How to Land Softly on a Hard Planet
Ron Baalke
How to Land Softly on a Hard Planet
Jet Propulsion Laboratory
March 25, 2002
Just one of the many problems in landing on another planet, after it's been
determined where to land and the method to get there, is landing safely. For
JPL, a safe landing is "the name of the game," as engineers work to prepare
two rovers for the journey to Mars.
The Mars Exploration Rovers scheduled for launch in 2003 are using the same
type airbag landing system that Mars Pathfinder used in 1997. The airbags
must be strong enough to cushion the spacecraft if it lands on rocks or
rough terrain and allow it to bounce across Mars' surface at freeway speeds
after landing. To add to the complexity, the airbags must be inflated
seconds before touchdown and deflated once safely on the ground.
"The 2003 rovers have a different mass [than Sojourner, the Pathfinder
rover], so we've made changes in the airbag design," said John Carson,
cognizant engineer. "Our requirement is to be able to land safely on a rock
extending about a half-meter (about 18 inches) above the surface. Extensive
testing gives us a process for trial and error before the final design."
How to Build a Better Airbag
While most new automobiles now come with airbags, spacecraft don't. The
fabric being used for the new Mars airbags is a synthetic material called
Vectran that was also used on Mars Pathfinder. Vectran has almost twice the
strength of other synthetic materials, such as Kevlar, and performs better
at cold temperatures.
Denier is a term that measures the diameter of the thread used in the
product. There will be six 100-denier layers of the light but tough Vectran
protecting one or two inner bladders of the same material in 200-denier,
according to Dara Sabahi, mechanical systems architect. Using the 100-denier
means there is more actual fabric in the outer layers where it is needed,
because there are more threads in the weave.
Each rover uses four airbags with six lobes each, which are all connected.
Connection is important, since it helps abate some of the landing forces by
keeping the bag system flexible and responsive to ground pressure. The
fabric of the airbags is not attached directly to the rover; ropes that
crisscross the bags hold the fabric to the rover. The ropes give the bags
shape, which makes inflation easier. While in flight, the bags are stowed
along with three gas generators that are used for inflation.
Testing, Testing, Testing
Since the airbags are composed of many layers, some tearing in the outer
layers is acceptable and even expected. Engineers test the bags to make sure
there will be no catastrophic problems that would prevent a safe landing.
Mars airbag testing is done in world's largest vacuum chamber at the Plum
Brook Station of NASA's Glenn Research Center in Ohio. "The Plum Brook
facility is pretty impressive, along with all the people who operate it,"
said Carson.
The test chamber used for the tests is a little over 30 meters (100 feet)
across and about 37 meters (120 feet) high -- big enough that three railroad
tracks go through it. A test spacecraft and airbag system weighing about 535
kilograms (about 1,180 pounds) are accelerated with a bungee cord system
onto a platform with rocks that approximate the Mars surface. The drop is at
landing speed, about 20 to 24 meters (yards) per second.
Tests are documented thoroughly with high-speed and video cameras, in
addition to visual inspections. Engineers even built a clear dome, studded
with rocks, that has a camera that documents tests from a rock's-eye view.
During testing, a crew from ILC Dover, the airbag's manufacturer, stands by
to make quick repairs and to note any changes required.
"We do extensive testing," said Tom Rivellini, deputy mechanical systems
architect. "We want to break the bag on Earth, not on Mars. If we see a tear
that is unexpected or goes too deep, we can make changes now [before the
final design]."
Carson added, "We'll go over all the data we've accumulated so far, do some
more testing, and decide on a design configuration."
And then on to Mars in 2003!
Sitard
Until active optics is cheap, I don't think I'm "robotics".
toadmonkey
TM
On 25 Mar 2002 19:36:37 GMT, baa...@zagami.jpl.nasa.gov (Ron Baalke)
wrote:
>http://www.jpl.nasa.gov/solar_system/features/airbags.html
>
>How to Land Softly on a Hard Planet
>Jet Propulsion Laboratory
>March 25, 2002
>
--Toadmonkey: "Now now. Brain popping and world crashing may be hazardous to ones perception of reality.
Very dangerous business that can lead to madness or something worse for some, truth."
Sir Charles W. Shults III
to 1000 to 1030 or so on Earth at sea level. Parachutes have limited uses in
near vacuum. Mars' atmosphere is about 1/125th as thick as ours. Because of
this, a parachute is only useful for shedding orbital velocity and doing a
controlled slowdown. A soft landing with a parachute on Mars would require it
to be inordinately large.
