Told You So

guest blogger john hare

Posting here has been a privilege with the quality of people responding. You all deserve a chance to say I told you so on my minor hardware attempt. Make your best guess at what happens for the privilege of saying it later. Anybody that always says nothing before, and then says they could have told me about that problem later, don’t bother this time.

You know I have an interest in pumps in general and turbopumps in chamber in particular. Yesterday I left the drawings and deposit with the machine shop for the rotating portion of a cold flow test rig.

agua pump

The intent is to use shop air at 125 or so psi to pump water into the test chamber. The water will be in an unpressurized barrel with initially 4 feet of head pressure, or less than 2 psi for initial feed. The water enters through the bearing shaft and exits through the holes in the turbine blades that serve as pump passages. The turbine disk is 2 inches diameter with the blades 3/8 inch long. Air and water exhaust through a single port under the turbine blades. The compressed air enters through a diffuser where the injectors would be in a real engine. The diffuser is to simulate the chaotic flow in an engine without turbine nozzles.

If everything works exactly right, I should get a lot of water introduced to the chamber at 50-75psi. We know that it won’t work right. Your predictions as to where it goes wrong should be good for a little fun. My prediction is that I get all the parts together, and then see a gotcha before hooking up the water.

If the unit spins and produces anything at all, I intend to try various numbers of ports and turbine nozzle arrangements to see what happens. I might be able to build a compressed air/kerosene hot fire unit in a year or three if this kludge works. A real rocket ain’t happening on my budget, skill level, and general cowardice towards dangerous things I have no experience with.

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I do construction for a living and aerospace as an occasional hobby. I am an inventor and a bit of an entrepreneur. I've been self employed since the 1980s and working in concrete since the 1970s. When I grow up, I want to work with rockets and spacecraft. I did a stupid rocket trick a few decades back and decided not to try another hot fire without adult supervision. Haven't located much of that as we are all big kids when working with our passions.

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13 Responses to Told You So

  1. Habitat Hermit says:

    You had me worried you were leaving or something with that first sentence! Hopefully not going to happen.

    I can’t see any reason why this pump design shouldn’t work but I’m having difficulties figuring out if the shape/limitation of the exhaust port could create trouble/strangeness compared to having a circularly uniform/annular exhaust port. I kind of suspect it will however that can be fairly easily remedied if it should be the case (recalculate the exhaust area into a “filled circumference” with the same total area and roughly centered under the turbine blades, and add some extra structural support needed for the casing to bridge the gap some distance away but that should be all). Could be interesting to do that even if there is no trouble just to compare.

  2. Habitat Hermit says:

    All that is assuming your calculation of the exhaust port size is correct in order to keep chamber pressure low enough that the liquid is allowed to spin out of the blades at the start and other stuff like that, or alternatively that the air pressure isn’t 125 psi from the get-go. Could be your future involves a lot of fiddling around ^_^

  3. MG says:

    I think I am not visualizing this correctly.

    125 psi air pushes water through hollow turbine blades. The water transfers momentum into the turbine (similarly to some lawn sprinklers operate). Some other high pressure air source pushes air through a diffuser, so that the “chamber pressure” is below 125 psi. A single (is that correct) outlet allows air and water to exhaust, which turns the turbine.

    From the illustration, it appears that your turbine blades will absorb momentum from the exhaust through a small rotational angle. Is that your intent?

  4. johnhare john hare says:

    The 125 psi air is diffused through the top and powers the turbine on the way out the exhaust port. The water is pumped through the ports in the turbine blades into the chamber. The only thing that will force the water into the chamber is the centrifugal pumping action. The back pressure in the chamber will define the success or failure of the pumping action. As HH suggests, if the air is exhausting back through the water port, it didn’t work.

  5. My prediction is that you will not get nearly as good of performance out of your turbine blades as what you are expecting; at least, not with this configuration. The pump will work, more or less, as you expect, for the a given amount of RPMs that you are able to induce (either through the turbine blades or other means).

    As I have mentioned before, I have no turbo-machinery experience, but I can see a couple of potential problems with this design just by looking at it.

    First of all, by your own numbers you are attempting to inject water at a lower pressure (50-75 psi) than that of the chamber (125 psi). Unless you mean 50-75 psi above chamber pressure, I think you may have a problem coaxing the water out of the pump.

