nitrogen and ozone in the atmosphere to produce nitric acid, which harms plant and animal life.
Efficient, advanced performance rocket engines would operate at a stoichiometric mixture ratio. They would not pollute because all the hydrogen would be burned by producing only steam. One-half pound of payload could be added for every pound of hydrogen that is offloaded, which means 41,000 lb. of additional payload. And because less hydrogen would be carried, the launch vehicle will be smaller, less expensive to build and have less drag. The public is not getting its moneys worth because we continue to fly outmoded, polluting and inefficient rocket engines.
I really doubt that slightly fuel-rich LH2/LO2 are going to produce much nitrogen oxides at altitude. The reason being is that most of the free hydrogen will leave the exhaust column almost immediately (because of the high temperatures in the flame) and will diffsue very quickly by lifting and scattering to higher altitudes. The hydrogen won't stop until it reaches the very top of the atmosphere, at which point solar UV can ionize the hydrogen, and it will depart the upper atmosphere for the ionosphere...
I would love to see some chemical reaction kinetics on this one--but I have to say: The Shuttle Solid Rocket Boosters are a lot more polluting than old LH2/LO2 ever will be.
I always thought that LOX/H2 engines were set to burn slightly rich in order to improve ISP and overall performance! (though I concede that it would reduce peak temperatures aswell I done belive that was the primary reason)
They burn slightly rich so that the average molecular weight of the products is slightly reduced (which improves the overall performance of the engine by increasing the exhaust velocity.) Peak Isp occurs very near 6:1 O2/H2 mixture ratio--so that is the standard atleast for most cryogenic LH2/LO2 engines...
Hydrogen polution...is that a problem? The fuel mix is an issue that I ran into on my spaceplane studies. If you are carrying all that LH2 inside your re-entry surfaces the volume leads to a big increase in overall weight. In those case I've even flirted with the idea that basic KP-1/LOX is the way to go even with the lower Isp. But for conventional booster this is not much of an issue.
Increasing the bulk density of propellant results in smaller and thus lighter propellant tanks.
Usually the weight savings is most by shifting to a denser propellant combination in the booster: thus Saturn IC of Apollo-Saturn 5 fame used LOX/RP-1 because this was the single largest component of the launch vehicle. The Saturn IIC second stage had much less LH2/LO2 than the first stage (had it used LH2/LO2 instead of LO2/RP-1) and so its performance was best by staying with liquid hydrogen.
And this is why the Shuttle solid rocket boosters are using a very dense NH4ClO4 (ammonium perchlorate,) aluminum dust, PBAN (polybutadiene-acrylinitrile) composite propellant system: its bulk density (even with the 1 m wide flame tunnel through the center of the booster) is just about 1.2-1.5 g/cm^3 on average which gives it a very high impulse-density for a chemical rocket propellant.
From what I do understand of rocket engineering (and I am not an engineer--only a wannabe!) is that when designing a vehicle many factors (initially dozens to hundreds of factors; later-thousands) must be very carefully considered before something approaching an optimal design evolves. Propellant combination is usually one of the initial parameters explored in 'roughing' out something that will become a design.
For instance--just a little side story from the Apollo days:
The original Saturn IC booster was to have 4 F-1 rocket engines and not 5. After the intial calculations were going along an engineer wondered about thermal radiation coupling from the engine flames to the bottom of the vehicle bulkhead, where the engines physically attached to the airframe. He became concerned because it looked like a large tank of water was needed to cool that section because it might get hot enough to melt (despite thick thermal insulation blankets.) A meeting was held, and another engineer had brought up a very important point on a completely different note: the booster was getting too heavy to fly with 4 F-1's. So a third person proposed a 'simple' solution to both: put a fifth F-1 rocket engine in the spot where it was getting too hot: the engine itself would block the thermal radiation and would of course increase thrust...it elegently solved both problems and we ended up with an Apollo Saturn "5" instead of a "4!"
The moral of the story is sometimes in rocket engineering seemingly subtle factors can conspire to result in big systems changes...
Aviation Week & Space Technology, 10/15/2007, page 8
Al Knutson, Burnaby, British Columbia
Dale L. Jensen, in his letter "Time To Ditch Old Style Engines" (AW&ST Sept. 24, p. 10), is out of touch with the realities and physics of liquid rocket propulsion.