For Viking, the parachute worked well enough to get it near the ground at a
controlled rate, where rocket engines were fired to make a soft landing. For
smaller probes, this is too expensive in terms of mass, so a lighter, more
robust system is needed. In the case of gas bladders, most of the size is just
that- gas- and very compressible.
Cheers!
Chip Shults
My robotics, space and CGI web page - http://home.cfl.rr.com/aichip
Richard Steven Walz
It uses them too, but Mars has only 1% of earth atmospheric pressure.
The terminal velocity if dropped might be as much as 500-800 mph!!
It needs parachutes, but they don't work so well, so they also need
the airbags. Parachutes get it down to 60 mph, the bags do the rest.
-Steve
--
-Steve Walz rst...@armory.com ftp://ftp.armory.com/pub/user/rstevew
Electronics Site!! 1000's of Files and Dirs!! With Schematics Galore!!
http://www.armory.com/~rstevew or http://www.armory.com/~rstevew/Public
Daniel O. Miller
> For Viking, the parachute worked well enough to get it near the
> ground at a controlled rate, where rocket engines were fired to make a
> soft landing. For smaller probes, this is too expensive in terms of
> mass, so a lighter, more robust system is needed. In the case of gas
> bladders, most of the size is just that- gas- and very compressible.
But they wound up using a rocket on Sojourner anyways, before the landing
bags inflated. The chute and airbags together didn't make it. The fuel
for that last 20 m/s would have been a good buy next to the mass of the
airbag system.
As an engineer I am rather uncomfortable with anything that's *designed*
to tumble uncontrollably.
Daniel O. Miller
"Does this look familiar? Do you know what it is? Neither do I! I made
it last night in my sleep. Apparently I used gindrogac - highly unstable!
I put a button on it, yes? I wish to press it, but I'm not sure what will
happen if I do..." - Gune
Robert Posey
hit and maintain an angle to get a alternative system to work. The problem I
have is that it would be better if the landing system was dual use, and thus
could be used for something on the ground. Quick and bad ideas, and
remembrances.
1. Parachutes or Bags made of material that could be used as fuel or some form
of building material.
a. Much easier if done in support of later manned or at least complex robotic
missions. It would be a really good idea to think of dual use items for at
least potential manned mission sites.
b. perhaps solar cell material of some low performance type that never less
saves mass.
2. Balloon braking that yields lighter than air system for either low level
scanning or precise site selection.(not my idea)
3. Wings for flying vehicle, perhaps as a post landing sub unit.
Richard Steven Walz
>
>3. Wings for flying vehicle, perhaps as a post landing sub unit.
Not in Mars pressure, won't work. That's the problem, you don't get
anything gliding or aerobraking to a low velocity in a nearly non-existent
atmosphere, it is a modified rock. The only advantage you have is lower
gravity and combo aero-rocket-cushion landing systems, AND to know how much
of each is optimum by repeated iterations on a computer.
-Steve
Tom Hiblar
inherently more robust. One small rock or dip in ground doesn't tip you
over, landing on an incline doesn't tip you over, stopping 15 feet too high
can be tolerated - it seems to me that the landing procedure is just more
fault tolerant.
Now, I have not see a fault tree analysis with probabilities and severities
assigned, and certainly it might be shown that current or soon to be
available rocket-all-the-way-down systems with obstacle avoidance have a
higher probability of sucess, and I'd be interested to see one, - but that
is my gut feel.
"Daniel O. Miller" <dmil...@ridgenet.net> wrote in message
news:Pine.SOL.3.95.102032...@owens.ridgecrest.ca.us...
johnb
Richard Steven Walz
Obviously not, Pathfinder/Sojourner worked fine.
They did simulate some of it in a huge vacuum chamber, but most of it was
strictly aerobraking and rocket-braking simulation.
Henry Spencer
Sir Charles W. Shults III <aic...@cfl.rr.com> wrote:
> For Viking, the parachute worked well enough to get it near the ground at a
>controlled rate, where rocket engines were fired to make a soft landing. For
>smaller probes, this is too expensive in terms of mass, so a lighter, more
Unfortunately, the airbag system is heavier, not lighter.
People *thought* an airbag landing system would be simple, versatile, and
lightweight. That's what Mars Pathfinder, nee MESUR Pathfinder, was
originally about: testing the technology for cheap Mars landers.
Unfortunately, when the Mars Pathfinder team had to deal with the problems
of actually doing it, they found that it wasn't that easy. It came out to
be complicated, because it needs a parachute *and* braking rockets (fired
by a radar altimeter) in addition to complex multi-layer airbags. It's
not nearly as versatile as originally hoped, because it doesn't work for
high-altitude landing sites (the air is too thin) and it requires very
tough payloads. Worst of all, it's actually heavier than landing rockets.
