Mars Rovers Landing ???

Sally posted:

The first rebound deltaV will be nearly twice the initial V.

I'm afraid that you must not confuse V (velocity) for speed. Velocity is a
vector with both a magnitude (speed) and a direction. It makes more sense to
just talk about the speed rather than bringing in vectors. The first change
in speed will be from the intitial contact speed as the spacecraft just begins
to hit the surface to zero, where the airbags are the most compressed. These
aren't big rubber balloons or soccer balls; they are tough multi-layer airbags
designed to cushion the impact on the lander and dissipate the impact energy.
The rebound change in speed from zero at the start of the first rebound to
when the lander clears the surface in its first bounce will be significantly
*less* in magnitude than the impact speed due to the inelasticity of the
airbags. I doubt it will be even 1/2 of the intial speed of impact. The
energy of impact is kinetic (1/2*mv^2). A dropped tennis ball probably
dissipates close to 60% of the energy of impact if dropped at a low speed onto
a hard surface (ie: it will rise roughly 40% as high as the height it was
dropped from). If 60% of the impact kinetic energy is dissipated in the
airbags on first impact, the rebound speed would be only about 63% that of the
impact speed. These airbags with the imbedded mass of the lander and rover
are very probably much less efficient at rebounding than a tennis ball, so
they probably dissipate 80% or perhaps somewhat more of the initial energy of
impact. Assuming an 80% dissipation would yield a rebound kinetic energy of
only 20% that of the kinetic energy just prior to first contact, so the
rebound maximum speed as the spacecraft just left the surface would be only
about 45% of the impact speed. Carrying this a bit further:

Impact speed v0
Impact speed at second impact: 0.45*v0
Impact speed at third impact: 0.20*v0
Impact speed at fourth impact: 0.09*v0

It would take just a couple of minutes to derive a simple formula for
determining deltaV for the nth rebound.

It doesn't take nearly that long.

The airbags do not do a good job of dissipating the energy of impact.

Really? How do you define "a very good job of dissipating energy"? A lowly
tennis ball manages to kill off 60% of the impact energy without really trying
very hard, and that is over half (looks pretty good to me). Did you do any
tests to back up your contention about the airbags? Did you sit at the vacuum
test chamber and observe the airbag prototype tests? Did you look at the
Pathfinder data? A spacecraft designer would *not* want a cushioning device
which did *not* disipate the energy of impact. The airbags do just that, as
was demonstrated by Pathfinder.

In a truly absorbing system the
energy would be dissipated as heat and none would remain to be re-imparted
to the spacecraft.

Energy can be disipated a number of ways which do not necessarily involve
creating heat (mechanical deformation is one).

As I mentioned elsewhere, self-expanding self-curing foams can be very
light. Even "low tech" foams from a DIY store will expand to many times the
original volume.

It takes too long for the foam to get to a consistency which would cushion the
impact enough. Solids (even foamy ones) and liquids would directly transmit
some of the force of hard impact directly to the frame in a sudden jolt rather
than in a more uniform decceleration.
--
David W. Knisely
Prairie Astronomy Club: http://www.prairieastronomyclub.org
Hyde Memorial Observatory: http://www.hydeobservatory.info/

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"Sally" wrote in message ...
"OG" wrote in message
...
One objection to a liquid 'goo' would surely be the shock wave within the
goo as the impacts took place. Impact on a gas filled balloon would
produce
a gentler increase in pressure on the lander, whereas the liquid filled
balloons would be subject to a more violent impact.
OK, I gave some figures to support the 'goo' bag idea and to show why it
would result in less stresses on the spacecraft. So, would you like to show
me the calculations that justify your shock wave statement? And would you
like to show me how you determined that the goo filled bags would result in
a more violent impact? Obviously, you disagree with the numbers I came up
with. In what way are they in error?

Your figures show the change of velocity as the lander bounces.
However, you don't address the more important issue of acceleration.
As you correctly point out, a bounce with low elasticity would reduce
deltaV, However an incompressible 'goo' would result in a much quicker
bounce, so the acceleration would be much greater for 'lander in goo'
compared to 'lander in CO2 gas'.

