Author Topic: Parallel Staging  (Read 59936 times)

Offline DonPMitchell

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Parallel Staging
« on: June 02, 2009, 10:24:47 AM »
I learned a lot from reading the 1951 Soviet study on rocket staging, done at the Steklov Mathematical Institute.  It was the basis for the design of the R-7, but it thuroughly considered the general problem of extending rocket range by discarding pieces of its structure during flight:

If you launch three identical rockets on the same trajectory, they should go the same distance in the same time.  If you bolt them together and launch them, you get the same result, no increase in range.  So how can you gain anything from putting rocket stages in paralle?




The simplest case is what the R-7 did, and what the Delta IV heavy does.  Make the boosters lighter, but give them the same engine thrust.  This means they contribute more to the acceleration, because they have a higher a = F/m thrust to mass ratio.  The Russians called this design the "carrying packet".

But Tikhonravov had an even better idea in the 1940s.  Imagine that when the Delta IV takes off, all three engines are fed from just the tanks in the two outer boosters.  When they run out of fuel, they are ejectged, and the central stage continues, using its own completey full fuel tank.  They called this design the "feeding packet", but to the best of my knowledge, nobody has ever implemented it.

YOu can argue that a feeding packet design is more efficient than sequential stages, because the second stage engine is working all the time, instead of being dead weight during the first-stage burn.

To optimize these concepts, the ratio of masses of the stages can be optimized, using three identical stages is not the best, it is better if the central "second stage" is smaller.
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Offline jdbenner

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Re: Parallel Staging
« Reply #1 on: June 02, 2009, 11:34:06 AM »
Three Identical Rockets allows better economies of scale, since more parts are of common design.  This could provide a cost savings, regardless of the sub optimized design.

Don, While it is definitely a thrust advantage, for the upper stage engine to be providing constant thrust, the upper stage engine is optimized for vacuum operation.  In sequential  staging the rocket engines do not have to survive as long, and they are optimized for their altitude of operation.

 P.S.  How would four Identical stages work?  The central core would be surrounded by three others.

« Last Edit: June 02, 2009, 03:55:25 PM by jdbenner »
Joshua D. Benner Associate in Arts and Sciences in General Science

Offline DonPMitchell

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Re: Parallel Staging
« Reply #2 on: June 02, 2009, 07:21:27 PM »
The work at Steklov Institute was very quantitative, using their best phenomenological models for spacecraft weigth, thickness of tanks and hulls versus acceleration load, etc.  I think one would have to look at it quantitatively to see if the issue of altitude optimized engines outweight the benefits of doing a "feeding packet" design.

The think the real problem with feeding packet design is just the engineering complexity of piping and switching between fuel tanks.  In the 1950s, another important factor that lead to the parallel staging was that nobody had ever ignited a rocket stage in a vacuum, and they didn't want to tackle that problem yet.
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Offline jdbenner

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Re: Parallel Staging
« Reply #3 on: June 02, 2009, 08:08:47 PM »
  In the 1950s, another important factor that lead to the parallel staging was that nobody had ever ignited a rocket stage in a vacuum, and they didn't want to tackle that problem yet.

Well, that is a valid concern.
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Offline ijuin

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Re: Parallel Staging
« Reply #4 on: June 02, 2009, 10:29:27 PM »
Yes, this is why the R-7 had the strap-on boosters and why the Atlas dropped two of its three engines in-flight as opposed to a two-stage design. It wasn't just the ignition-in-vacuum problem either--there's also the ullage problem. In simplest terms, when the first stage engine shuts down and the first stage separates, the upper stage is in weightless freefall, which means that the fuel can "float" around inside the tanks, which would result in the turbopumps gulping vapor, which would cause them to exceed their maximum safe speed and fail (to say nothing of the lack of fuel getting to the combustion chamber). The ullage problem is usually addressed by having small solid or pressure-fed rocket motors that provide a small amount of thrust for a few seconds in order to give the fuel enough "weight" to settle in its tanks.

Offline DonPMitchell

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Re: Parallel Staging
« Reply #5 on: June 02, 2009, 11:17:28 PM »
In sequential stages (like the Luna and Molnya stages added on top of the R-7), the Soviets solved the ullage problem by igniting the upper stage while the lower stage was still burning.  That's why you see the open cage between the stages.

