Rocket Engine Q&A

[JosephB]

Found the thread but if anyone wants to comment please do. Thanks.

[JosephB]

Looks like there is a good chance that both Ares I & V will be using expendable 5 segment HTPB composite boosters down the road eventually. Very nice payload improvement (See page 9).

Reasonable assessment?

[Antares]

5.5 seg SRB makes TO even worse.  According to one of Ross's posts, somewhere above 5.5 but less than 6 actually has worse performance for Ares 1 or 5 or both.


I'm not sure if it can blanket be said that a H2 engine is more expensive to design than kero.  However, H2 is certainly harder in operations.  You have to use helium purges and pressurant instead of N2 since N2 would freeze at LH2 temperatures.  H2 also freezes O2, so common bulkheads are much more difficult to design and build unless you're experienced at it (ahem).

As for the engine, since LH2 has such low density, it requires more pump stages in a turbopump.  That's hard by itself and really hard for a single shaft.

H2 burns cleaner, so combustion chamber design is easier.  Mostly I think it's just the temperature of LH2 that makes it harder, if that can be said universally.

[Antares]

Reasonable assessment?

No
1) Ares V is so out of bed on cost, nothing should be considered even remotely certain.

2) Composite and HTPB lose all semblance of human rated heritage.

[JosephB]

I originally was trying to find out how much extra performance an Ares I with a 5.5 segment booster would gain but found out that the higher alt. & heating on the casing makes reusability a big problem.

Which leads to a second question. Development cost & man rating aside, which would be cheaper to fly, a composite 5 seg HTPB booster or a liquid stage?

[Antares]

Any idea why ATK was not selected in the EELV competition back in 1996?  That might be the easiest place to find an answer.

[kevin-rf]

Okay heres one that has been rattling arround in my brain a little and could make an interesting discussion. We know solids give a very rough ride. I was wondering what liquid fuel gives the smoothest ride. (Asuming a properly designed POGO system).

I think I know the answer. LH/LOX because of how of it burns and the low molecular weight. But is that true?

Considering Kero/LOX seems to have the most violent combustion, I do not think it is it.
The first rockets used Alcohol/LOX because it was easy to work with. I wonder, lower ISP, but would the ride have been smoother?

Since Hypergols burn on contact do you get more complete combustion and a smoother burn?

I am sure there are other things we can throw into the mix, pressure fed verses turbo fed. How well the combustion chamber is designed.

[Jim]

I don't believe there is a difference in the ride due to propellants

[Patchouli]

5.5 seg SRB makes TO even worse.  According to one of Ross's posts, somewhere above 5.5 but less than 6 actually has worse performance for Ares 1 or 5 or both.


I'm not sure if it can blanket be said that a H2 engine is more expensive to design than kero.  However, H2 is certainly harder in operations.  You have to use helium purges and pressurant instead of N2 since N2 would freeze at LH2 temperatures.  H2 also freezes O2, so common bulkheads are much more difficult to design and build unless you're experienced at it (ahem).

As for the engine, since LH2 has such low density, it requires more pump stages in a turbopump.  That's hard by itself and really hard for a single shaft.

H2 burns cleaner, so combustion chamber design is easier.  Mostly I think it's just the temperature of LH2 that makes it harder, if that can be said universally.

I once read that the RL-10 would be a very cheap engine if it were mass produced due to it's simplicity.

You're probably right it's likely the handling of the fuel that makes it so costly.

Then there are nasty things like hydrogen embrittlement not sure how bad an issue this is for pad operations but maybe a shuttle or delta person fill that in.

[butters]

As a rocket tech newbie, I'm trying to get my head around the design trade-offs between LOX/LH2 and LOX/RP-1 (or some shorter-chain hydrocarbon) for the first stage of a heavy launch vehicle.

Obviously LH2 has a specific impulse advantage, greater cooling capacity, and cleaner combustion.  But everything else seems to favor RP-1: cost, density, pumping, plumbing, lubrication, insulation, tank pressure, boil-off, mass ratio, production, storage, logistics, etc.

What are the best arguments for why LH2 is less desirable for first stages than for upper stages?  Is there still a reason to use RP-1 first stages if LH2 is more appropriate for the upper stages?

How much room is there for specific impulse improvement, for example, with a LOX/LCH4 full flow staged combustion cycle engine with a chamber pressure of about 4000 psia?

[meiza]

Having worse ISP in the first stage makes the first stage bigger in wet weight. (It still might be lighter in dry weight, like Atlas is lighter than Delta.)
Having worse ISP in the second stage makes the second stage bigger in wet weight AND thus the first stage bigger in TOTAL (since it carries the wet second stage).

