posted
I have a piece that I've worked on several times over the years. I have one minor point that I am curious of and am not sure where to look for the information.
My character dives out of his craft as it is breaking up. He has no trust in his equipment and figures he is going to land hard even if everything works right. He plans to land in a small lake that is not directly below him. He is using old retro rockets to land instead of a parachute. The water will snuff out expected fires when he rocket pack fails mizerably.
My question, is that during free-fall, how far can one travel horizontally, while falling. Is it a few yards or a few hundred yards per thousand feet? The farther the better. How much is plausible for a given distance?
posted
The answer to that question will depend on a lot of things, but mostly, on the horizontal component of his momentum when he exits the craft. If he jumps out at 200mph, say, then he'll have a lot of momentum; he'll lost that speed quickly through air resistance, but it'll still make a big difference.
Height when he bails is also a factor. And so is whether he's got anything he can use as a wind deflector (think sky-surfing), because if so, he can use that to transmit some of his downward momentum into horizontal.
Plus, of course, if he has retro-rockets (on him, not on the craft), than he may be able to use them to actually give him nudges in the right direction. Depends how controllable they are.
posted
Does he know what he's doing? Does he understand how to use his body's shape as a wind foil?
An experienced Freefaller in the proper clothing can travel further horizontally than vertically.
Now, given that you are using rocket packs, I think the discussion is moot. You are already assuming an invalid technology, so you can pretty much make up the rules.
The jetpacks of the forties, fifties and sixties could lift a man off the ground, so they theoretically could stop his vertical fall at which point horizontal movement is infinite in comparison. However, once one of those jetpacks lost its verticality, they lost significant degrees of control.
posted
If there were no air resistance, and he were going (say) 3 miles per minute when he bailed, he'd continue at that rate till he hit the ground (and died).
I don't know how quickly air resistance stops you. Thing is, once the parachute's open, it *must* slow you down to a (vertical) velocity at which you can survive hitting the ground -- though you may break a leg -- so it would also have to slow you down to that velocity horizontally.
posted
The soviet space capsils used to use some small jets on their parachutes to slow them at the last instant for a softer landing.
My idea here is the character dives out of a escape capsule. It is falling apart because it is well beyond its useful life. Because of the conditions of everything else in the capsule, he knows the breaking jets of the jet pack is not going to operate for very long. he intends to activate them as low as he dares, and land in the water to snuff out the heat and fire of the jets. I think he has seen someone else dive out and burn up after a short couple minutes.
Anyway, he dives out, and spots the lake or ocean shore and tries to direct himself to that point. It is not until he is past the water's edge that he activates the jet pack. It slows him down enough, almost to a stop, to where he unhooks the jet pack and drops into the water. The jet pack explodes, the water protecting him. The idea though is that he freefalls with as much horizontal motion as he can get. The idea is there is extra excitement by his not being sure if he can make the distance to get to safety.
The image I was thinking of, and he dove out just at the edge of breathable air, is that his target is off in the distance ahead or to the side of his fall.
[This message has been edited by rstegman (edited May 04, 2006).]
posted
You'd pretty much have to see it below you. The horizon from 5 miles up (about where you described him ejecting) is several hundred miles away.
Without a proper sky diving outfit, he's not aiming far and aiming for a body of water directly below him would still be an impressive trick. The wind at ground level may not even be blowing the same direction as the wind 5 miles up.
He'd also have to really know the jetpack equipment. Turning it on full burn Might stop him too quickly and could be the same as hitting the ground at 125 mph.
But, you are writing fiction, make it as dramatic as it needs to be.
posted
Just for those curious, the distance to the "Horizon" is equal to:
y=1.17*sqrt(x)
Where x is your height in feet and y is nautical miles.
Plugging in 5 miles up, that makes the horizon about 190 miles away. If you're going for something semibelievable, I think that at the outset, you'd want nothing reachable over 1 mile. That means that it'd have to be almost straight down or a little off center from where he's falling.
