Re: question on hyrdogen launchers

In article <1136471022.754785.292100@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>,
Mike Lorrey <mlorrey@xxxxxxxxx> wrote:
>> No, you could not. Only very small tanks, with minimum-gauge walls, scale
>> by the square rule. Larger tanks, including most rocket fuel tanks, scale
>> by the cube rule - or more precisely, as tank volume times tank pressure.
>> Very large tanks, like your LH2 aircraft carrier, scale by a fourth-power
>> rule, as the internal pressure starts to be dominated by the weight of
>> the propellant.
>
>What, you are counting the mass and pressure twice each?

Nope. The mass of pressure-vessel walls scales with the volume enclosed,
not with the surface area, because a bigger vessel needs thicker walls to
contain the same pressure. (Consider... Slice the tank in half. The
area of fluid supplying pressure to push the halves apart scales with the
square of size, so the wall cross-section must scale with the square too.
But the wall length scales only linearly with size, so wall thickness must
be scaled with size too.) So you start out with a cube rule.

And if hydrostatic pressure is the dominant pressure in the tank, then the
pressure scales with depth, which scales linearly with size. Hence the
fourth-power rule for big tanks.

>If the volume
>is so large that the propellant weight is the prime determinant of
>internal pressure, then you don't need to pressurise it other than to
>keep the empty volume taut, and the rocket is automatically a pressure
>fed system.

This is a disadvantage, not an advantage. Pressure-fed systems have heavy
tanks. The fact that there isn't any significant pressure on top of the
liquid doesn't change the problem noticeably; assuming the tank is more
or less full at launch, the average pressure is half the pressure at the
bottom.

>...Why not call it a fifth power, since you need to
>take into account the mass times whatever G load you are putting on it?

G-load doesn't scale with size. In fact, bigger launchers have some
tendency to use lower accelerations, to keep structural loads down.
--
spsystems.net is temporarily off the air; | Henry Spencer
mail to henry at zoo.utoronto.ca instead. | henry@xxxxxxxxxxxxx
.

Relevant Pages

  • Re: question on hyrdogen launchers
    ... >>No. Tank mass scales by square rule, while contents mass scales by cube ... Only very small tanks, with minimum-gauge walls, scale ... > by the cube rule - or more precisely, as tank volume times tank pressure. ...
    (sci.space.policy)
  • Re: Shuttle explosion risk?
    ... NASA taking precautions in fear tanks could burst ... Center's launch pad 39A next week after NASA finishes pressurizing ... The spherical tanks provide pressure needed to push rocket propellants ...
    (sci.space.shuttle)
  • Re: Minimum development cost space transport
    ... > person SSTO. ... > low chamber pressure, a pressurized design gets ... A pressure fed design ... there are still methods possible for building small scale ...
    (sci.space.policy)
  • Re: Electric Motorglider Flies
    ... Of course gasoline also requires tanks, but they are often just sealed ... appear that carbon fiber or Kevlar composite pressure cylinders may be ... tensile strength, up to the point where it cracks open ("Hydrogen ...
    (rec.aviation.piloting)
  • ARM: Review - Armorscale 1/35 scale Armor Accessories
    ... Armorscale 1/35 Scale Armor Accessories ... : price US $8.00 ... Second is a pair of the rear fender polygonal fuel tanks from the late ... offers two exquisitely molded tanks in resin - the seams are ...
    (rec.models.scale)
Loading