That's why there were no plans to use it after MP. The political fallout
of the MPL failure pressed it back into service for the MER rovers... but
the MER team are having to redesign parts of it, and are struggling to
keep the rovers light enough and durable enough.
Its one strength is that it's better at making safe landings on very rough
surfaces. It may have a niche role to play in the long-term scheme of
things, but it's never going to replace rocket landings.
--
Many things changed on Sept. 11, but the | Henry Spencer he...@spsystems.net
importance of freedom did not. -SpaceNews| (aka he...@zoo.toronto.edu)
ma...@the-light.com
>That's why there were no plans to use it after MP. The political fallout
>of the MPL failure pressed it back into service for the MER rovers... but
>the MER team are having to redesign parts of it, and are struggling to
>keep the rovers light enough and durable enough.
>
>Its one strength is that it's better at making safe landings on very rough
>surfaces. It may have a niche role to play in the long-term scheme of
>things, but it's never going to replace rocket landings.
I have to wonder when switching from reading discussions like this to watching a
television ad for a remote controlled toy that can speed over all kinds of
obstacles, overcoming being turned over, etc. NASA could get some pointers from
the toy industry, I think.
Joseph Lazio
m> he...@spsystems.net (Henry Spencer) wrote:
>> That's why there were no plans to use it after MP. The political
>> fallout of the MPL failure pressed it back into service for the MER
>> rovers... but the MER team are having to redesign parts of it, and
>> are struggling to keep the rovers light enough and durable enough.
>>
>> Its one strength is that it's better at making safe landings on
>> very rough surfaces. It may have a niche role to play in the
>> long-term scheme of things, but it's never going to replace rocket
>> landings.
m> I have to wonder when switching from reading discussions like this
m> to watching a television ad for a remote controlled toy that can
m> speed over all kinds of obstacles, overcoming being turned over,
m> etc. NASA could get some pointers from the toy industry, I think.
How often do these commercials show these toys hitting the ground at
5000 mph?
--
Lt. Lazio, HTML police | e-mail: jla...@patriot.net
No means no, stop rape. | http://patriot.net/%7Ejlazio/
sci.astro FAQ at http://sciastro.astronomy.net/sci.astro.html
Dan Messenger
(eg the moon)
Joseph Lazio <jla...@adams.patriot.net> wrote in message
news:llzo05b...@adams.patriot.net...
Matthew F Funke
>
>Joseph Lazio <jla...@adams.patriot.net> wrote in message
>news:llzo05b...@adams.patriot.net...
>> >>>>> "m" == mars <ma...@the-light.com> writes:
>>
>> m> he...@spsystems.net (Henry Spencer) wrote:
>> >> That's why there were no plans to use it after MP. The political
>> >> fallout of the MPL failure pressed it back into service for the MER
>> >> rovers... but the MER team are having to redesign parts of it, and
>> >> are struggling to keep the rovers light enough and durable enough.
>> >>
>> >> Its one strength is that it's better at making safe landings on
>> >> very rough surfaces. It may have a niche role to play in the
>> >> long-term scheme of things, but it's never going to replace rocket
>> >> landings.
>>
>> m> I have to wonder when switching from reading discussions like this
>> m> to watching a television ad for a remote controlled toy that can
>> m> speed over all kinds of obstacles, overcoming being turned over,
>> m> etc. NASA could get some pointers from the toy industry, I think.
>>
>> How often do these commercials show these toys hitting the ground at
>> 5000 mph?
>
>And also on planets / natural satalites with a lower gravity than earth ?
>(eg the moon)
Note that if you're coming in from far enough away (and it doesn't
take very far at all to be "far enough"), you'll be coming in to the Moon
at a *minimum* of 300 mph, and Mars at a *minimum* of 11,200 mph. I don't
think the toy industry has devices that can withstand those kinds of
impacts.
Of course, these values change if you bother to do an orbital
insertion before descent.
--
-- With Best Regards,
Matthew Funke (m...@hopper.unh.edu)
Jonathan Silverlight
<jla...@adams.patriot.net> writes
>>>>>> "m" == mars <ma...@the-light.com> writes:
>
>m> he...@spsystems.net (Henry Spencer) wrote:
>>> That's why there were no plans to use it after MP. The political
>>> fallout of the MPL failure pressed it back into service for the MER
>>> rovers... but the MER team are having to redesign parts of it, and
>>> are struggling to keep the rovers light enough and durable enough.
>>>
>>> Its one strength is that it's better at making safe landings on
>>> very rough surfaces. It may have a niche role to play in the
>>> long-term scheme of things, but it's never going to replace rocket
>>> landings.
