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Total Members Voted: 10

Azure Priest on August 03, 2010, 07:49:32 am
I'm curious, if they're planning to use the mass driver as a makeshift rocket, how do they plan to turn it? It needs to be turned around for the deceleration on final approach if nothing else.

J Thomas on August 03, 2010, 07:57:03 am
I'm curious, if they're planning to use the mass driver as a makeshift rocket, how do they plan to turn it? It needs to be turned around for the deceleration on final approach if nothing else.

That isn't a problem because of conservation of angular momentum. If something rotates one way, something else has to rotate the other way. So if Bert and Ernie grab the mass driver and try to push against it to turn themselves, it will turn too.


Brugle on August 03, 2010, 09:33:39 am
And it is the one way for absolutely private communication. 
Not necessarily.  A laser beam reflected off a helmet might pick up the conversation, just as a laser beam reflected off a window can pick up conversations of people on the other side.

I wouldn't expect that to happen in this situation, but you did say "absolutely".

rclocher3 on August 03, 2010, 11:51:03 am
Anyway, I suspect the story about QRP on NASA moon suit radios is apocryphal.  Assuming those were radiating 250 mW omnidirectionally (unlikely; they probably actually had a very strong bias toward horizontal with very little vertical radiation -- and only vertical would be pointed anywhere near Earth when the astronauts are, as all Apollo missions were, within 45 of the geo-pole at the center of nearside), the effective power at Earth's distance would be approximately 8 nanowatts.  In practice, given a reasonable antenna design for suit-to-suit and suit-to-LEM communication, you'd need to trim that figure by another factor of at least twenty, giving about 400 picowatts.  That's certainly possible to detect -- NASA picks up a more attenuated signal than that from the one still-functioning Voyager spacecraft -- but the antenna required to do it with real-world detectors would be bigger than a 1980s vintage satellite TV dish by several times (NASA uses 20 meter and larger dishes to communicate with outer planets missions).

Nobody told Larry Baysinger that intercepting Apollo 11 suit radio communications from Earth was impossible back in July 1969, and incredibly, he managed to pull it off.  Proof has just recently come to light: http://www.arrl.org/eavesdropping-on-apollo-11 .  What a great story!

Rocketman on August 03, 2010, 12:50:33 pm
Brugle your right.  Didn't think of that.  You got me.   ;D

ZeissIkon on August 03, 2010, 06:38:56 pm
Given the distance between Ida and Dactyl, it's tempting to wonder why Bert and Ernie's suit jet packs aren't up to the job;

While the two asteroids may LOOK stationary relative to each other, they are moving at very high rates of speed. Even if the suit packs had sufficient fuel, which they don't, it would take a decaday or more to complete the trip given the proper acceleration and deceleration requirements.

Ummm...

Very high rates of speed?  Really?  Yes, they're booking around the sun at something like half a kilometer per second, but that velocity doesn't matter for getting from one to the other (any more than the Earth's velocity in Solar orbit matters for launching a satellite).  What matters is the orbital speed of Dactyl around Ida, as well as their separation, both of which are pretty small.

"Dactyl's orbital period is about 20 hours" and "its orbital speed is roughly 10 m/s".  http://en.wikipedia.org/wiki/243_Ida  The same article gives their separation as 90 km, only about 3x the average diameter of Ida.  With Ida's rotation at about 4.63 hours, you could just about run into orbit from the "ends" (Ida is shaped roughly like a russet potato), if you run "with" the rotation and have some way to keep your feet planted long enough to sprint fifty meters.  Assuming a Hohmann orbit from surface-grazing to Dactyl, the half-period must be less than ten hours, probably under eight; if you take longer than that, you've over jumped.  Suit jets ought to easily make up  less than 10 m/s  to circularize at apo-Ida -- and if they have enough fuel to be worth wearing (and hadn't already been partially depleted by doing actual work on Ida's surface), should be able to handle the launch, too (which should only require a delta-V of, at most, about 20 m/s).  Matching with Dactyl, you can just about ignore Dactyl's gravity -- just not enough mass there to matter unless you're falling in from Solar orbit.

