Author Topic: Why Hydrocarbon for the First Stage?  (Read 2933 times)

Offline SolarPowered

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Why Hydrocarbon for the First Stage?
« on: 06/14/2007 06:22 pm »
I've seen several people comment in various threads that in their opinion an ideal rocket (based on current chemical propulsion technology) would have a kerosene first stage, an LH2 second stage, and an LH2 third stage if it has a third stage.

The philosophy apprears to be validated by the Atlas and Delta rocket families, where the Boeing pulled out of the commercial market because they couldn't compete with Atlas pricing.

What I don't understand is, why is this the case?  In playing with spreadsheets for modeling rocket performance, I find that, for a given mass-to-orbit, a kero rocket is going to have a lot higher liftoff weight, because of the lower Isp of the fuel. (Duh)  I understand that the LH2 rocket is going to have bigger tanks because of the low density of LH2 compared with kerosene.  But the LH2 rocket is also is going to get by with a lot less engine, because is doesn't have to lift nearly as much propellant.  And my (possibly flawed) understanding is that the cost of tanks is essentially "free" when compared with the cost of engines, so I would expect the LH2 rocket to be cheaper.

So, the best I can figure is that LH2 engines cost a great deal more, per pound of thrust, than kerosene engines.  Either that, or tanks actually do cost a lot in comparison with engines, so the extra tank cost overwhelms the savings on engines.

Could someone enlighten me about why a kerosene first stage is better?

Thanks!

Offline meiza

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Re: Why Hydrocarbon for the First Stage?
« Reply #1 on: 06/14/2007 06:34 pm »
It's easier to make high thrust kerosene engines. Turbopumps are sized by volume of flow, not mass, and hydrogen is really light. Also hydrogen is tricky to deal with in a big stage.
The lightness is more important in upper stages where their mass affects the size of the below stages too, or if the lower stages are kept the same, then the payload more directly.

Offline Propforce

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Re: Why Hydrocarbon for the First Stage?
« Reply #2 on: 06/14/2007 08:08 pm »
Quote
meiza - 14/6/2007  11:34 AM

It's easier to make high thrust kerosene engines. Turbopumps are sized by volume of flow, not mass, and hydrogen is really light. Also hydrogen is tricky to deal with in a big stage.

Huh?  What?  So I make the LH2 turbopump a little bigger, what is the big deal?


Offline Zachstar

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Re: Why Hydrocarbon for the First Stage?
« Reply #3 on: 06/14/2007 08:25 pm »
Quote
Propforce - 14/6/2007  3:08 PM

Quote
meiza - 14/6/2007  11:34 AM

It's easier to make high thrust kerosene engines. Turbopumps are sized by volume of flow, not mass, and hydrogen is really light. Also hydrogen is tricky to deal with in a big stage.

Huh?  What?  So I make the LH2 turbopump a little bigger, what is the big deal?


Much larger first stage.

Hydrogen boils off quickly and must be managed.

Hydrogen burns quite easily and starts easily.

And it has to be chilled as well.

Offline WHAP

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Re: Why Hydrocarbon for the First Stage?
« Reply #4 on: 06/14/2007 09:18 pm »
Quote
pad rat - 14/6/2007  2:53 PM

...and when you spill it, it cracks your launch table ;)

No, that was LOX  ;)
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Offline Antares

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Re: Why Hydrocarbon for the First Stage?
« Reply #5 on: 06/14/2007 10:25 pm »
For me it's an operability question.
Hydrogen volatility and leak tendency
The helium required to purge hydrogen systems,
helium is an increasingly scarce commodity,
boosters/tanks that have to get barged instead of flown (at least for ILV class),
-423F is a lot worse than -290ishF,
another large set of cryogenic flight valves
......  From a purely technical stance, LH2 is great.  But when total launch system cost and ease of use are added, it's RP all the way.

Heck, I'd imagine Elon did a pretty hard cost analysis since he had a clean sheet, and SpaceX didn't even use a hydrogen second stage.

I think the reason Delta IV went LH2 was the company still had experience with the SSME and Shuttle MPS.
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Offline quark

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Re: Why Hydrocarbon for the First Stage?
« Reply #6 on: 06/14/2007 10:53 pm »
Quote
Antares - 14/6/2007  4:25 PM

For me it's an operability question.
Hydrogen volatility and leak tendency
The helium required to purge hydrogen systems,
helium is an increasingly scarce commodity,
boosters/tanks that have to get barged instead of flown (at least for ILV class),
-423F is a lot worse than -290ishF,
another large set of cryogenic flight valves
......  From a purely technical stance, LH2 is great.  But when total launch system cost and ease of use are added, it's RP all the way.

Heck, I'd imagine Elon did a pretty hard cost analysis since he had a clean sheet, and SpaceX didn't even use a hydrogen second stage.

I think the reason Delta IV went LH2 was the company still had experience with the SSME and Shuttle MPS.

Agree with everything Antares listed.  Another big swinger is size.  Size ripples back through all the infrastructure.  The scale of production, the size of tooling, the difficulty of transportation (the Atlas booster can be flown, the DIV requires a ship),  the size of the assembly building, the size of GSE, the size of the launch mount and MST.  Everything is driven in the more expensive direction by LH booster.  It matters far less on upper stages due to the lower prop mass required.

Take a tour of LC-37 then LC-41 back to back and you'll see what I mean.

Further, from a performance POV, thrust is more important for first stages while ISP is king for upper stages.  That's why you see the wide use of solids for boost even though they have even lower ISP than hydrocarbon.

Offline CFE

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Re: Why Hydrocarbon for the First Stage?
« Reply #7 on: 06/15/2007 06:44 am »
In terms of the decision at Space X to go LOX-kerosene on both stages, I'm certain that the difficulties of developing hydrogen-burning engines factored into that decision.  SpaceX also must have considered the studies showing that using the same propellants in all stages leads to cost savings.

When McDonnell Douglas initially designed Delta IV, they also wanted to cut costs by using the same fuels in both stages.  The difference vs. SpaceX is that developing new hydrogen-fueled engines wasn't a problem for McDD or its engine vendors, P&W and Rocketdyne.  The Delta IV upper stage was a bright spot to emerge from the disastrous Delta III program, and it used an evolved member of the long-running RL-10 family.  On the lower stage, Rocketdyne was able to develop the fairly-simple RS-68 by leveraging work from the STME program, adopting a heavy but simple ablative nozzle, and reducing the chamber pressure (limiting the area ratio of the RS-68 nozzle and sacrificing Isp.)

LockMart didn't seem to be concerned about the cost impact of different fuels in the two stages of Atlas V.  The result is a rocket (Atlas V 401) that has better performance than Delta IV medium in a smaller package.  Kerosene has lower Isp than hydrogen, but the impulse per unit of propellant volume is better than hydrogen.  On upper stages, hydrogen makes more sense because the bulk of the rocket's liftoff mass has already been expended during the first stage burn.
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