>
>m> I have to wonder when switching from reading discussions like this
>m> to watching a television ad for a remote controlled toy that can
>m> speed over all kinds of obstacles, overcoming being turned over,
>m> etc. NASA could get some pointers from the toy industry, I think.
>
>How often do these commercials show these toys hitting the ground at
>5000 mph?
But they don't, surely? (with the possible exception of Mars Polar
Lander and its secondary probes) ISTR that the hard disk in my computer
is rated for a higher "g" than the Mars landers are expected to take, if
their retro-rockets and parachutes work.
RedX
"Dan Messenger" <m...@me.com> schreef in bericht
news:a9f3po$434$1...@newsg2.svr.pol.co.uk...
> And also on planets / natural satalites with a lower gravity than earth ?
> (eg the moon)
>
If you drop something on earth, it accelerates until the acceleration from
the gravitation is equal to the decceleration from bumbing into all those
airparticles in the atmosfeer. This means that an object in free fall has a
maximum speed.
When you do the same thing on the moon, you have a lower acceleration
because of the lower gravity, but there is no atmosfeer to slow down the
object, so it can reach a lot higher speeds than on Earth.
RedX
Matthew F Funke
Good idea, but no.
Consider an object coming in from "far away" to the Earth. When it
gets to the top of the atmosphere -- which we'll arbitrarily put up at 210
nautical miles (388920 m), the altitude at which the Space Shuttle orbits
-- it's screaming in at about 24,300 mph (10,900 m/s). The force
*backward* on the object would have to average about 15.4 gees all the way
down... which might not be the case for a given object.
In contrast, an object coming in from "far away" from the Moon is
going to be coming in at a paltry 5300 mph by the time it hits the
surface. (For reference, to slow from 24,300 mph to 5300 mph in Earth's
atmosphere would still require an average backwards force of about 14.7
gees.)
Jonathan Silverlight
<m...@hypatia.unh.edu> writes
So? It happens every day, when a meteorite lands. They usually fall at
terminal velocity, which is probably not more than a couple of hundred
miles an hour for small rocks or space probes (of course, most objects
hitting the atmosphere break up or burn up first).
I take my hat off to the Japanese, who are trying to build probes to
drop onto the Moon. As they say, it's not the fall that kills you.
Matthew F Funke
Right... but we're talking about toys here, right? How many toys are
built to withstand a shock like 15.4 gees (average)?
I acknowledge that rocks can be built to take the punishment, and so
can specialty instruments. But the thread was discussing building probes
like *toys*. You take one of those toys "built to take punishment" and
just drop it out a twentieth-story window, and it won't seem to be built
quite so tough.
>I take my hat off to the Japanese, who are trying to build probes to
>drop onto the Moon. As they say, it's not the fall that kills you.
Interesting... I hadn't heard anything about that. Can you provide a
URL or paper source?
Alex R. Blackwell
>
> Jonathan Silverlight <jsi...@merseia.fsnet.co.uk> wrote:
> >I take my hat off to the Japanese, who are trying to build probes to
> >drop onto the Moon. As they say, it's not the fall that kills you.
>
> Interesting... I hadn't heard anything about that. Can you provide a
> URL or paper source?
I presume he is referring to the proposed ISAS Lunar-A mission. See
http://nssdc.gsfc.nasa.gov/database/MasterCatalog?sc=LUNAR-A or
http://www.isas.ac.jp/e/enterp/missions/lunar-a/cont.html
--
Alex R. Blackwell
University of Hawaii
Mark
refer to the cost to lift it (mass) into orbit, or to accelerate it toward
Mars? I mean if mass were needed purely for weight, would it be feasible to
propel "space junk" toward Mars?
"Henry Spencer" <he...@spsystems.net> wrote in message
news:GuKz5...@spsystems.net...
ma...@the-light.com
Where are these numbers coming from? If your coming from far enough out, you'll
be going essentially the same speed approaching the moon or the earth since it
will be the gravity of the combined system drawing you in.
>>So? It happens every day, when a meteorite lands. They usually fall at
>>terminal velocity, which is probably not more than a couple of hundred
>>miles an hour for small rocks or space probes (of course, most objects
>>hitting the atmosphere break up or burn up first).
>
> Right... but we're talking about toys here, right? How many toys are
>built to withstand a shock like 15.4 gees (average)?
When I mentioned toys, I was analogizing to the rover, not the lander. I don't
think the rover would survive these kind of conditions either.
Bee Admin
eveywhere softly up and out there, and such thing kill inteligent.