IOW, the only reason Bert and Ernie even need to consider the mass driver is because they've partially or mostly depleted their rocket packs getting their excavation set up.

BTW,  for most of us, the place we read about astronauts being able to talk by touching helmets was probably Have Space Suit, Will Travel, by Robert A. Heinlein.

Rocketman on August 03, 2010, 09:46:29 pm
BTW,  for most of us, the place we read about astronauts being able to talk by touching helmets was probably Have Space Suit, Will Travel, by Robert A. Heinlein.
I don't think I've ever read that particular Heinlein story and I started the question.  I guess great minds think alike.  ;)

terry_freeman on August 03, 2010, 10:26:47 pm
I have previously heard of the idea of touching helmets together, but have not read Have Space Suit, Will Travel - though I have probably read almost all the rest of Heinlein's oeuvre.

Unfortunately, I cannot recall when I first heard of the idea. 

wdg3rd on August 04, 2010, 01:02:08 am
OK, any who haven't read Have Space Suit Will Travel, go buy a copy (unless by slim chance you have a local library with a copy).  It's one of the better juveniles.  (Of course, it is now [well for at least a quarter century] impossible to find a Heinlein juvenile in the juvenile [or the adjacent "Young Adult"] section of any "public" library).
Ward Griffiths        wdg3rd@aol.com

Men will never be free until the last king is strangled with the entrails of the last priest.  --  Denis Diderot

Sio on August 04, 2010, 08:42:49 am
QRP is almost like the virology of radio.  It deals with the very, very small, so small you wouldn't think it could do much of anything, yet it is effective at distances that are downright amazing.  It's easy to put up huge stations that crank out so much power that you can hear them six blocks away by holding a piece of pyrite in your teeth (joking).  What's hard is making yourself heard with a tiny, one-transistor transceiver powered by a single battery, with an antenna made of wire thrown into a tree by hand with a rock at the far end.

Yes, you're right, nowadays most amateurs don't know Morse -- they got rid of the requirement to know it to get the license, so most new hams don't bother.  Some still do because it's fun, and like you said, it's the best propagation method under the most adverse conditions.  Though some of the digital methods, such as PSK-31, are starting to catch up.  I've got a kit for an 80-meter PSK-31 transceiver called a "Warbler" that only puts out 3 watts max.  A conversation of 300 miles is not unreasonable for this device, when paired with a computer soundcard for encoding and decoding. (http://www.njqrp.org/warbler/index.html)  Heck, there are even programs for translating morse code using a sound card, and converting typed input into keyer output.  Cheating, but hey, it gets you that wonderful propagation.

Interesting idea...speech recognition, converts to text, then gets sent by morse code, then reverse it into a speech synthesizer at the other end.  Rube Goldberg would be proud!

I don't think the bad guys would know how to listen in once the boys reduced their power.  These aren't rocket scientists -- if they were, they'd be working instead of stealing, and wearing something other than cheap rental suits.

---Sio, extra-class amateur radio operator

Azure Priest on August 04, 2010, 09:18:56 am
Given the distance between Ida and Dactyl, it's tempting to wonder why Bert and Ernie's suit jet packs aren't up to the job;

While the two asteroids may LOOK stationary relative to each other, they are moving at very high rates of speed. Even if the suit packs had sufficient fuel, which they don't, it would take a decaday or more to complete the trip given the proper acceleration and deceleration requirements.

Ummm...

Very high rates of speed?  Really?  Yes, they're booking around the sun at something like half a kilometer per second, but that velocity doesn't matter for getting from one to the other (any more than the Earth's velocity in Solar orbit matters for launching a satellite).  What matters is the orbital speed of Dactyl around Ida, as well as their separation, both of which are pretty small.

