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Herr Doktor von Braun requests your technical advice

  1. Sep 26, 2004 #1

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    You are called into the office of Wernher von Braun on a fine summer day in 1940. He tells you he has been working with an engineering team on the early design phase for what will be “a very large sounding rocket, the A-4, which will be used for peaceful purposes to investigate the upper atmosphere. This rocket will be capable of carrying one metric ton of, um, some meteorological equipment known as a ’gonkulator,’ to an altitude of 90 kilometers.” The fuel tank will hold ethanol, and the oxidizer tank will hold liquid oxygen. The issue at hand is the placement of the propellant tanks inside the fuselage of the rocket. Von Braun tells you that the team looked at, and discarded the idea of making single cylinder with a vertical septum to separate the two liquids from one another. Instead, one cylindrical tank will be immediately above the other, which will in turn be immediately above the turbo pump, which in turn is above the thrust chamber. Once that decision was made, this idea was considered but ultimately discarded: run a pipe from the bottom of the upper tank down on the exterior of the fuselage to the position of the turbo pump inside the fuselage, where the pipe re-enters the fuselage to feed the pump. Instead, it has been decided that the liquid in the upper tank, in order to get to the pump, will pass through a lagged pipe running down through the lower tank close to its centerline. He asks you to determine whether the upper tank ought to be the one holding the oxidizer or the fuel.

    Here is a cutaway view of the pipe running through the lower tank:
    http://www.v2rocket.com/start/others/003-a-tank_017-b-delivery_pipe.jpg

    A view of region between the two tanks:
    http://www.v2rocket.com/start/others/004-ab_tank-010.jpg

    Von Braun tells you, “Your decision should balance the objectives of rocket performance, reliability, and the safety of the launch crew. It is intended that the rocket will be stored at the launch site with the ethanol fuel in its tank at ambient temperatures prevailing in the Fatherland--or perhaps temperatures prevailing along the coast of the North Sea, since we are anticipating that authorities might wish to launch our research rocket out over that body of water, in order to meet certain objectives that I am unfortunately not at liberty to discuss with you.“

    The liquid oxygen will be transferred to the oxidizer tank from a tank truck a few minutes prior to launch. The oxidizer tank will of course be vented at its top to allow the evaporating oxygen to escape so that the tank does not burst under pressure. The fuel tank (but not the oxidizer tank) will be kept under pressure during the rocket’s ascent by nitrogen gas, in order to assure consistent feed of fuel to the pump inlet. The turbo pump is powered by its own monopropellant, and is in that sense independent of the rocket engine’s propellant tanks. Some physical parameters that you might wish to make use of in deciding how to arrange the tanks are:

    Height of rocket: 46 feet
    Diameter: 5 feet 5 inches
    Thrust: 52,000 pounds
    Burn time: 65 sec
    LOX loaded onboard: 4900 kg
    Ethanol loaded onboard: 3710 kg
    Boiling point LOX: -297.3 deg. F
    Freezing point ethanol: -227 F
    Specific gravity LOX: 1.105
    Specific gravity ethanol: 0.789

    {Actually a 75/25 mixture of ethanol and water was used in the fuel tank, so the freezing point and specific gravity of the fuel were somewhat different than the numbers for pure ethanol given above.}

    I know the answer as to which way the German design team went, but I do not know their motivations for making the choice, and I am curious to see what ideas people here can come up with.
     
    Last edited: Sep 27, 2004
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  3. Sep 27, 2004 #2

    enigma

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    My guess would be to put the oxidizer tank below so you would have the center of mass lower. It would also be easier to load the liquid oxygen that way... you wouldn't need to pump it up 30 feet.
     
  4. Sep 27, 2004 #3

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    I figured I could count on enigma to respond to this topic! I included specific gravities of the two liquids for the reason that the choice of which way to place the tanks would affect the center of mass of the loaded rocket. With the rocket resting on the firing table, the rocket would be more stable against wind loads if the CG is lower, as it would be with the LOX tank below the alcohol tank. On the other hand, I know that a rocket flies more stably through the atmosphere if the CG is far above the center of pressure, and that would favor putting the LOX tank above the alcohol tank. Mitigating this is that the V-2 had both graphite vanes in the exhaust and rudders on the fins for steering, so that if it was close to neutral stability, it may not have been a problem as far as keeping the rocket on course.