Sir Charles W. Shults III
of sawdust or a kilogram of exotic alloys and microchips. The real cost is in
launch, so in many cases, the material itself is, well, immaterial! On that
"democratic" footing, the bottom line it the determining factor. Functionality
is the most important thing about whatever you launch, and dead weight is a
complete waste of your resources.
That having been said, you can see that anything with more than one function
is popular for spacecraft. This is one of the reasons that hollow stainless
steel tanks appeared in spacecraft boosters- while they were heavier than light
alloy tanks, they were far tougher and could withstand the rigors of load
bearing and fuel containment simply because they could be pressurized and made
to maintain their shape- something like metal balloons.
Now, clearly, anything that is sent to a remote destination will have to
give the best possible performance. That is because lofting it is an expensive
proposition. The real cost of any spacecraft is in mass, not material per se.
If some small mass item can help decelerate something from interplanetary travel
speeds to soft landing, it is going to be used. If it is massive, it cuts into
the essential payload space, so the designers are going to try to make it do as
much as possible.
Some landing systems are going to retain most of their mass all the way
down, some will shed mass as they approach. Two such systems with opposing
philosophies are balloons versus rockets. If it is practical to use small
rockets to bring the craft down to low velocity, they will be used. But rockets
require split second timing and so the spacecraft needs sensors that will detect
the ground with great precision- thus you need a radar sensor or something in
that sensory "ballpark". This means that you have to add the cost of a radar,
processing power, battery power, and wiring to the spacecraft to use rocket
deceleration to a soft landing. Think of this as an "invisible" piece of
overhead mass that has to be present to make rocket landings work.
Now, let's look at parachutes.
Parachutes are just about the simplest, "dumbest" system you can imagine for
bringing something down. However, Mars has very thin air. And, to make matters
worse, the parachute used at low velocities will probably not work at all at
hypersonic velocities. This means you need something like a two-parachute
system. This also means you need two canisters and two deployment systems,
perhaps a system to eject the first chute and canister entirely (to cut down on
mass, of course, during the actual landing), and also an atmosphere thick enough
to do the job of supporting a mass hanging from that parachute. Think about how
Mars is pretty darn close to being in a vacuum. Its "sea level" air pressure is
about 8 millibars, where Earth has a sea level atmospheric pressure of about a
thousand millibars. That is a factor of 125 difference. Even a back of the
envelope estimate says that a parachute will likely have 125 times the area to
safely land the craft, compared with a parachute meant for Earthly loads.
That means that the parachute will have much more mass to do a specific job.
In the end, analyzing the mass and deceleration factors of all these systems
is the only sure way to arrive at a useful landing approach. The Russians used
ballutes to help decelerate some of their craft- an inflatable bladder that
increased the drag cross section of a spacecraft in orbit so that it would shed
some of its velocity. This is a wonderful system, but it can be very tricky to
use with precision. You can be assured of coming down, but perhaps not the
exact location.
Now, rockets can be excellent for low gravity bodies, and Mars has roughly
38% of the gravitation of Earth. And, rockets excel in space or near vacuum,
where drag is not present or only marginally so. And if you work out the
integration of acceleration versus time, you quickly conclude that short, hard
bursts of rocket power are far more efficient than long drawn out low power
bursts. Even a kid can see that- try figuring how long a rocket can hover at
one gee near the Earth and how long it has to fight that one gee overhead at two
gees of thrust, or at 5 gees, or at 50.
Now you see why projectiles are shot from a gun- all the acceleration occurs
in a burst, imparting the most efficient use of thrust against gravitation. The
most efficient rocket thrusters for landing will shoot HARD and fast, at the
exact moment needed. But now, you have to ask whether your landing craft will
tolerate such treatment! It means hardening it with more structural metal,
making it heavier, and once again adding to the mass expenses.
So designing the landing system is a delicate balancing act- getting a
system that is light, durable, and capable all at once.
ma...@the-light.com
> Now you see why projectiles are shot from a gun- all the acceleration occurs
>in a burst, imparting the most efficient use of thrust against gravitation. The
>most efficient rocket thrusters for landing will shoot HARD and fast, at the
>exact moment needed. But now, you have to ask whether your landing craft will
>tolerate such treatment! It means hardening it with more structural metal,
>making it heavier, and once again adding to the mass expenses.
Didn't the Russians use such a scheme for manned missions? I remember seeing a
picture of a capsule returning to earth and a meter or so-long feeler, sticking
out the bottom of the capsule fired retro-rockets when it felt the ground.
Sir Charles W. Shults III
viable. Just think about how many gees it would take to stop that sucker! Now,
if it was at the bottom of a parachute, that is another matter. In that case,
it would work very nicely.