If you doubt this, try putting filling a couple of long balloons with
air and taping them around an egg. See how well the egg survives a
drop onto concrete. Then try it with two balloons filled with water.

What might make a difference is if the air bags are fitted with valves
that would allow gas to escape when the compression is at its
greatest. In this way, when the bag hits the ground, the air is
compressed, some of it is forced out and the energy of rebound is
reduced.

Another objection would be the polluting effect of the goo as it has to be
released from the balloons onto the martian surface when the lander is
freed
from the protective 'shell'
Good point, that would be another design constraint for engineering the goo.

I think the proposal to use polystyrene as a shock absorber would be
unacceptable simply on pollution grounds.
Polystyrene?? Why would polystyrene be used?

Apologies - I now see you originally suggested formaldehyde foam. What
that would do to the tests for organic materials I can't imagine.

"Sally" wrote in message ...
"OG" wrote in message
...
One objection to a liquid 'goo' would surely be the shock wave within the
goo as the impacts took place. Impact on a gas filled balloon would
produce
a gentler increase in pressure on the lander, whereas the liquid filled
balloons would be subject to a more violent impact.
OK, I gave some figures to support the 'goo' bag idea and to show why it
would result in less stresses on the spacecraft. So, would you like to show
me the calculations that justify your shock wave statement? And would you
like to show me how you determined that the goo filled bags would result in
a more violent impact? Obviously, you disagree with the numbers I came up
with. In what way are they in error?

Your figures show the change of velocity as the lander bounces.
However, you don't address the more important issue of acceleration.
As you correctly point out, a bounce with low elasticity would reduce
deltaV, However an incompressible 'goo' would result in a much quicker
bounce, so the acceleration would be much greater for 'lander in goo'
compared to 'lander in CO2 gas'.

If you doubt this, try putting filling a couple of long balloons with
air and taping them around an egg. See how well the egg survives a
drop onto concrete. Then try it with two balloons filled with water.

What might make a difference is if the air bags are fitted with valves
that would allow gas to escape when the compression is at its
greatest. In this way, when the bag hits the ground, the air is
compressed, some of it is forced out and the energy of rebound is
reduced.

Another objection would be the polluting effect of the goo as it has to be
released from the balloons onto the martian surface when the lander is
freed
from the protective 'shell'
Good point, that would be another design constraint for engineering the goo.

I think the proposal to use polystyrene as a shock absorber would be
unacceptable simply on pollution grounds.
Polystyrene?? Why would polystyrene be used?

Apologies - I now see you originally suggested formaldehyde foam. What
that would do to the tests for organic materials I can't imagine.

"G=EMC^2 Glazier" wrote in message
...
bed,and when this water mattress hits it breaks open and out comes the
rovers from a puddle of water. This would be great for you and NASA.
That way there would be no question of water on Mars surface. Its a
Bert,
You are a rogue g
Sally

"G=EMC^2 Glazier" wrote in message
...
bed,and when this water mattress hits it breaks open and out comes the
rovers from a puddle of water. This would be great for you and NASA.
That way there would be no question of water on Mars surface. Its a
Bert,
You are a rogue g
Sally

"Sally" wrote in message
...
What might make a difference is if the air bags are fitted with valves
that would allow gas to escape when the compression is at its
greatest. In this way, when the bag hits the ground, the air is
compressed, some of it is forced out and the energy of rebound is
reduced.
Yes! Or introduce a system of interconnections between the bags. As the
gas
moves around it would move through restrictions and some of the energy
would
be dissipated as turbulence and then heat. I think this is how the
Pathfinder system was constructed. But if so then why did it have to
bounce?

Cost, reliability, mechanical simplicity. Maybe a solution could be made
that avoided bouncing. However, the tolerances needed to provide a single
impact landing are much less than a multi-bounce landing. The key
requirement is to provide a gentle dissapation of the kinetic energy from
the descent.

Once the lander has landed it has to free itself from the bags. Air bags
that can be completely deflated will be much simpler to escape from than goo
filled bags.

What is the problem with air bags?