"Dactyl's orbital period is about 20 hours" and "its orbital speed is roughly 10 m/s".  <snip> Suit jets ought to easily make up  less than 10 m/s  to circularize at apo-Ida/


Shouldn't the suite's be going MORE than 10 m/s? If not they're not going to "catch up" to the asteroid that's rapidly fleeing from the would be space walker.
-- and if they have enough fuel to be worth wearing (and hadn't already been partially depleted by doing actual work on Ida's surface), should be able to handle the launch, too (which should only require a delta-V of, at most, about 20 m/s).  Matching with Dactyl, you can just about ignore Dactyl's gravity -- just not enough mass there to matter unless you're falling in from Solar orbit.

Wait, didn't you just say the suits needed less than 10 or they'd "over jump?" And a rocket pack going 20 m/s in micro gravity..... SPLAT!

IOW, the only reason Bert and Ernie even need to consider the mass driver is because they've partially or mostly depleted their rocket packs getting their excavation set up.

No, it's NOT the "only" reason. As Bert pointed out, even if they had enough fuel, the delta-V is SO MUCH SMALLER than the orbital rate, that it would take a decaday to complete the trip, much more time than the available "consumables" in their suits. When talking about speeds per second, please multiply the number by 3600 for the per hour speed and you'll quickly see just how fast a "mere" 20 m/s velocity is. (Note it's approx 40 mph. That's highway speeds. Unless Bert and Ernie are part cheetah, they certainly can't run that fast especially with those cumbersome suits.)

ZeissIkon on August 04, 2010, 08:10:08 pm
No, it's NOT the "only" reason. As Bert pointed out, even if they had enough fuel, the delta-V is SO MUCH SMALLER than the orbital rate, that it would take a decaday to complete the trip, much more time than the available "consumables" in their suits. When talking about speeds per second, please multiply the number by 3600 for the per hour speed and you'll quickly see just how fast a "mere" 20 m/s velocity is. (Note it's approx 40 mph. That's highway speeds. Unless Bert and Ernie are part cheetah, they certainly can't run that fast especially with those cumbersome suits.)

Clearly, you don't understand orbital mechanics well enough to have followed a couple places where I compressed some steps.  Allow me to expand.  Ida's rotation period of 4.36 hours, and long axis 53.6 km give a rotation speed at the "tips" of about 17 km/hr (like any free-spinning body, over time the rotation has tumbled to maximize angular momentum -- that is, Ida tumbles like a football flying end over end after a kickoff, not like a spiral forward pass), or about 4.7 m/s.  These "tips" are almost a third as far from the center of mass as Dactyl is, so orbital speed there is something like twice what it is at Dactyl's distance (very rough approximation) -- that is, Ida supplies about a quarter of the needed velocity to get into a surface-grazing orbit.  A good sprint is around 10 m/s, perhaps a little more for forty of fifty meters (as opposed to a hundred).  If the suit jets can't manage the remaining 5 m/s pretty promptly, they're pretty much useless (even for self-rescue in case you come unmoored from your ship unexpectedly).  You might also notice I didn't say they could sprint that fast in a pressure suit, only that one could almost sprint into orbit -- looks to me like a good sprinter has 3/4 of orbital velocity, barring suit encumbrance and the much larger problem of being unable to keep feet planted in a couple milligee field.