In a similar vein, I once did the math for a personal rocket decelerator for
people who might jump out of planes without a parachute. If you had a folding
frame that would spring open and lock, and at the far ends there were two small
zinc-sulfur solid rockets, you could in theory bring yourself down to a safe
velocity very quickly. All you need is to lose that crucial last 100 miles per
hour or so in free fall velocity.
Here are some figures.
Imagine that you are comfortable with a fall from 10 feet. This is often
pretty easy to do, and if you are in good physical shape, not at all difficult.
Using the standard acceleration formula, S=1/2 AT^2, we can calculate how fast
you will be moving.
S = displacement- how far you will go. Now, A is acceleration, T is time in
seconds, the rest is simple. Since I used 10 feet as an example, I'll use
English units throughout. 3 meters will work just as well.
Since S = 10 feet, and A is 32.16 feet per second^2 at sea level, we can
eliminate the 1/2 term by rewriting it as:
20 = 32.16 * T^2
Divide both sides by 32.16 to get ~0.622 = T^2, and then we take the root.
It will take us 0.789 seconds to fall 10 feet from a dead start. Now, how fast
are we going? Our speed will be acceleration multiplied by time, or 0.789
seconds times 32.16. This yields ~25.4 feet per second, or 17.3 miles per hour.
Since the terminal velocity for a falling human being is about 120 miles per
hour, you need to dump about 105 miles per hour to land softly. And, you need
to do it quickly. How many gees are you willing to take? 5 perhaps? Let's say
that this is a reasonable figure, just for the sake of argument.
To decelerate 105 miles per hour at 5 gees, we treat it just like
acceleration. We just reverse the sign when it comes to the crunch (good grief!
Now I remember why I called the rocket frame folding, instead of collapsible!)
Since A is now 5 gees, or 160.8 feet per second^2, we can simply divide to
get our time. 105 miles per hour is also 154 feet per second. Clearly, we will
spend only slightly over a second at that 5 gee figure to drop our velocity to
something reasonable. (Translation: hey, I can jump from here!)
Now, what height should we be at when we start our engines? This is
critical! Too low, and you crunch anyway, only to rebound to some height and
fall again- remember that the rockets will keep firing! Too high, and you reach
a nearly dead stop in midair, only to start falling all over again. Timing is
indeed crucial!
Since we know that we will be accelerating at 160.8 feet per second for
(rounding) 1 second (which gives us 109 miles per hour, and a slightly softer
landing, and a good reason to round), T^2 is still1, and A is 160.8, so one half
of that is the height we should be at when the rockets get lit. In other words,
80.4 feet.
So, if you have a couple of small rockets that could provide 5 gees of
combined thrust for 1 second, and they could do that for your body weight, your
gear, the folding (not collapsible, thank goodness) frame and the rockets
themselves, then you would fire them when you were 80 feet above the ground from
a free fall to stop safely with no parachute. Be sure you are upright when it
happens!
Why would you want to do this?
Hmm. Imagine a scenario where you need to drop people in some place
silently, maybe at night. Parachutes would show up against the sky, maybe, even
if they were black. Paratroopers drifting to earth at night would be vulnerable
to a sniper with a body heat scope. That's not so exotic, some of us have body
heat and infrared hardware lying about on our workbenches these days. So how do
you get your guys into the area and on the ground fast?
Dress them in black, soak the outfits in something that cools as it
evaporates- water, alcohol, whatever. That will mask their body heat
signatures. Put small black drogue chutes on them to cut some of the velocity
and help orient them. When they jump, they pull the cords and the chutes pop
out as the frame unfolds. A pocket radar ground ranger starts and controls the
rocket decelerators. Attach it to the frame so the position of their moving
bodies will not screw up the readings.
Add thick crushable foam pads to the shoes- something like packing foam, but
more aggressive. You would be amazed to see how much kinetic energy that can
take up. They fall like rocks, avoiding detection, heat sensing, snipers, etc.
Near the ground, boom! They are hard decelerated and on the ground running.
When the shoes crush the pads, the frame ejects and the drogue chute and rockets
are gone. You may be able to literally hit the ground, take a breath, and run
for the woods or whatever.
On a lighter note, this same technology can be applied to resource locating
robots, or mine sweeper robots, or almost any sort of autonomous hardware that
for whatever reason, cannot use a parachute. Fun with numbers, and fun to
tinker with.
Take notice- if you try this and blow your fool head off, don't say you
weren't warned. I do not condone the average guy tinkering with this sort of
stuff without first getting his figures and physics in place very tightly! You
could end up hurt or worse. You might even end up working for NASA.
Martin Brown
Matthew F Funke wrote:
Quite a few. Even the humble IBM microdrive is rated to 3g non operating.
