Re: Engineering Newsletter

[T.L.G.vanderLinden@student.utwente.nl (Timothy van der Linden)]

Timothy replies to Kelly:

>> I think that staging would not solve a more fundamental problem: Pressure
>> One can see the engine as a force pressing on the back of the ship. We want
>> a certain acceleration a. The heavier the ship gets, the bigger the force
>> that is needed to get that acceleration (F=m*a). But assuming the backside
>> of the ship stays about the same, the pressure (p=F/Area) gets bigger and
>> bigger. ---
>
>Do you mean the thrust loads?  If so we will need to take that into
>consideration.  But we can use some of the fuel or reaction mass as structure
>(ice perhaps?) and the structure needed to carry the load shouldn't be a
>major fraction of the fuel mass.

I'm not sure of what you mean with thrust loads.
What I meant was that the beams that connect the engine with the ship may
not be able to stand the load. It's like you were pulling a very heavy cart
with a sewing thread. For most metals the pull- or press-strength is about
the same and in the magnitude of 4E8 N/m^2

You have a solid beam of iron (7.8E3 kg/m^3) with sizes 10x10x5000 metres.
It would weigh 10*10*5000*7.8E3=3.9E9 kg
Now you want to accelerate is with 9.8 m/s^2 you do that by equally
excerting pressure at the 10x10 area.
That would mean a pressure of (3.9E9*9.8)/(10*10)=3.8E8 N/m^2
As you can see the pressure is almost to is maximum and any unperfection
will probably have caused it to snap earlier.

I thought of a way to overcome this problem though, just add some (probably
more than 10) extra engines. Then the load per engine could be less and so
could the stresses. These extra engines should NOT be placed at the back
(that would not change the problem) but along the side of it.


>>I don't agree with you on that (yet?), I will expect an answer on
>>my letter of 01-05 10:55.
>
>What letter?  The ones in my inbox don't have that time?

Maybe you have one of 6 hours earlier due to timezones, but I will send it
to you again.

>> >>      * Power from installations at Solar system
>> >
>> >Beamed power or fuel launchers have the advantage of offloading the need
>to
>> >carry the heavy fuel (and power systems) with the ship.  That improves the
>> >ships power to weight ration significantly.  But the systems are difficult
>to
>> >do, limit range, and don't seem to help us to slow down.
>> 
>> Also they have a not so good efficiency. (A big part of the beam just flies
>> along the ship without being used.)
>
>Thats true of beamed power, not launched fuel.

The advantage of a lower ship:fuel ratio is that it needs less energy. If
one decides to launch that fuel instead of adding it to the ship from the
start, there is not much gain: Although the weight of launched fuel is less,
there is a large extra amount of energy needed to launch that fuel.
It follows from calculations that the total gain is not so big as you would
have thought. But what you have added is a complicated launch method.

Timothy