    I have more to say, but I will wait a day or two in case others want to weigh in before I tell whether enigma "got it right."
     
  5. Sep 28, 2004 #4

    GENIERE

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    I also would place the LOX tank below but for a different reason. LOX flowing through the center of the fuel tank may freeze the surrounding ethanol or would require much space wasting insulation to prevent it. If the ethanol were pumped through the center of the LOX tank at a high velocity the insulation requirement would be much less. As far as safety, a failed relief valve would create a less spectacular explosion if the oxidizer was not piped through the fuel tank.
     
  6. Sep 28, 2004 #5

    enigma

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    With the rocket geometry of the A4 (or most to all other rocket geometries), you're not going to the center of mass behind the center of pressure. You need big, blunt leading edges (like Gemini or Mercury spacecraft) if you want that.
     
  7. Sep 28, 2004 #6

    BobG

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    I chose the opposite config for similar reasons. The rocket will sit motionless on the pad longer than it will be in motion with fuel flowing through the pipes. The fuel in the surrounding lower tank will have more of an effect on the fuel in the pipe - there's more mass to spread the temperature change across. The liquid oxygen has a higher density. It will take longer to raise it's temperature to match the surrounding fuel, reducing the insulation requirements for the pipe.
     
  8. Sep 28, 2004 #7

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    Hey, thanks to all of you for the feedback!

    The A-4, better known by its ‘vengeance weapon’ designation V-2, was the first really big rocket. Its designers gave credit to Goddard for pioneering the use of turbo pumps and gyros and such, but in terms of size and performance parameters, the V-2 dwarfed all of Goddard’s rockets. Captured V-2s were fired in the New Mexico desert by teams that included captured German engineers, and it arguably got the US started on the road to putting men on the moon.

    The Germans put the alcohol tank above the LOX tank, as the diagram at the bottom of this linked page shows:

    http://www.v2rocket.com/start/makeup/design.html

    My own thoughts, which mesh more or less with those expounded on by other posters, were that the LOX tank truck driver would not have to climb as high on the Meillerwagen steps to make a connection to the LOX filler tube if the LOX tank was below--as in fact it was. I am figuring LOX filling took place after the rocket was already standing upright in place on the firing table. It appears that the filler tube for the LOX, but not for the alcohol, was in fact placed down below the tanks near the combustion chamber, bearing in mind that A-stoff was the German code word for LOX, and the nose of the rocket would be to the left in this photo:

    http://www.v2rocket.com/start/others/007-016-a-stoff_filling_valve_pump.jpg

    I too had thought about potential alcohol freezing problems that could arise if the rocket sat long enough with the LOX aboard, if the pipe was full as the rocket sat there prior to starting the engine as BobG may be assuming. With the alcohol tank below the LOX tank, a sheath of frozen alcohol clinging to the LOX pipe might reduce the range of the rocket, since not all of the alcohol would wind up getting to the engine. On the other hand, with the LOX tank below the alcohol, the alcohol in the pipe might freeze solid and plug the pipe up such that the rocket would fail to ignite, or maybe a trickle of alcohol would get through the pipe and there would be some wimpy engine burn that would fail to get the rocket off the ground. But the second picture I linked to in my initial post calls out a valve between the tanks, so maybe the pipe contained only air until ignition, which I think is what Geniere is assuming.

    For the heck of it, I will provide one more link. It looks like this photo depicts the top of the upper (alcohol) tank, with a filler tube and a manhole hatch being visible:

    http://www.v2rocket.com/start/others/usafm016.jpg
     
    Last edited: Sep 28, 2004
  9. Oct 10, 2004 #8

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    "Burning Ring of Fire"

    If you go to the following site, you will find that the upper link (where it says “left click here”) shows several V2 launches:

    http://www.v2rocket.com/start/others/aud_vid.html

    The penultimate launch on the video shows a V2 clearly in trouble from the very start. A ring of flame can be seen at about the section of the rocket where the turbo pump was located. I am guessing that a fuel line broke under pressure or vibration and sprayed some of the fuel inside the rocket, though obviously there was enough flow remaining to the combustion chamber to get it off the ground. (A fully-functioning V2 had a thrust that was twice the loaded weight of the rocket, so even at half of the rated thrust it would launch itself.) Would the pressure have built up due to the pumping of fuel into the fuselage so as to pop some of the rivets and allow burning fuel to spew out of the side of the rocket? Within a few seconds the rocket has guidance problems. I imagine the controls going to the vanes and rudders at the rear of the rocket were burned up such that this V2 became unguided.
     