And to over 1000g for instantaneous shock impulse (according to their datasheet).
Kit designed carefully out of strong composites and titanium should not find 15g
steady acceleration that hard to survive. The problem comes if there is too much
turbulence and vibration.
The first "toy" vehicle designed for reentry and recovery were the spy satellite
film canisters for the Corona project in 1959 (first successful mission with film
recovered the next year).
The thing that really gets inbound objects in the Earth's atmosphere is the hot
shockwave in front of them. Hitting air at 24000mph generates a lot of frictional
heating and deceleration. Safe trajectories have to be just right - go in too
shallow and you bounce off like a stone skimming a pond, go in too steep and the
forward shockwave pressure build up and frictional heating destroys you.
> I acknowledge that rocks can be built to take the punishment, and so
> can specialty instruments. But the thread was discussing building probes
> like *toys*. You take one of those toys "built to take punishment" and
> just drop it out a twentieth-story window, and it won't seem to be built
> quite so tough.
You get a rather higher acceleration than 15g when it stops so suddenly.
15g continuous is bad, but not all that bad for hardware. Humans would not survive
long.
Regards,
Martin Brown
Matthew F Funke
>m...@hypatia.unh.edu (Matthew F Funke) wrote:
>
>>Jonathan Silverlight <jsi...@merseia.fsnet.co.uk> wrote:
>>>Matthew F Funke writes:
>>>> Consider an object coming in from "far away" to the Earth. When it
>>>>gets to the top of the atmosphere -- which we'll arbitrarily put up at 210
>>>>nautical miles (388920 m), the altitude at which the Space Shuttle orbits
>>>>-- it's screaming in at about 24,300 mph (10,900 m/s). The force
>>>>*backward* on the object would have to average about 15.4 gees all the way
>>>>down... which might not be the case for a given object.
>>>> In contrast, an object coming in from "far away" from the Moon is
>>>>going to be coming in at a paltry 5300 mph by the time it hits the
>>>>surface. (For reference, to slow from 24,300 mph to 5300 mph in Earth's
>>>>atmosphere would still require an average backwards force of about 14.7
>>>>gees.)
>
>Where are these numbers coming from? If your coming from far enough out, you'll
>be going essentially the same speed approaching the moon or the earth since it
>will be the gravity of the combined system drawing you in.
The numbers are coming from the escape velocities of both bodies --
which, because an equals sign is nice and symmetrical, is also the
velocity at which you'd hit the surface if dropped from infinity (or "far
enough away" to an engineer like myself <grin>).
Now, granted, I was ignoring the effects of the Earth when
considering the impact on the Moon and vice versa. However, at the Moon's
average orbital radius (384403 km), an object would only have an
additional 1440 m/s (3220 mph) imparted from Earth if dropped from
infinity -- provided Earth was in line with the fall and the acceleration
vector it added was completely tangential to the direction of travel.
Given the earlier escape velocity quoted for the Moon, and provided I can
add escape velocities like this (Can I? Could someone who knows better
please correct me if I'm wrong?), that still only adds up to an impact
velocity of 8520 mph, while Earth's (ignoring the Moon!) is still 24,300
mph.
Actually -- again assuming that escape velocities can be added -- the
*Moon's* escape velocity at the Moon's orbital radius is about 160 m/s
(360 mph), so that would add in to Earth's escape velocity (with the
Moon's full help) at 24,700 mph or thereabouts. (I'm rounding off to
reflect the fact that the numbers I used didn't have all that much
precision... only good to one part in a thousand or so.)
Matthew F Funke
Thanks for that. I'll go look them up straight away. :)
Matthew F Funke
>> built to withstand a shock like 15.4 gees (average)?
>
>Quite a few. Even the humble IBM microdrive is rated to 3g non operating.
>And to over 1000g for instantaneous shock impulse (according to their
>datasheet).
<nod> Granted. But careful using this as an example. A lot of
things can withstand a *much* higher instantaneous shock impulse than a
steady average load. The 15.4 gees I mentioned were not an instantaneous
shock impulse (which you address below, actually, so I guess I'm just
mentioning this to be pedantic).
>Kit designed carefully out of strong composites and titanium should not
>find 15g steady acceleration that hard to survive. The problem comes if
>there is too much turbulence and vibration.
In which case, there are much higher acceleration forces than 15.4
gees; I've heard those in mechanical engineering circles refer to the
"jerk", which is represented by the derivative of acceleration with
respect to time, when speaking of such things.
>The first "toy" vehicle designed for reentry and recovery were the spy
>satellite film canisters for the Corona project in 1959 (first successful
>mission with film recovered the next year).
Excellent cite of precedent!