"Sally" wrote in message
...
What might make a difference is if the air bags are fitted with valves
that would allow gas to escape when the compression is at its
greatest. In this way, when the bag hits the ground, the air is
compressed, some of it is forced out and the energy of rebound is
reduced.
Yes! Or introduce a system of interconnections between the bags. As the
gas
moves around it would move through restrictions and some of the energy
would
be dissipated as turbulence and then heat. I think this is how the
Pathfinder system was constructed. But if so then why did it have to
bounce?

Cost, reliability, mechanical simplicity. Maybe a solution could be made
that avoided bouncing. However, the tolerances needed to provide a single
impact landing are much less than a multi-bounce landing. The key
requirement is to provide a gentle dissapation of the kinetic energy from
the descent.

Once the lander has landed it has to free itself from the bags. Air bags
that can be completely deflated will be much simpler to escape from than goo
filled bags.

What is the problem with air bags?

Sally posted:

I will accept your figures for the sake of this discussion. But why should
there be *any* rebound? As soon as spacecraft downward motion is zero that
should be the end of the braking phase...job done. Why does the airbag
system then have to relaunch the spacecraft into a rebound at all?

Because if you dissipate the energy in "one shot", you may subject the
spacecraft to a level of deceleration which is too high for the spacecraft to
survive intact. Its better to "bleed-off" the energy. To create a system
where there was zero bounce would require some sort of huge airbag system as
well as a complex deflation system to release the air as the spacecraft
reached near zero speed. Its not quite the same as a car air bag, where the
bag is fixed and inflated to cushion the impact. A smaller system of airbags
makes more sense from a simplicity, stability and coverage end of things.
--
David W. Knisely
Prairie Astronomy Club: http://www.prairieastronomyclub.org
Hyde Memorial Observatory: http://www.hydeobservatory.info/

**********************************************
* Attend the 10th Annual NEBRASKA STAR PARTY *
* July 27-Aug. 1st, 2003, Merritt Reservoir *
* http://www.NebraskaStarParty.org *
**********************************************

Sally posted:

I will accept your figures for the sake of this discussion. But why should
there be *any* rebound? As soon as spacecraft downward motion is zero that
should be the end of the braking phase...job done. Why does the airbag
system then have to relaunch the spacecraft into a rebound at all?

Because if you dissipate the energy in "one shot", you may subject the
spacecraft to a level of deceleration which is too high for the spacecraft to
survive intact. Its better to "bleed-off" the energy. To create a system
where there was zero bounce would require some sort of huge airbag system as
well as a complex deflation system to release the air as the spacecraft
reached near zero speed. Its not quite the same as a car air bag, where the
bag is fixed and inflated to cushion the impact. A smaller system of airbags
makes more sense from a simplicity, stability and coverage end of things.
--
David W. Knisely
Prairie Astronomy Club: http://www.prairieastronomyclub.org
Hyde Memorial Observatory: http://www.hydeobservatory.info/

**********************************************
* Attend the 10th Annual NEBRASKA STAR PARTY *
* July 27-Aug. 1st, 2003, Merritt Reservoir *
* http://www.NebraskaStarParty.org *
**********************************************

Bert posted:

The rover inside also hits the bottom bag,and at that(very
close) instant the bag comes up to hit the rover and change its downward
acceleration to go in the up direction.

Bert, the rover sits *inside* a protective lander structure. The airbags are
on the *outside* of this lander.

David maybe the best idea would be to have the rovers inside a water
bed,and when this water mattress hits it breaks open and out comes the
rovers from a puddle of water.

This is a lousy idea for a variety of reasons. Water is not compressible and
thus would transfer the full shock of impact to the lander. It is also heavy.

Its a
little like cheating,but NASA is run by thieves,and propagandist

And you know little about NASA! Quit being insulting.
--
David W. Knisely
Prairie Astronomy Club: http://www.prairieastronomyclub.org
Hyde Memorial Observatory: http://www.hydeobservatory.info/

**********************************************
* Attend the 10th Annual NEBRASKA STAR PARTY *
* July 27-Aug. 1st, 2003, Merritt Reservoir *
* http://www.NebraskaStarParty.org *
**********************************************