All this, of course, negates wind velocities at the different levels of altitude, further, it doesn't take into account any other issues that might be faced by a 5 mile bailout. As you are writing fiction, your readers wouldn't notice this if you make the rest of the fall interesting. Dan Brown had a guy fall from a few hundred feet into a river at night without a parachute and live. It was riveting, and nobody really questions its possibility, they just remark that Langdon was lucky as all get out.
If you tell it convincingly, no matter how false/impossible, people will believe it because you are convincing.
posted
A freefall from about sixty feet will shatter bone like glass, even with a parachute (which cannot open fully in sixty feet, much less slow one down enough to survive impact without serious injuries).
They say the best possible way to land is on your back, oddly enough...the body should bend naturally, like a hinge, absorbing much of the impact energy. I'd still expact major injuries, though.
Hitting water at any speed over 15mph (give or take a few units) is very much like hitting concrete at the same speed. You need to calculate how long the rocket pack will burn to know at which altitude the character must fire it to survive. I'd say between 100 and 180 feet up, if the pack exactly simulates a parachute's velocity reduction.
This is all guesswork for the most part. Just make sure he doesn't lock his knees on impact (assuming he doesn't land back-first, but feet-first) or he'll shatter the knee joints. Also, consider the design of the rocket carefully...you wouldn't want something of excessive mass strapped to your body when both hit the ground (water) at relatively high speed.
Inkwell ----------------- "The difference between a writer and someone who says they want to write is merely the width of a postage stamp." -Anonymous
[This message has been edited by Inkwell (edited May 04, 2006).]
First is whether the guy is an experienced skydiver. No matter how you look at it, this is a very high level jitsu he's pulling. Basically an impossiblity for your everday Joe.
Looking over your last post, it should be an impossibility for anyone who isn't a registered pre-cog as well. How does he know exactly when the rocket pack will explode? Also, why bother to protect himself from the explosion when he's obviously has to be fireproof to survive the detachment maneuver? Oh, and he has to be capable of controlling his mid-air momentum without the jetpack, but who cares about a little thing like that.
Humans (tough humans) can survive falls of about 120 feet onto gravel or sand. They can also die by tripping over their own feet while walking across a meadow. They cannot do anything like what you describe in your last post, and that's that. Scale it back.
posted
Suvivor, I completely agree that it would most likely be completely and 100% impossible to pull off. However, what I think he is asking for is how to make it sound believable and encorporate a little bit of science, not asking if it is possible.
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posted
So, from what I understand, that when a person dives out of the airplane, he has little travel ability in freefall other than the forward momentum given by the craft he is in. Diving out at ten thousand feet (two miles) would give him at most,from the way it sounds, about a quarter mile variance in choosing where he will come down. Basically, forget about aiming during the ree fall.
The actual landing part will require some skill in writing, I do admit that. The main thing I am after is that he gets out of the lake with nothing but a knife and the clothing on his back, and there is nothing that can be gathered, even small debris, to help him. Having the jet pack explode above him as he enters the water of a shallow lake was one way of doing this.
posted
I can think of plausible ways that your scene can happen. All it takes is imagination.
First, the rockets could have some sort of electornic monitoring of their internals. This could be as simple as a teperature sensor in a critical spot or as complex as a self-aware computer that controls their firing. For story purposes the system cannot stop the rockets from failing, but it can give the human the "pre-cog" information he needs to choose the moment to release the backpack.
Second, the human character could be trained for this. It sounds like he is an astronaut, so it seems reasonable that he is an experienced skydiver. With proper training, a skydiver at terminal velocity can achieve significant horizontal movement and control. This won't take him to the horizon, but it could take him a few miles.
Third, have him land in the softest water you can find. Living on the Great Lakes I have more experience with fresh water than salt water, and I think hitting fresh water is a gentler impact. Making the water warm might further soften the blow. Can someone with experience diving into salt water weigh in here?