My post was more to point out that designing something to withstand a
landing on a hard surface was much more challenging than designing a
remote-controlled toy (whom it was suggested NASA should take some advice
from when they design their probes). A previous poster pointed out that
the shock of hitting the ground was incredible already; I only meant to
point out that even *reentry* is difficult in its own right, before you
even hit the ground. Not as simple and powerful a point, but still...
>The thing that really gets inbound objects in the Earth's atmosphere is
>the hot shockwave in front of them. Hitting air at 24000mph generates a
>lot of frictional heating and deceleration. Safe trajectories have to be
>just right - go in too shallow and you bounce off like a stone skimming a
>pond, go in too steep and the forward shockwave pressure build up and
>frictional heating destroys you.
<nod> Yes, I know. But even those on "safe trajectories" have their
own difficulties to deal with, not the least of which is slowing down a
great deal before meeting the surface. Your average remote-controlled
buggy might be severely messed up by the forces involved just getting to
the surface, never mind the jostling when it gets there. (But then, I've
never actually tried to subject an RC buggy to 15.4 gees... I'm gonna
have to go find a Radio Shack for the sake of science.)
>> I acknowledge that rocks can be built to take the punishment, and so
>> can specialty instruments. But the thread was discussing building probes
>> like *toys*. You take one of those toys "built to take punishment" and
>> just drop it out a twentieth-story window, and it won't seem to be built
>> quite so tough.
>
>You get a rather higher acceleration than 15g when it stops so suddenly.
>15g continuous is bad, but not all that bad for hardware. Humans would
>not survive long.
Admittedly, the 20-story drop was a bad analogy for the continuous
force idea. My apologies for being murky.
I was under the impression that even hardware would be severely
punished if placed under a continuous 15-g load without special
consideration, and that toys were outside that level of consideration.
-=shrug=- Maybe not. It'd be interesting to try it, though.
ma...@the-light.com
> Wow, I hope not! Perhaps if the cosmonauts ejected first, that would be
>viable. Just think about how many gees it would take to stop that sucker! Now,
>if it was at the bottom of a parachute, that is another matter. In that case,
>it would work very nicely.
Obviously, there was a parachute as well. The feeler/rocket combo was the
counterpart of the ocean for the USA, I believe.
Henry Spencer
Matthew F Funke <m...@hypatia.unh.edu> wrote:
> Now, granted, I was ignoring the effects of the Earth when
>considering the impact on the Moon and vice versa. However, at the Moon's
>average orbital radius (384403 km), an object would only have an
>additional 1440 m/s (3220 mph) imparted from Earth if dropped from
>infinity -- provided Earth was in line with the fall and the acceleration
>vector it added was completely tangential to the direction of travel.
>Given the earlier escape velocity quoted for the Moon, and provided I can
>add escape velocities like this (Can I? Could someone who knows better
In fact, you can't. What a fall through a specific gravitational field
adds is a specific amount of *energy*, so to add the effects of two
fields, you have to add energies, not velocities. After some common
factors cancel out, what you do is not x+y but sqrt(x^2 + y^2).
The practical effects of this are that "adding" two similar velocities
gives a velocity that's larger than either but not as big as the sum,
while "adding" a small velocity to a big one gives a velocity only very
slightly larger than the big one.
> Actually -- again assuming that escape velocities can be added -- the
>*Moon's* escape velocity at the Moon's orbital radius is about 160 m/s
>(360 mph), so that would add in to Earth's escape velocity (with the
>Moon's full help) at 24,700 mph or thereabouts...
As per the above, the effect would be slight.
Matthew F Funke
>Matthew F Funke <m...@hypatia.unh.edu> wrote:
>> Now, granted, I was ignoring the effects of the Earth when
>>considering the impact on the Moon and vice versa. However, at the Moon's
>>average orbital radius (384403 km), an object would only have an
>>additional 1440 m/s (3220 mph) imparted from Earth if dropped from
>>infinity -- provided Earth was in line with the fall and the acceleration
>>vector it added was completely tangential to the direction of travel.
>>Given the earlier escape velocity quoted for the Moon, and provided I can
>>add escape velocities like this (Can I? Could someone who knows better
>>please correct me if I'm wrong?)...
>
>In fact, you can't. What a fall through a specific gravitational field
>adds is a specific amount of *energy*, so to add the effects of two
>fields, you have to add energies, not velocities. After some common
>factors cancel out, what you do is not x+y but sqrt(x^2 + y^2).
Ah. Of course. That makes sense. Thanks for the correction. I'll
reconsider the problem as one of reducing a pair of potential energies
(and, of course, gaining a net kinetic energy) as an exercise to confirm
this in my own head. :)