  10. Oct 10, 2004 #9

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    Additional engineering and historical questions

    Were the graphite steering vanes on the same shaft as rudders, or at least mechanically engaged together such that if one was at 17% of its deflection, so was the other? Did rudders get used for trajectory control on descent? Did the 28V battery supply power for moving those control surfaces? Or the compressed nitrogen? I believe from looking at pictures of the steam turbine that it powered only the propellant pumps, i.e. there was no takeoff shaft for powering other systems.

    Did vanes and rudders steer independently as four controlled units (i.e. a vane & rudder pair on one particular quadrant move as a single unit, independently of what the units are doing in the other three quadrants), or were they pair-wise linked so that only two signals were sent down to that part of the rocket from the guidance? To control the orientation of the rocket along its longitudinal axis would require four independent units, my intuition says. Was it important for the launch crew to set up the rocket on the stand so that a certain plane of the fuselage was oriented north-south? If they were off by two degrees, did the rocket fly two degrees off of the intended azimuth? (Maybe the answer to this changed late in the war when I think they started using radio signals to guide the rocket.)

    If the target was short range, did they load less propellant? Or fill the tanks, but cut off the propellant valves early? Or run fully-loaded tanks dry but go to higher apogee than they really needed to?

    Was the turbine for the turbopump on/off or throttled? Did it have a speed governor?

    Given von Braun’s later love of kerosene, was that fuel ever tried in the V-2? (I seem to remember reading that furfuryl alcohol, made from fermented potatoes, was used on occasion for V2s during the war.) Was the LOX made at facilities that were built just for V2s, or did they already exist for other applications?

    Were spotters used on the ground in the target area to send feedback to the designers as to accuracy? Multiple ones, triangulating to pinpoint impact? Were they considered spies if caught? Or did they rely on aerial reconnaissance by the Luftwaffe?
     
    Last edited: Oct 10, 2004
  11. Oct 18, 2004 #10

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    I saw an item in the paper last week saying that Maxime Faget died of cancer. He was the one who designed the shape of the Mercury spacecraft. Gemini, shapewise, was an upsized Mercury, though its internal systems were substantially different.
     
  12. Nov 27, 2004 #11

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    Some answers from an operator's guidebook

    In order to be able to answer some questions about the A-4 (or V-2, if you prefer), I ordered the A4 Fibel from the address given at the bottom of this website page:

    http://www.v2rocket.com/start/others/reference.html

    This is a reprint of the American A-4 manual used in the 1950s by the Redstone Arsenal, which is to say that it is an English translation of the manual given to German field troops starting in 1944. Its purpose is not to describe how the rocket actually works, but to guide the various crews in transporting the rocket and the fluids that it requires to the launch site, and to set up the rocket for launching, and to set up ground equipment for guiding the rocket. A percentage of the hand-drawn illustrations feature scantily clad German frauen. There is also a peppering of motivational sayings such as, A soccer player has to observe the other playing members of his group so that he will turn the ball over to them at the right moment. Otherwise--victory will be questionable.

    The five fluids, referenced by codename letters (A=liquid oxygen, B=alcohol, T=hydrogen peroxide, Z=potassium permanganate, P=compressed nitrogen at 200 atu) needed by the rocket were loaded after the rocket was erected onto the launch table. For any hard-drinking handlers of the rocket, there is this warning: B-substance is very poisonous. One shot glass will blind you, four shot glasses will kill you.