Now you can see that it doesn't matter that much if a first stage is a bit heavy when wet if it can compensate that with good thrust and low dry weight (which kerolox does). But if a second stage is heavy when wet, it must not only thrust more itself, but the first stage must deliver more impulse (more thrust, more fuel, ie a bigger stage). Having a good thrust or a good mass fraction don't help in the second stage as much as in the first stage, since the penalty of high wet weight is so drastic.

Hence it's smarter to spend more effort/money on wet mass optimization in the top part of the rocket than at the bottom. And hydrogen gives lower wet mass while kerosene gives lower dry mass.

[gospacex]

Having worse ISP in the first stage makes the first stage bigger in wet weight. (It still might be lighter in dry weight, like Atlas is lighter than Delta.)
Having worse ISP in the second stage makes the second stage bigger in wet weight AND thus the first stage bigger in TOTAL (since it carries the wet second stage).

Now you can see that it doesn't matter that much if a first stage is a bit heavy when wet if it can compensate that with good thrust and low dry weight (which kerolox does). But if a second stage is heavy when wet, it must not only thrust more itself, but the first stage must deliver more impulse (more thrust, more fuel, ie a bigger stage). Having a good thrust or a good mass fraction don't help in the second stage as much as in the first stage, since the penalty of high wet weight is so drastic.

Hence it's smarter to spend more effort/money on wet mass optimization in the top part of the rocket than at the bottom. And hydrogen gives lower wet mass while kerosene gives lower dry mass.

Another consideration is that not only dry mass of kerolox stage is smaller, but the stage is smaller itself. When you build a big rocket and/or you need to adapt an existing infrastructure, this becomes a factor too.

An example: VAB has limited dimensions. Biggest possible LH/LOX rocket that fits in the VAB is going to have lower performance that biggest possible kerolox one. (Assuming only 1st stage is kerolox)

[edkyle99]

As a real world example, the AtlasV provides better performance than DeltaIV. 

This isn't all due to kerosene, etc.  If Delta IV Medium had a Centaur second stage, it would nearly match Atlas V 401 performance to GTO.  The only limiting factor would be the liftoff thrust to weight ratio.  RS-68A will solve that problem. 

Delta IV has a heavy upper stage compared to Centaur, but it avoids the balloon tanks.

The real difference between these two launch vehicles is cost.  Delta IV is a bigger rocket.  It has to be to carry all of that low-density first stage propellant.  That means bigger ground processing and launch facilities, which means more dollars.  (Although Atlas has "smarter" launch facilities too.  If Delta used a mobile platform rather than a massive mobile service tower, it's facilities would probably have cost less than they did.)

 - Ed Kyle

[Antares]

I was wondering what liquid fuel gives the smoothest ride.

The difference is really only in near-pad acoustics and engine section acoustics.  It's very small compared to the difference between solid and liquid.

[JosephB]

Came across an interesting link in another thread...
http://www.californiaspaceauthority.org/html/press-releasesandletters/pr060718-1.html

I'm sure IPD is old news to veteran posters but for those on the learning curve it seems a fair amount of effort has been put into reusable engines just a few years back. RS-84 kerolox and then the IPD demonstrator (250,000 lb class).

Were these to be used in flyback first and/or second stages?
What applications did they have in mind?

EDIT: Oooops! Never mind. Read up on SLI history.
Yet another "late to the party" post.
In this case, by how many years?

[mmeijeri]

Is NASA Glenn still involved in spacecraft propulsion research or has that all been moved to MSFC?

[mmeijeri]

How are the code names for US rocket engines (RS-68, RS-84 etc) assigned? Is there some government agency that maintains these names or are they just chosen by their manufacturers??

[gin455res]

With some first stages having as many as 9 separate engines, is there any reason why you couldn't  have a mixture of two types of engines. Engines with different propellants, e.g. 6 merlins and 1 SSME, so that you could gradually throttle down the merlins to approximate the function of a true tri-propellant engine.

Has this idea ever been tried, and if not why not?

thanks   

[ugordan]

With some first stages having as many as 9 separate engines, is there any reason why you couldn't  have a mixture of two types of engines.

Energia used 4 kerosene boosters (Zenit) and a hydrogen core so that's that.

[gin455res]

With some first stages having as many as 9 separate engines, is there any reason why you couldn't  have a mixture of two types of engines.

Energia used 4 kerosene boosters (Zenit) and a hydrogen core so that's that.

I meant on a single stage together as a unit.