So my daughter was asking me (after having seen that beast of a Saturn V at KSC recently), why the fuels were different in Stage 1 (Kerosene and LOX) from Stage 2 and 3 (LH and LOX).

My answer was something like "because the Kerosene won't burn in the upper atmosphere or in space." She's 8, so the argument works (and I thought it was an excellent question).

Now, I'm no engineer, chemist, or physicist, nor do I play any of these on TV, but:

Then I got to thinking.... that's not true at all.... you've still got the LOX as an oxidizer. But (1) you'd have to carry a whole heck of a lot more LOX to burn the kerosene in vacuum than you could possibly carry, or (2) the kerosene weighs much more than the LH, or (3) the kerosene, not being supercooled, would boil off in a vacuum.

Which one is right?

Thanks.

Kevin

(PS I apologize for the crosspost on the other "major" rocketry forum).

It's cheaper, safer, easier to handle, and much more dense. By volume it has more energy.

Luke Strawwalker will be here soon to type out a more detailed 20,000 word answer for you.

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Kevin,

Google is your friend:

Search terms - Kerosene liquid oxgen 1st stage

Results:

http://en.wikipedia.org/wiki/Rocket_propellant

Everything you need to know about ANY propellant combination including the math and the reasoning to go along with it.

Google is our friend Well unless its, 'A Logic named Joe'! That's an old X-Minus One radio show program episode were a 'Logic' (Computer) named Joe becomes self aware and starts causing havoc for humanity

Jonathan

Thanks. That helps tremendously. Just bein' lazy today!!!

That's a much better way of phrasing the question that I did: why the kerosene AT ALL?? The answer you gave about its cost makes sense. And of course the fact that putting as much LH2 into an SI as you would need would create an ungodly dangerous ticking time bomb (see: Challenger, 1986).

Thanks.

Kevin.

Humble old kerosene is a remarkably good fuel for big 1st stage rocket motors, just as it is for jet engines. If the S1-C had been designed to use liquid hydrogen instead, it would have needed to be many times larger than its already massive size, due to that fuel's much lower density. I would hazard a guess that the rocket (along with the launch support structures and the cranes used to assemble it) might very well have become the tallest free-standing structure in the world at that time, once it was fully assembled for launch. The logistics of fueling a rocket with that much LH2 would have been incredibly daunting and stupendously dangerous.

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Yep... predictable am I??

See the same thread over on TRF... I posted the 20,000 word answer there... LOL

Later! OL JR

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Quite possibly... (tallest structure). The S-IC was about 140 feet tall, off the top of my head... to carry LH2, it would probably have had to be at least half again if not double that tall... or else increase the diameter to about 40 feet instead of 33 feet. As it was, Michoud couldn't build anything much past 33 feet in diameter... this was a problem that was REALLY hurting the Ares V (which would have used LH2 in the core stage). They kept adding RS-68's and STILL didn't have enough power to meet the performance goals... (due to RS-68 being a pretty poor-ISP LH2 engine due to its cheaper, throwaway design, and some inefficiencies in the design of the LH2 turbopumps, as I understand it). They went from 5 to 6 RS-68's and were thinking of adding a seventh! Problem was, when they went from five to six, they increased the core diameter from the 27.5 feet (331 inch) diameter of the shuttle ET, to the 396 inch (33 foot) diameter of S-IC/S-II... which would have required totally new tooling at Michoud. When they started looking at adding a SEVENTH RS-68 to the Ares V, they found that they simply didn't have enough room for the propellant... even with the stretched core stage tanks (to match the five segment boosters) and the diameter increase. SO, the only options were ANOTHER core stretch (which doesn't work because the SRB's require a thrust beam between the top of the two SRB's where they support the core vehicle on the pad, and transfer their thrust into the structure via the thrust beam underneat the LOX tank in flight, since the LOX tank contains MOST of the weight of the propellants. The thrust beam moves up and down as much as 6 inches in flight, so it cannot go through the wall of the LH2 tanks, of course... so that only left the possibility of increasing the core diameter... after a bit of head scratching, NASA figured they COULD just squeeze an 11 meter diameter stage (433 inch) in the MAF (Michoud Assembly Facility) with INCHES to spare between the roof and floor with the stage laying on its side... but anything bigger would require raising the roof of the MAF, and that would be INCREDIBLY expensive.

Another issue is, at some point the rocket gets SO tall that it starts bumping up against the limits of the VAB building height... basically the VAB can only handle vehicles up to 410 feet in height on the MLP (LUT in Saturn parlance). Anything taller won't fit through the VAB doors. Of course, such an INCREDIBLY HUGE hydrogen powered first stage is also incredibly inefficient, and MASSIVELY expensive! Ares V had basically started hitting the point of diminishing returns... the more engines they added, the more segments they added to the SRB's, the smaller the performance increases were becoming... it finally reaches a point where adding more simply REDUCES total performance, which was basically where Ares V was when it was canceled. AS it was, some base heating environment and base plume recirculation studies, which hadn't been done when the switch was made from the more efficient (and regeneratively cooled) SSME's to the less efficient (and ablatively cooled) RS-68's, showed that basically the RS-68's couldn't handle the heat environment of such a big cluster underneath a 33 foot diameter tank, especially when flanked on either side by a pair of huge SRB's with highly- heat radiating exhaust plumes... the RS-68's rely on radiative cooling as well as ablative cooling... so large clusters tend to radiate heat ONTO EACH OTHER instead of away into the atmosphere or into space... and having white-hot glowing alumina-particulate rich SRB plumes (with the particulates all radiating heat) just compounds the problem. Hence the switch back to SSME's in the Ares V's final days before its cancellation... and the choice of SSME's for the SLS launcher that replaced Ares V.

