Re: "Rockets not carrying fuel" for orbital transfer.

From: Robert Clark (rgregoryclark_at_yahoo.com)
Date: 11/21/04 

Date: 20 Nov 2004 17:04:01 -0800

"George Dishman" <george.dishman@clara.co.uk> wrote in message news:<1100550828.8114.0@damia.uk.clara.net>...
> "Robert Clark" <rgregoryclark@yahoo.com> wrote in message
> news:832ea96d.0411141957.1a521553@posting.google.com...
> >...
> >
> > I came up with two other ideas for reducing the weight of the fluid
> > that had to be supported by the rocket as the tube trails behind the
> > rocket.
> > Firstly, I wanted to investigate both the possibilities of using
> > gaseous hydrogen or liquid hydrogen for the fluid carried by the tube.
>
> That drops the density so you need much higher
> speeds for the same flow rate so makes everything
> more difficult.
>
>
> > However, the liquid hydrogen scenario just gave too much weight. But
> > suppose the rocket didn't have to provide the propulsion for the fluid
> > in the tube? This is what I envision:
>
> <Snip pictures>
>
> All you have done is use a compound engine. The
> same mass is being accelerated to the same speed
> so will need the same fuel. You are forgetting
> the engines not only lift the craft but also the
> fuel needed to lift themselves. In fact with more
> engines, you have greatly increased the mass and
> the fuel needed, and all these schemes create a
> huge drag with air friction on the tube which also
> needs more fuel.
>
> Instead, imagine using a nearly rigid pipe as the
> arm of a trebuchet to pump fuel only over the first
> few seconds. That might be practical though the
> risks during disconnection are significant.
>
> George

 I'm also investigating the possibility of using a rigid structure to
reach into LEO. However, I think the efficiency of the tube method is
better than you suggest.
 Let's go back to the case of launch from Earth to LEO. I'm still
considering here that you're not using engines to combust fuel but are
only conducting a high pressure fluid up the tube to provide
propulsion. So the weight of the exhaust ports is quite small, not
that of a full blown engine.
 Let's estimate the the size of these exhaust ports.

          ^
          |
          |
Towards the rocket.

       | |
       | |<----Fluid carrying tube.
       | |
       | |
       | |
       |___ ___|
       /__ | | __\
      // || || \\
     //| |\\
    // | | \\
       | |
       | |
       | |
       | |
       | |
       | |
       |___ ___|
       /__ | | __\
      // || || \\
     //| |\\<---Exhaust ports directed aft.
    // | | \\
       | |
       | |
       | |
       | |
       | |
       | |
       |___ ___|
       /__ | | __\
      // || || \\
     //| |\\
    // | | \\
       | |
       | |
       | |

 Let's say you put a pair of these ports every 100 meters. Then each
pair of ports would only have to provide the thrust to support the
weight of 100 meters of the tube and fluid. Let's use liquid hydrogen
now. Its density is 71 kg/m^3. The volume of a 100 m tube, .3m wide is
Pi*(.15)^2*100 = 7.07 m^3. So the mass is 71 kg/m^3 times this or
about 502 kg, 1104 lbs.
 We're still using the presumption that we can communicate, say, a
pressure like the 6400 psi pressure produced by the shuttle liquid
hydrogen turbopumps up the tube. (Whatever type of pumps we use would
be located on the ground not the rocket so can be quite large.) Now we
want two exhaust ports to support 1104 lbs., or 552 lbs. each. So 552
lbs = (pressure)*(square area of ports) = 6400 * Pi * (1/4)*(diameter
of ports)^2 . We get a diameter of .33 in or 8 millimeters. Actually
they might even be smaller than this by using convergent-divergent
type nozzles used with rockets.
 Now remember the entire tubes weight is supported by these exhaust
ports so the great majority of the fluid that reaches the rocket will
be driving only the payload and rocket. For a .3m = 12in wide tube
this could be a thrust of 6400 * Pi * 6^2 = 723,824 lbs. that is
solely used to loft the payload and (engineless) rocket, and again we
can probably do better than this using the nozzles normally used on
rockets.
 Note that we can get even more thrust from the exhaust ports by
making them wider or by using more than 2 at each level. This is
important since we can also solve the hypersonic drag problem. These
exhaust ports are not engines but it would be a simple (and light
weight matter) to give them directional ability. Then you could have
them automatically direct their thrust to counteract the drag caused
by each portion of the tube.

    Bob Clark

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