    Secondly, as Habitat Hermit noted, you are severely restricting the amount of useful surface area where momentum may be imparted to the blades. Also, I think your high-pressure air will escape around the ends of the blades just as much as through them.

    You may need to design a separate chamber for the turbo-pump that will allow you to isolate the flow paths of your shop air and your injected water. Perhaps something like this may work: (If you can’t make this out, try copy/pasting it into a fixed width text editor, like notepad.)

    ||^ ^||
    || __ _____ __ ||
    ||^|__| V |_____| V |__|^||
    || ||
    ||____| V _|^|_ V |____||
    |_____| |_| |_| |_____|
    V |^| V

    The trick here is to prevent, as much as possible, the leakage of the high-pressure gasses from the main chamber into the pump injection chamber through the turbine. This may require the use of a solid ring around the outer edge of the turbine blade (with injector holes appropriately located). For the outlet of the main chamber to the turbine blades, expose as much of the turbine’s surface area as possible.

  6. MG says:

    Ah. The sentence about 125 psi air pumping water into the test chamber confused me.

    You will get *some* pumping action. I have to go to the NewSpace 2009 opening talk, but the relevant hurdle is whether you will get enough power out of the turbine to elevate the pressure of the water at the blade exit above the pressure of the air at the blade exit.

    BTW, do you mind my asking which machine shop, and how much they charge for machinining the turbine?

  7. johnhare john hare says:

    The machine shop is Terry Lesnet in Lakeland Fl. Older equipment and no CNC or CADCAM. We’ve done business off and on for years. It will be a under a grand for everything. He said he would treat me right on the bill, and always has. The brighter side of cost plus.

  8. MG says:

    Ok, before I get a few hours sleep, here is a quick and dirty energy analysis:

    Delta P = rho v^2 / 2g [Bernoulli, applied to water in turbine blade]

    Set Delta P to 4 bar ~ 60 psi ~ 400kPa

    rho = 1000 kg/m3
    g = 9.817 m/s2
    v~88.6 m/s

    v = omega *r; r~.025m

    Omega = 3544 radians / s = 33900 RPM

    Which seems a bit high.

  9. johnhare john hare says:

    I expect to moderate the chamber air pressure to various values below the 125psi available. The actual pressure observed in the chamber while it is pumping water into the chamber will give the reading on pump delivered pressure.

    33,900 rpm is fairly low compared to some of the RC jets out there. I think they use real bearings and not a bushing oiled before each test.

    This rig will be cheap enough to play with that I expect to do a lot of fiddling (HH) in the manner suggested (Eric) to get any useful information. I think getting multiple “I told you so” is in my future.

  10. Ack.. I forget how much the blogger reply window mangles plain text. None of the spacing was kept in my ASCII art drawing. Let’s try this:

    ||^ ^||
    || __ _____ __ ||
    ||^|__| V |_____| V |__|^||
    || ||
    ||____| V _|^|_ V |____||
    |_____| |_| |_| |_____|
    V |^| V

    If this doesn’t look good either, then feel free to delete this comment.

  11. Well sorry about that failed second attempt. For what it’s worth, I’ve been able to post the diagram over at my blog (Turbo pump ASCII artSpaceflight Sandbox).

  12. johnhare john hare says:

    Kewl. After my initial attempt fails I can use a plastic shrink ring around the turbine blades to get those results.

    Of course I am going to blast straight shop air through a nozzle one time also.

    I think I am going to get fair entertainment value for my money.

  13. johnhare john hare says:

    Fiddling around has started before the parts left the machine shop. Terry spun the turbine up to (Terrys’ estimate) 30,000-60,000 rpms dry with shop air. I increased exhaust ports from 1 to 6 spaced under the turbine perimeter. Based on the air required to do this, I changed the air feed to the 1″ used on the jackhammer air compressors rather than shop air.

    The rpm estimate is based on his experience that I have no intuitive feel for, so I’m a bit of a sceptic. If it actually reaches those rpms in loaded use, it corresponds to pressures of 300-1,200 psi, given reasonable pressure recovery. I’m starting to think it possible that the unit will actually push water up to over 75 psi. If it does, then it seems that the air mass will be less than the water mass being pumped.

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