One of the US's biggest problems now with SLS (or ANY Heavy Lift Vehicle) for that matter, is the lack of a TRULY high-thrust high efficiency engine. SSME only produces about 500,000 lbs of thrust... although it does have EXCELLENT ISP for a large engine... but the 'low' thrust makes it less suited for first stage duties... particularly on an expendable vehicle, and particularly for such an expensive and complicated engine that was designed for reuse. The RS-68, on the other had, has quite a bit more thrust (on the order of 750,000 lbs) but the ISP is MUCH lower-- the engine was designed as a cheap, throwaway version of a thrust-uprated SSME (harking back to the NLS studies of the mid-80's into the early 90's, to an expendable version of SSME called the STME-- Space Transportation Main Engine... STME was never built, but served as a basis for the RS-68.) Lower ISP means more fuel to do the same work, and with LH2, since it's SO light and requires SUCH big tanks, this is an absolute KILLER. Basically, for an HLV, you really NEED a million pound-plus thrust engine to avoid having to have massive clusters, SRB's (or LRB's), or both... Besides, RS-68 isn't exactly cheap... especially when you figure in the additional costs for the modifications to manrate it... and if you had to redesign it for regenerative cooling, well, you might as well build a whole new engine from scratch for the fuel and specs you want... it would cost about the same... when you figure in the cost advantages of producing SSME's in quantity and throwing them away (unlike shuttle that only built a few at a time, very infrequently, meaning they were VERY expensive on a per-unit basis) the SSME's really aren't THAT much higher than the RS-68's... or so goes the theory anyway... plus, its easier to "dumb down" and simplify a complicated SSME than it is to increase the capabilities of the RS-68.

Later! OL JR

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Remember, Challenger's failure (it wasn't an accident-- NASA KNEW well in advance from damage on previous flights that the O-rings were faulty and suffered more problems the colder the weather was at launch... but the gambled on the launch anyway, and LOST). Challenger's failure was NOT due to the LH2... it was due to the SRB... the O-ring seals failed and created a leak, which partially plugged by "gunk" in the exhaust, until that gunk blew out at altitude (due to wind shear and other factors) and hot exhaust from the SRB started cutting through the casing like a blowtorch... I read that the hole in the side of the SRB was about THREE FEET in diameter when the shuttle "exploded". The orbiter's TVC (engine steering) system was pegged "hard over" trying to keep the stack flying straight, and the SRB TVC had thier nozzles NEARLY hard over, trying to counterbalance this 'side thrust" from the huge hole in the booster... the hole was pointed toward the back side of the tank, just above the struts holding the back end of the SRB's to the tank... the strut and tank wall burned through... but even if the hole had been pointed AWAY from the tank, Challenger would only have survived a few more seconds than it did... once the SRB TVC was pegged hard over, with the hole getting larger every second, the thrust from the SSME's and SRB's would have been unable to keep the stack stable, and it would have cartwheeled out of control, broken up, and "exploded". As it was, the blowtorch flame from the SRB hole burned through the strut and tank wall, fatally weakening the tank, allowing the SRB to rip the other strut free, causing the lower LH2 tank dome to fall off spilling the entire load of LH2, while simultaneously the upper end of the loose SRB pivoted on the upper ball mount on the end of the thrust beam, allowing the nosecone of the SRB to rip into the LOX tank above like a giant can opener, spilling the LOX... when the two clouds of icy gases combined-- WHOOF! Shuttle didn't explode-- the LH2 and LOX merely burned off in massive fireball cloud... it didn't "detonate". The ET ripped to shreds, the orbiter (whose engines were in the midst of shutdown due to LH2 starvation, once the tank bottom ripped off) turned sideways at ~ Mach 2, and subsequently simply broke up into a million pieces... the SRB's continued flying, with their TVC systems still locked nearly hard-over (which is why they flew a "lazy spiral" out of the cloud) for a few seconds until detonated by Range Safety.

LH2 is incredibly hard to seal, because even the tiniest pinholes or micro-fractures in a weld will leak LH2 like a garden hose (or worse), and its extremely low temperature means it is VERY hard on materials and wants to make them brittle as glass... but for "explosive" power, it's not particularly worse than any other rocket fuel... somewhat, but not MASSIVELY worse... the main thing is, safely venting and burning off the boiloff vapors (GH2, or Gaseous H2) from boiloff on the pad. IIRC, Saturn V actually used about nearly twice as much hydrogen as was in the tanks at liftoff... because the propellant had to be constantly replenished on the pad as heat leaked into the tanks (despite their insulation) and boiled the hydrogen off into gas, which was then vented through umbilicals and piped to a burn pond nearby...

A buddy of mine from the police academy, who was a fire chief and was getting his peace officer license to be an arson investigator, also pointed out the insidious fact that HYDROGEN BURNS CLEAR-- you cannot see a hydrogen flame right in front of you... he learned that the hard way when he was a company firefighter at a chemical plant years before and got severely burned in a hydrogen fire... that's why SSME's flame is clear... pure water vapor at 6,000 degrees F. Delta IV's flame is sorta reddish orange, not because of the hydrogen, but because of the ablative coating inside the engine nozzle burning off to keep it from melting, which creates glowing orange particles contaminating the exhaust plume...

later! OL JR

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These are excellent explanations.

Which begs the question (not to go off-topic): why not just use Saturn V-derived rockets for orbital escape lift? I know that the tooling and infrastructure investment would be big, but the thing worked.

Kevin.