Was watching a documentary of when they discovered a monolith on the
moon in the late 1990s.

I a "moon bus" that travels at low altitude to span great distances
possible?

is the moon's gravity low enough that the fuel cost of "floating" above
surface would not be that great? (and allow greater horizontal speed
and greater comfort than if traveling on wheels on ground)

or would the only way to accomplish this to inrease horizontal velocity
to near orbital speed at which point you're looking at cost of
accelerating and decelerationg which would make travel between 2 points
on Moon too expensive?

On May/30/2021 at 12:34, JF Mezei wrote :
> Was watching a documentary of when they discovered a monolith on the
> moon in the late 1990s.
>
> I a "moon bus" that travels at low altitude to span great distances
> possible?

I saw that documentary :-) a long time ago. But I don't remember the
moon bus.

> is the moon's gravity low enough that the fuel cost of "floating" above
> surface would not be that great? (and allow greater horizontal speed
> and greater comfort than if traveling on wheels on ground)
>
> or would the only way to accomplish this to inrease horizontal velocity
> to near orbital speed at which point you're looking at cost of
> accelerating and decelerationg which would make travel between 2 points
> on Moon too expensive?

For long distances I think that going suborbital would be more practical
and probably even cheaper energy-wise. For shorter distances, the old
buggy on wheels would be better.

I don't think that for any distance it would make sense to build a ship
that would use rocket power to provide lift while moving horizontally.
Even if you only want to go a few kilometres away and what you have is a
rocket ship, not a wheeled vehicle, you would basically do a very short
suborbital flight instead staying close to the ground with the rocket
keeping you up.

Alain Fournier

On 31-May-21 2:34 am, JF Mezei wrote:
> Was watching a documentary of when they discovered a monolith on the
> moon in the late 1990s.
>
> I a "moon bus" that travels at low altitude to span great distances
> possible?
>
> is the moon's gravity low enough that the fuel cost of "floating" above
> surface would not be that great? (and allow greater horizontal speed
> and greater comfort than if traveling on wheels on ground)
>
> or would the only way to accomplish this to inrease horizontal velocity
> to near orbital speed at which point you're looking at cost of
> accelerating and decelerationg which would make travel between 2 points
> on Moon too expensive?
>

The moon bus never made sense to me, with suborbital transits seeming
more practical.

Sylvia.

On 2021-05-30 20:06, Sylvia Else wrote:

> The moon bus never made sense to me, with suborbital transits seeming
> more practical.

If gravity becomes low enough, is there a point where small vertical
trusters to maintain low altitude over ground become feasable?

If you are doing a survey or search and rescue, you don't want to be
going too fast or too high.

I take it moon gravity at 1/6s of earth is still too much for such
horizontal travel above ground ?

On 31-May-21 1:30 pm, JF Mezei wrote:
> On 2021-05-30 20:06, Sylvia Else wrote:
>
>> The moon bus never made sense to me, with suborbital transits seeming
>> more practical.
>
> If gravity becomes low enough, is there a point where small vertical
> trusters to maintain low altitude over ground become feasable?

If it was so low that ion thrusters were sufficient, then I suppose.
Otherwise I don't see it happening.

>
> If you are doing a survey or search and rescue, you don't want to be
> going too fast or too high.
>
> I take it moon gravity at 1/6s of earth is still too much for such
> horizontal travel above ground ?
>

Consider that the rockets supporting the moon-bus have to provide thrust
that would, in free space, impart an acceleration of 1/6g.

So in free space, the delta-v would be

0.5 * g/6 * t^2

where t is the duration in seconds.

Applying the Tsiolkovsky rocket equation to this gives

0.5 * g/6 * t^2 = Isp * g * ln(m0/mf)

where Isp is the specific impulse, m0 is the initial mass (full
fuelled), and mf is final mass (all fuel gone).

Rearranging, and cancelling the g's gives,

ln(m0/mf) = 0.5 * 1/6 * t^2/ Isp.

Pull a reasonably typical Isp of 300 out of the air, and try for a 30
minute (1800 second) mission, gives
ln(m0/mf) = 0.5 * 1/6 * 1800^2 / 300

Or about 7. That is, six sevenths of the vehicle's initial mass is fuel.
That's not unfeasible as far as the vehicle is concerned, but it does
represent an awful lot of fuel that has to be shipped to the moon, or
manufactured there, just to run moon-bus missions.

Sylvia.

In article <LLYsI.7050$EW.2711@fx04.iad>, jfmezei.spamnot@vaxination.ca
says...
>
> On 2021-05-30 20:06, Sylvia Else wrote:
>
> > The moon bus never made sense to me, with suborbital transits seeming
> > more practical.
>
> If gravity becomes low enough, is there a point where small vertical
> trusters to maintain low altitude over ground become feasable?