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Offline Bob B.

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Re: Parallel Staging
« Reply #6 on: June 03, 2009, 10:09:50 AM »
In sequential stages (like the Luna and Molnya stages added on top of the R-7), the Soviets solved the ullage problem by igniting the upper stage while the lower stage was still burning.  That's why you see the open cage between the stages.

The American Titan used "hot separation" as well.  Although they didn't use an open cage, the interstage adapter included vent holes to allow the exhaust gas to escape.

Offline jdbenner

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Re: Parallel Staging
« Reply #7 on: June 03, 2009, 03:46:29 PM »
In theory the stage could be spun to solve the ullage problem.  The centripetal acceleration would keep the propellant near the walls.

In a pressure fed engine, opening the propellant valves to the engine, would settle the propellant, by a combination of cold gas rocket thrust, and  gas flow in the tank.   
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Offline DonPMitchell

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Re: Parallel Staging
« Reply #8 on: June 04, 2009, 03:53:17 PM »
In 1937, Leningrad GIRD designed a rocket that used centrifugal force to pump fuel to a pair of widely separated engines.
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Offline jdbenner

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Re: Parallel Staging
« Reply #9 on: June 04, 2009, 07:48:41 PM »
That sounds kind of like the roton.

http://www.scaled.com/projects/roton.html
Joshua D. Benner Associate in Arts and Sciences in General Science

Offline ijuin

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Re: Parallel Staging
« Reply #10 on: June 04, 2009, 09:54:36 PM »
In theory the stage could be spun to solve the ullage problem.  The centripetal acceleration would keep the propellant near the walls.

In a pressure fed engine, opening the propellant valves to the engine, would settle the propellant, by a combination of cold gas rocket thrust, and  gas flow in the tank.   


Either of those would work for a small to mid-sized stage (probably up to Centaur-sized) on an unmanned rocket, especially if the stage above it is a solid motor that would benefit from being spin-stabilized anyway (as with GEO satellites and many interplanetary probes).

Offline DonPMitchell

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Re: Parallel Staging
« Reply #11 on: June 05, 2009, 12:28:36 AM »
Pressure feed requires a lot more mass than pump feed, as rockets get bigger.  The volume, mass and engine thrust of a rocket scale up as the cube of the rocket length.  The mass of the fuel tank scales as the volume times pressure, and I can make a hand waving argument that pressure would have to increase as the 3/2 power -- flow must increase as the cube, but cross section of the fuel lines scales as the square.

Here's a real engineering study on how pressure feed increases: http://www.flometrics.com/rockets/rocket_pump/pfcalcs.pdf

For small rockets, pumps are a bad idea, as Goddard found.  He wasted years trying to build a fuel pump for the first liquid fuel rocket, and then at the last minute just used pressure feeding.  For big rockets, like the V-2, engineers saw right away that pressure feed wouldn't work.

Goddard made a similar argument against solid fuel rockets -- where the entire rocket body must withstand the combustion chamber pressure.  The US and ESA still have build very big solid boosters, but the Russians stuck with pump fed liquid fuel rockets -- the Zenit rocket basically played the role that the solikd boosters do in our shuttle.
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Offline jdbenner

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Re: Parallel Staging
« Reply #12 on: June 05, 2009, 05:24:13 PM »
Pressure feed could still work on large rockets, but the mass fraction per stage, would go down.  Given the fact that propellant is only a small fraction of the launch cost, pressure feed could actually save money.  On the other hand if cheap pumps are available, pump fed rockets are a very good idea.

 http://www.astronautix.com/lvs/searagon.htm
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Offline DonPMitchell

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Re: Parallel Staging
« Reply #13 on: June 05, 2009, 09:27:02 PM »
The thing that stands out in Sea Dragon is the idea of a single engine producing 36,000 tons of thrust.  That is way beyond anything people know how to build.  Could it be cooled?  Could it achieve combustion stability?
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Offline jdbenner

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Re: Parallel Staging
« Reply #14 on: June 05, 2009, 09:36:08 PM »
Cooling would be easy, since the surface aria to volume ratio goes down as the engine grows.

I do not know about combustion stability.  That engine is obviously much larger than any constructed.
Joshua D. Benner Associate in Arts and Sciences in General Science