Non-stoichiometric mixture ratios have always been used in rocket engines, not to lessen engineering challenges but to maximize exhaust velocity and thus maximize propulsive thrust, hence efficiency. The original J2 engine had a molecular mixture ratio of 2.9 of fuel to oxygen to provide a specific impulse of 418 seconds, an astonishing high value for the era.
The modern space shuttle main engine has a fuel mixture of 2.66 to 1 of fuel to oxidizer, representing close to the maximum power available for any chemically fueled rocket engine, and providing a vacuum impulse of 455 seconds. Although a ratio of 2 to 1 is chemically stoichiometic, this would be wasteful due to the governing laws of chemistry and physics. Early rocket scientists knew of these certainties long before the first orbital space flight.
Combustion objectives vary in application. Car engines do run slightly oxygen (air) rich to minimize hydrocarbon and soot emissions. Aside from objectives of maximum thrust, there is no evidence that residual hydrogen of the exhaust stream combines with anything other than its preferred reactant of atmospheric oxygen. The slightly lowered exhaust temperature would make its reaction with atmospheric nitrogen even less likely.
Actually the space shuttle uses 6:1 LO2/LH2 by mass. The old F-1 rocket engines of the Saturn-5 used a mixture ratio closer to 2.6:1 LO2/RP-1 which left a small amount of soot in the exhaust--detectable by an easily visible exhaust trail even in clear air...
The slightly reduced molecular weight of slightly non-stoichiometric mixtures results in a maximum Isp slightly high than dead-on stoichiometric mixtures. The chemistry is pretty straightforward...
Regarding J2X rocket engines for Ares launch vehicles and your In Orbit item "Billion-Dollar Baby" (AW&ST July 23, p. 15), it is an unfortunate result of the rush to return to the Moon and venture to Mars that the U.S. will spend $1.2 billion for eight engines, or $150 million each. This is an exorbitant amount for powerplants that have been developed and successfully flown...
That right there is damning enough. There's no doubt that they're milking this "Shrub Space "Vision" for all it's worth.
I have little enough hope for the "Ares" 1 & 5 as it is, and as time goes on, less & less. (elsewhee I've linked to articles which are dubious at best, about this plan and these vehicles. Maybe too pessimistic, or can you be too pessimistic?)
Long past time to kill the Shuttle "Golden Goose" Flying Cost Overrun. Divert funding from it to start things slowly by 1) throwing together some sort of COTS Shuttle-C. Forget the SSME, accept less payload than perhaps the ultimate, & use something else expendable (Would this lift more than a Delta-4H equivalent?) 2) build a simple reliable (probably expendable) ground-LEO-ground crew vehicle.
Right there, we'd cut the cost to lift cargo, vastly increase safety for crew, and pehaps have the combination of vehicles for Lunar and maybe Mars efforts.
I like the old Shuttle-C because it uses everything exactly the same as we've been using it, except the housing for cargo and the expendable engines. Nothing else new, and what we'd be using is vastly simpler than the current Shuttle stack because it's only unmanned. Accept an ugly retro expendable capsule for now, because it get us there and starts to repair the mistakes we've been making. Eventually build the HL-20 or 42, for the "slick sexy fun" crew vehicle of the future (based on '80s work which was based on the old USSR Uragan...) Eventually build a BDB for cargo. Preferrably the Sea Dragon...
But so much for dreaming. What we've got is another contractors dream of a huge expensive space program, designed by Carl Rove and the White House staff. Bet that it goes nowhere. The next administration & future congresses will finally allow it to be recognized that it serves no purpose except milking the taxpayers, and dies ignominiously, leaving us with the Soyuz, possibly extending the Shuttle because we've got nothing else. Maybe we'll get a CEV, but maybe even it's beyond NASA's current ability to build without it costing tens of bilions to keep contractors and NASA centers alive.
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"A devotee of Truth may not do anything in deference to convention. He must always hold himself open to correction, and whenever he discovers himself to be wrong he must confess it at all costs and atone for it."
Monhandas K. Gandhi