Problems:
To move horizontally, he needs to use the areodynamics of his body, and if the rocket pack is bulky this won't work. You need to make it a small, slender, lightweight rocket pack. Stubby wings (ala Buzz Lightyear) or a cape (ala Dracula) would help.
Also, his body must be horizonal, head slightly down, to achieve horizontal control. But it must be vertical, head up, for his final breaking maneuver. How will he achieve this? It's probably possible, but I know I couldn't do it.
posted
One more thing: if the rocket pack is simply falling, then both astronaut and rocket pack will hit the water at the same time, and pretty close together.
You could make the rocket pack gyroscopically stabilized. When the astronaut unstraps himself he it vertical, and the rockets will continue to point upward. This means they will shoot straight up before exploding. That would look cool.
posted
Or just substitute the rocket pack with an antigrav harness that drains its power cell completely slowing him down (justify it by saying the unit was never designed to be used above 5,000 feet, it's old, your character's obese, etc.). That way he'll hit the water at a relatively low velocity, yet cannot use the harness for survival improvisation...as per your desired plot element.
If that's too advanced tech-wise, just give him a pack that deploys a glider system of some kind, then have its memory-composite wings damaged by fragments when the craft above explodes (you could have your character injured by the 'shrapnel,' if you really wanted to stack the odds against him).
Despite the damage, he could glide even closer to the horizon than simple freefall before descending over a lake...at which time he could lose control of the glider (for the inevitable splashdown) due to freak air currents, or something. I dunno...the possibilities are virtually endless. As long as you have a good reason for dumping him into thin air, go with what sounds moderately believable. Just stay away from Eraser as a reference. Ah-nuld woulda splintered every bone in his body and juiced his organs into mush.
Another thing to consider is that some humans have the tendency of developing hypoxia above 8,000 feet, and it can be lethal. If he's jumping at 10k or higher, he might need an oxygen bottle and mask. Oh yeah...the temperatures are also sub-zero at high altitudes. Your character definitely doesn't want to be a man-sized ice cube cracking in that nice, warm lake.
Inkwell ------------------ "The difference between a writer and someone who says they want to write is merely the width of a postage stamp." -Anonymous
posted
Cut the rocket pack out altogether. He falls a few thousand feet and survives by diving into the water. Problem solved.
Okay, you might not think it's very realistic, but at least we got rid of the damn rocket pack. And it's not less realistic than what you had.
Or here's an idea, give him a parachute but have it catch on fire or something. He lands in the water and has to take the thing off so he can avoid drowning. Or he lands in or near a brushfire and doesn't have time to do anything but unstrap and run.
If you want to seriously soften the water, you need to introduce bubbles somehow. I don't know what kind of guy aims to land in a bubbling hot spring or waterfall pool, and I don't think it's necessary if he has most of a parachute (albeit one that is on fire), but hey!
Just lose the exploding rocketpack. A non-exploding rocketpack I'll buy.
posted
Or you could have him do something like this. I think it would be a lot of fun to write a scene like that, especially if the guy was improvising as he went.
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posted
The trouble with bubbles and water, is that bubbles are made of air. If you've ever gone white-water rafting before, you know the one spot you do NOT want to get stuck in, is a spot where the river is churning up a lot of bubbly froth. Why not? You'll sink to the bottom like a rock and most likely be stuck there.
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posted
Perfect! One problem substituted with another...
Back to a different topic that rstegman didn't ask about but did mention: just a couple of inches of water is probably sufficient to protect your guy from shrapnel. Mythbusters did a segment on this with bullets, and of course, they know everything.
I don't know how well bubbly water would protect him, though.
posted
It wouldn't. The point of bubbles in the water is that they reduce the tendancy of water to slow things down. This keeps impact with the water from killing you (though the drowning problem Elan mentioned is a concern), but it also keeps impact with the water from stopping shrapnel very effectively.
