Am 31.07.2015 03:28, schrieb Sylvia Else:
>>> The thrust line has to pass through the craft's centre of gravity to
>>> avoid making the craft rotate. The direction of motion (which is
>>> ill-defined anyway) has nothing to do with it.
>>
>> Actually no!
>>
>> There are two components: inertia and gravity.
>>
>> To decelerate the craft, the thrust should point directly into the
>> direction, where the ships flies. (That's what I called 'direction of
>> motion'.) This is caused by inertia, what is not altered by the lower
>> gravity of Moon.
>>
>> To compensate acceleration caused by gravity, the engine should generate
>> a force in opposite direction (up), hence the thrust should point down.
>>
>> This makes two accelerations that combine to the needed thrust, (what
>> the crew had to direct properly).
>>
>> Any minuscule error would cause a rotation or other unwanted features of
>> the flight (like hitting the ground).
>>
>> TH
>
> What I said is true. To the extent that some particular thrust direction
> is required for other reasons, the crew had better ensure that the craft
> is oriented so that the thrust line goes through the centre of gravity.
No, it's not true:
gravity is the force, by what the Moon attracts the craft. This force is
pointing down - of course.
The centre of gravity of that spaceship is a certain point (within the
limits of this ship), where all gravitational forces balance out and we
could assume a single centre of gravity and a single vector acting upon
this point.
The other force is caused by deceleration. This is pointing in direction
of flight and has an own centre, where the forces add together like one
single vector acting upon the spaceship.
Only: these points do not need to be the same. This would only be the
case, if the craft would be a solid homogeneous piece of material.
This is clearly not the case, hence we had two different points for
centre of inertia (in direction of flight) and centre of gravity (vertical).
Also problematic: the craft is inhabited by astronauts, who could
possibly move. Also difficult to estimate is the influence of the
decreasing amount of fuel, while slowing down the ship.
You need to direct to thrust exactly opposite to the current sum of both
points. This is a little difficult, since the craft turns in vertical
direction, what has an effect upon this centre.
To actually control direction and strength of the thrust is way too
difficult to be done manually. But a relatively simple guidance system
could eventually do this. This would continuously measure the deviation
from the desired course and adjust the engine accordingly.
This would also enable to calculate the effect of corrections on the
place of landing, the landing velocity in vertical and horizontal
direction and the consumed fuel. From this a new course could be
calculated and feed into the guidance system, while still on descent.
Its quite a challenging task, especially if you try to land at a certain
spot, but is in the range of possibilities even with a cheap rasperry
pie and a moon-based equivalent to GPS.
TH