No. If you are optimizing the trajectory to be the most efficient,
suborbital is always most efficient.
> If you are doing a survey or search and rescue, you don't want to be
> going too fast or too high.

If that is a hard requirement, then you'd accept the inefficiency. But
that will limit the duration of the trajectory, so the scientists would
have to accept that. The reality is you'd end up with both sides
compromising.
> I take it moon gravity at 1/6s of earth is still too much for such
> horizontal travel above ground ?

It has less to do with the level of gravity than it does with trajectory
optimization.

Jeff
--
All opinions posted by me on Usenet News are mine, and mine alone.
These posts do not reflect the opinions of my family, friends,
employer, or any organization that I am a member of.

On 2021-05-31 00:46, Sylvia Else wrote:

> Or about 7. That is, six sevenths of the vehicle's initial mass is fuel.

Thanks for the math reality check. Just about every science fiction
series from UFO , Space 1999 (which was actually an unofficial sequel -
the plans for space station Alpha were shown on paper on last episode of
UFO) and 2001 all had vehicle capable of traveling at low altitude over
the moon and was wondering how feasable this would be considering the
low gravity.

But despite that low gravity, your math showed "not even close".

But i have to wonder: if to travel from US moon base to the Russian or
Chinese moon base, you use suborbital trajectory, wouldn't that also use
up a lot of fuel because you have to accelerare upwards a lot and then
you have to use fuel to slow down just as much as you land at destination.

If you "hover", you may need a lot of fuel to hover, but when you get to
destination, you only need to stop your horizontal speed and gently drop
on ground.

On 01-Jun-21 3:39 am, JF Mezei wrote:
> On 2021-05-31 00:46, Sylvia Else wrote:
>
>> Or about 7. That is, six sevenths of the vehicle's initial mass is fuel.

Unfortunately, I made a really stupid mistake. I'll come back to it.

>
> Thanks for the math reality check. Just about every science fiction
> series from UFO , Space 1999 (which was actually an unofficial sequel -
> the plans for space station Alpha were shown on paper on last episode of
> UFO) and 2001 all had vehicle capable of traveling at low altitude over
> the moon and was wondering how feasable this would be considering the
> low gravity.
>
> But despite that low gravity, your math showed "not even close".
>
>
> But i have to wonder: if to travel from US moon base to the Russian or
> Chinese moon base, you use suborbital trajectory, wouldn't that also use
> up a lot of fuel because you have to accelerare upwards a lot and then
> you have to use fuel to slow down just as much as you land at destination.
>
> If you "hover", you may need a lot of fuel to hover, but when you get to
> destination, you only need to stop your horizontal speed and gently drop
> on ground.
>
>

On 31-May-21 2:46 pm, Sylvia Else wrote:
> On 31-May-21 1:30 pm, JF Mezei wrote:
>> On 2021-05-30 20:06, Sylvia Else wrote:
>>
>>> The moon bus never made sense to me, with suborbital transits seeming
>>> more practical.
>>
>> If gravity becomes low enough, is there a point where small vertical
>> trusters to maintain low altitude over ground become feasable?
>
> If it was so low that ion thrusters were sufficient, then I suppose.
> Otherwise I don't see it happening.
>
>>
>> If you are doing a survey or search and rescue, you don't want to be
>> going too fast or too high.
>>
>> I take it moon gravity at 1/6s of earth is still too much for such
>> horizontal travel above ground ?
>>
>

OK, let's try that again, this time, shaking out the aging-brain
cobwebs, and using the equation for change in velocity rather than
distance travelled.

> Consider that the rockets supporting the moon-bus have to provide thrust
> that would, in free space, impart an acceleration of 1/6g.
>
> So in free space, the delta-v would be
>
>     g/6 * t
>
> where t is the duration in seconds.
>
> Applying the Tsiolkovsky rocket equation to this gives
>
>     g/6 * t = Isp * g * ln(m0/mf)
>
> where Isp is the specific impulse, m0 is the initial mass (full
> fuelled), and mf is final mass (all fuel gone).
>
> Rearranging, and cancelling the g's gives,
>
>     ln(m0/mf) = 1/6 * t / Isp.
>
> Pull a reasonably typical Isp of 300 out of the air, and try for a 30
> minute (1800 second) mission, gives
>
>     ln(m0/mf) = 1/6 * 1800 / 300
>
The right side happens to come to 1. And, presumably again for aging
brain reasons, I made a second mistake here in the interpretation.

Anyway, that means that m0/mf is about 2.7, meaning the vehicle is
initially 63% fuel.

Assuming that I've got this right this time, it's still a lot of fuel to
be obtained from somewhere, but not as bad as I previously thought.
Perhaps the documentary wasn't so far wrong.

Note though that things get progressively worse for longer missions. The
natural log (ln) on the left means that the fuel fraction goes up
exponentially with mission length, and if there's no fuel at the
destination, the mission length is the round-trip time.

Sylvia.