Now, a Hohmann orbit is a minimum-energy transfer from one orbit (say, the surface-grazing one at 20 m/s and about 23.8 km radius) to another (say, Dactyl's, at around 90 km radius and 10 m/s).  One characteristic of this type of orbit is that it is exactly tangent to the other two orbits at its inner and outer points (periidea and apoidea, in this case), and for low-eccentricity orbits, the Hohmann transfer has a period very close to half the sum of the two parent orbit periods (so, a Hohmann transfer from Earth orbit to Mars orbit would have a period of almost exactly 18 months, thus taking nine months for either the inward or outward leg).  Once Bert and Ernie have run/jumped/jetted into a surface-grazing orbit, the delta-V required to make a Hohmann transfer to Dactyl's orbit is small (should be on the close order of 5 m/s), and the wait time can't be any longer than half Dactyl's orbital period -- hence less than ten hours.  Once they reach Dactyl's orbit, they match velocities with a circularizing burn, which can't require as much delta-V as Dactyl's orbit speed, because they already have some velocity from their own orbit -- hence, they need add less than 10 m/s.  Note that their "parking" time, waiting to be in the correct position for trans-Dactyl insertion, need not exceed one orbit, or at most around nine hours, so they'll spend less than twenty hours from the time they reach one of Ida's tips (hiking thirty or so km in effective weightlessness should be no big strain, nor take a long time) until they have to worry about terminal maneuvering at Dactyl.

Now, notice that even if Bert and Ernie don't jump or run at all to start, the largest single maneuver their suit jets are required to make is about 15.3 m/s to go from one of Ida's tips to orbit, with a following one of (I think, it's been 35 years since I did the math for this transfer) less than 5 m/s and a circ burn the same as the insertion burn.  This could all be greatly complicated, of course, if Dactyl's orbital plane is highly inclined relative to Ida's rotation.   Fortunately for Bert and Ernie, Dacty orbits prograde and is only inclined about 8 -- there'll be more error in their burns than the correction needed to change inclination by that amount, unless Bert can find a way to let his ock directly control their suit jets (unlikely, if it doesn't have bluetooth).

Mind you, the mass driver is much more interesting, as a story, than simply using suit jets to go retrieve their shuttle and ship, and much better at giving them a weapon effective against ships when they get to Dactyl -- but I say again, if those rocket packs are good enough for tween-ship maneuvering and such, and have enough propellant to be a good match for a suit that can recycle for multiple days, the only reason the rocket packs couldn't be used for the trip is if they're already depleted from setting up the excavator and mass driver.

quadibloc on August 04, 2010, 10:26:24 pm
Ah. Since Ida is in orbit around Dactyl, the delta-V values involved are unusually low. I hadn't taken the time to look that up, but now that I'm aware of it, I can think of a plausible reason why the rocket packs are not adequate. Even if they aren't depleted, using them would require a course that sticks to the minimum delta-V. So they would be moving slowly for hours, giving the other fellow plenty of time to notice them and shoot at them. Using the mass driver might let them arrive quickly, with less chance of being noticed.

Of course, that plan really only works if their suit radio communications had not been intercepted, since if they know to look for the mass driver, it's much easier to see.

SandySandfort on August 04, 2010, 11:02:50 pm
Ah. Since Ida is in orbit around Dactyl...

Nope, little Dactyl orbits around the much larger Ida. Beyond that, these a just piddly-ass little gas jets. They are intend for space-walk-like missions, not long travel (in the delta-V sense). Remember, these guys were using them all day before they discovered that their shuttle was gone.

ZeissIkon on August 05, 2010, 04:21:01 pm
Ah. Since Ida is in orbit around Dactyl...

Nope, little Dactyl orbits around the much larger Ida. Beyond that, these a just piddly-ass little gas jets. They are intend for space-walk-like missions, not long travel (in the delta-V sense). Remember, these guys were using them all day before they discovered that their shuttle was gone.

Exactly -- even though this is a job well within the ability of a vintage 2000 NASA MMU, Bert and Ernie have been shoving stuff around with their jet packs for long enough to get pretty tired, even before they started.  And they'll still probably want the ability to chuck a few high velocity rocks when they get to Dactyl.

Mind you, I was wondering if the impact of the first couple rocks wasn't going to cause some trouble at the mass driver's exit -- even if the rocks only exit at a couple hundred meters per second (far above Ida-Dactyl system escape velocity), one rock hitting another at that speed will tend to shatter and spray fragments in random directions.  Maybe a carbon nanotube construction is strong enough that doesn't matter, but I bet that first jump was a little rough...

 

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