It takes more than a couple of inches of water to stop bullets, though. More like a couple of feet.
My point is that if you don't want the guy to have a rocketpack, just don't give him one in the first place. Give him a small emergency parachute, designed for single use. Make the landing bad enough to ruin it totally, or have him land in water so that he has to cut it apart to avoid drowning.
You're a writer. That means that the audience must imagine what you propose. If you propose things that are idiotic in concept...you don't have radical visuals or jazzy music to come to the rescue, you've just got the concept, sitting there imprinted on the page.
posted
Doc Brown, I liked that video. That was fun.
My thoughts is that, at least on TV, some devices will start to hum or squeel at ever increasing tones leading up to the moment of the explosion. The lift pack would be stabilized and designed for people who never seen one before. He stayed with the craft long enough to see others who bailed out, and used the lift packs from the begining, explode. He knows his will fail. He freefalls until he feels he has to use the jet pack. The tones rise. Just before he is sure it will explode, he releases the harness and falls into the water. The lift pack could have some form of nuclear power, or something more powerful, and the water protects him from the worst of the energy too. The main thing is to get him on the ground with essentually nothing. ONe thing I could do is have him coming down over the ocean and try to get close to the beach.
posted
Back in the 80's there used to be these shows that were like Reader's Digest. One was called "That's Incredible" and then there was "Real People." I think in one they featured a skydiver whose chute failed but landed in a fish pond and lived. I also know a guy who survived a failed chute by landing in a pine tree... he lost his two front teeth... but this was in a military training exercise where they are dropping you from the lowest altitude at which a chute will have any effect.
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posted
If the main thing is to get him to the ground with essentially nothing, then give him an ordinary parachute and have him lose it to the water or a brushfire or something.
I don't know why you're so resistant to this idea, you haven't mentioned any objection to it. But the exploding rocketpacks are just plain stupid. If you want to necessitate a water landing, you could have it so that the local gravity is too high or the atmosphere is too thin. Or you could have the chutes be a little defective (though all of them being defective seems a bit unlikely, I suppose that you're positing sabotage).
But the exploding rocketpacks...as you describe them, they are just not cutting it with me. Why are you really so set on them? If you tell me, then perhaps I can accomodate you.
A person has a terminal velocity of about 200 mph when balled up and about 125 mph with arms and feet fully extended to catch the wind.
"However, by diving or "standing up"in free fall, any experienced skydiver can learn to reach speeds of over 160-180MPH. Speeds of over 200MPH require significant practice to achieve. The record free fall speed, done without any special equipment, is 321MPH. Obviously, it is desirable to slow back down to 110MPH before parachute opening."
The terminal velocity of a skydiver is about 124 mph (200 kph)
posted
I actually think the original concept can be saved. Add temperature gauge/alarm to jetpack, have character cut out of the harness while the temp alarm is going off and while he is over a lake. This takes out the problem of going vertical to get out of the jet harness. I also want to know how many people posting in this topic took physics. A parachute slowing the horizontal movement, yeesh (okay it would but not to a large extent, and winds be more your competitor than the parachute). If you drive a car over a cliff the distance you travel horizontally depends on the hieght of the fall (how much time you have to move) and the speed you at which you left the cliff. Distance=Speed*Time
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quote:I also want to know how many people posting in this topic took physics.
I did. I guess this wasn't the topic where I expanded on some of the wacky physics fun. Oh, yeah, that was in "It really Happened!" Another thing my physics teacher did as a boy scout was ride in the back of a pickup truck and stand behind the cab with a blanket stretched between the arms as a sail. You jump and your buddies at the tailgate catch you. I think the only other real "How is this man still alive?" story was when someone shot an arrow straight into the sky and when it came back down they suddenly, intuitively, grasped the return acceleration princple and ran like hell. I think it buried itself like 9" in the dirt.
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