    Approximately one minute after firing, the fuel supply of the combustion unit will be cut off… Two methods may be employed for cut off. 1. Cut off from radio signal given from the ground. 2. Cut off via the installed impulse device in the missile itself. The firing is regulated by an earlier or later cut-off of the fuel supply. Propulsion is therefore cut off at a lower or higher velocity. Method 1 involved Doppler shifting, whereas Method 2 involved an electronic integrator attached to the gyro. There is no indication in the manual that the various tanks of liquid were filled to anything less than capacity for short-range flights.

    T-substance will flow through the 8-ton pressure valve and the 25-ton valve to the steam generator in such a quantity that the produced thrust of the A-4 will amount to approximately 8 tons, or as the case may be 25 tons. So it seems that the engine was throttleable only in the sense that at ignition the thrust was kept low enough that the rocket remained on the launch table, and only when the controllers decided that it was working properly did they raise the thrust suddenly to full in order to send it on its way.

    The direction in which the missile will fly is regulated by the turning of the launching platform. The steering operates in such a way that the A-4 flies in the direction of fin Number 1. The manual gives instructions to the “measuring troop” on how to survey such that the rocket is set up perfectly vertically and with the Number 1 fin pointing toward the target. There was also the option of setting up to control the azimuth from a ground radio installation: The guide beam transmitter is set up approximately 10 to 16 km behind the launching site. The guide beam transmitter affords a more accurate lateral guidance of the A-4.

    Current is needed in order to operate the radio and steering devices of the A-4... Your source of current in the A-4 is: 1. The 27-volt course gyro battery. 2. The 50-volt course gyro battery… KEEPING THE SERVO MOTORS WARM... At regular intervals turn on Switch (10), after 3 to 5 minutes turn it off again. Otherwise--the oil will become thick. So the rudders and vanes were operated by servomechanisms which meant their operation was electric rather pneumatic.

    The fins are numbered 1, 2, 3, 4 in counterclockwise order as seen from above the nose of the rocket. The air rudders 2 and 4 are called trim tabs. You cannot adjust them by hand. They are to even out constructive errors. The translation is quirky in places; I imagine they meant ‘construction errors.’ So here it sounds like only two rudders could be moved by the rocket guidance signals. However it also says, Check if the trim tabs move in the same direction that rudder No. 1 and 3 are standing. Then report: “Trim tabs move correctly.” If the trim tabs return to the zero position immediately, then report: “Trim steering clear.“ I am puzzled by this statement that the trim tabs do in fact move, when earlier the manual seemed to be saying they were fixed into place at the factory. Photos I have seen show fairings on all four fins, presumably enclosing the mechanism that operated the air rudders. That leads me to believe that all four air rudders could in fact be moved by signals. Regarding testing the jet rudders prior to launch: Check if both [jet] rudders move equally fast towards Fin No. 3 and then towards Fin No.1...Check if the rudders No. 1 and 3 move about the same from 0 to Fin No. 2 and then to Fin No. 4. So at least in this particular pre-launch test, the vanes moved pair wise, i.e. opposite vanes moved synchronously. That does not preclude the possibility that the guidance system of the rocket during its ascent could put out signals that moved all four jet vanes independently, though. Addressing the issue of whether air and jet rudders were coupled to one another, there is this statment in the manual: Sometimes the air and jet rudders operate together, sometimes separately. I am not sure what to make of this: For take-off jet controls No. 1, 2 and 3 must be fixed. Why three of the four? And in what sense were they fixed into place?

    Approximately one minute after firing, the fuel supply of the combustion unit will be cut off. After that the A-4 will fly like a normal shell that has left the gun barrel. Apparently after engine cut-off there was no further guidance.

    Finally, this photo shows a cutaway exhaust tube coming from the turbo pump. (Note that the unit is displayed upside down from how it would have been placed in the rocket.)
    http://www.v2rocket.com/start/others/v2engine111.jpg
    The copper coil must be a heat exchanger, but the website gave no explanation of its purpose. The manual says, Fetch the Z-substance container form the pre-heating device. Be sure that the container has been heated well. I gather that that the Z-substance (permanganate solution which catalyzed the concentrated peroxide [T-substance] into steam and oxygen) works best when warm, so I am guessing that the permanganate was what flowed through the tubing.
     
    Last edited: Nov 27, 2004
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