Rocket lift-off questions.

Escape velocity 11 km/s. But certainly rocket does not blast off with this speed. On tv, i see rocket slowly lifting upward. How much
distance it covers in initial 4-5 seconds?
And is it possible to blast off rocket with acceleration much slower than g? Will it fall or go upward?
Can we propel rocket or any body in upward direction with 'constant velocity', say just 5 centimeter per second?
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neo wrote:

Ignoring the significant practical matter of propelling a rocket at constant speed up from the Earth into space, yes a rocket could "escape" Earth's gravity going a constant 5 cm/sec.
Escape velocity is defined relative to a particular place in the gravitational field--typically the surface of the gravitational source. So 11 km/sec is the escape velocity from the Earth's surface. At a point further from the Earth the escape velocity is lower. At many thousands of km from Earth the escape velocity will be less than the 5 cm/sec of this example.
Escape velocity is also defined for an object under the influence of only gravity, no propulsion and no drag or other forces. In other words, an object is given enough kinetic energy to bring it up to the escape velocity and then left alone. The object in that case will slow down, coasting to rest at infinity (theoretically speaking). The rocket in your example is NOT left alone, but is continually propelled--with diminishing impulse--to keep it at the constant speed of 5 cm/sec.
It is better to think of "escape energy" rather than escape velocity. It sounds so easy to consider we could escape Earth's gravity just ambling along at 5 cm/sec. But the energy required is at least M*V*V/2, (V is the escape velocity, M is initial mass) regardless of how that energy is expended.
http://en.wikipedia.org/wiki/Escape_velocity
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Oh, there are a few things to keep in mind there:
First of all, 11 km/s is only the escape velocity at Earth's surface. The escape velocity goes down as you get further away, because gravity gets weaker as you get further away. Escape velocity is the minimum speed that you need to go away *and never come back* (unless, of course, you have rockets to turn around with).
Second, a lot of rockets don't achieve escape velocity. A lot of them simply go into orbit, which requires less speed than escape velocity. A satellite in orbit isn't in freefall because it's moved fast enough to escape Earth's pull; in fact, it's Earth's pull that's holding it in orbit! It's just that a satellite is moving fast enough *sideways* that while it falls, it never actually intersects the Earth's surface (the Earth's surface curves away too fast). As it turns out, for a given altitude, the escape velocity is always the square root of 2 times the circular orbit velocity.

Interesting question. Yes, you can accelerate much slower than one g upward. The astronauts inside will still feel more than one g no matter what, though, because any acceleration you generate during launch will be felt in addition to Earth's pull. If you can keep doing that for long enough, you'll eventually get to a point where Earth's gravity has "died off" enough where even your meager velocity will allow you to escape forever.

You'll have to accelerate it from rest to 5 cm/s, but once you've done that, there's no reason you couldn't eventually get to escape velocity this way, too. (The escape velocity of Earth is 5 cm/s when you're about 320 trillion km away from the Earth, if I've done my math right. That's over thirty light-years -- by that time, other gravitational fields will be much more predominant.)
Of course, in real life, this is somewhat impractical. Rocket engines during launch tend to have a set mass flowing out the back at a set velocity; the amount of *push* remains pretty constant, but the mass of the rocket is constantly decreasing. Therefore, as the rocket engines continue to fire, the rocket will accelerate more and more. But as a thought exercise to see how rockets and physics work, your 5 cm/s rocket is perfectly feasible.
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Hello Neo,
Here are your answers:
1) Rockets look slow because the observer/camera is a such a distance to make the angular displacement look small. If you were standing near the rocket, its motion would look much faster.
2)Distance covered depends upon the performace characteristics of the vehicle ... are you trying to launch ABMs to intercept ICBM/MERVS 40 miles above a city, or launching men and women into space? It all depends.
3)NO you can't get into space at an acceleration less than a g (9.8m/s^2), until you over come the retarding force due to gravity, you will just sit there on your pad buring fuel. Think about a ball held by a string. Fire a motor that provides 5cm/s^2 constant acceleration ... cut the string. You now have 9.8m/s^2 pulling the ball down and a far weaker 5cm/s^2 pushing upward. The ball will fall to Earth at a slighty slower speed than if you had simply droped it. You have in affect given the ball RETRO ROCKETS.
So check the mass:thrust ratio ... in order to see forward motion into the air, you have to AT LEAST provide enough force to cancel the gravity gravity force. Will an acceleration of (980 + 5)cm / s^2 get you into the air ... YES!
Salvage One tried and succeeded in reaching the moon with a slooooooow moving rocket ... but that is TV ... go figure. Just like the A-Team firing hundreds of rounds and no one gets hit ... STILL a great show ... both of them! I Pitty da fool who don't fly rockets!
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On Mon, 01 May 2006 12:56:41 -0700, snipped-for-privacy@juno.com wrote:

Hey! Joel Higgins *proved* the theory was sound. Remember? He drove Andy around the race track, in his convertible!
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There's actually a junkyard (OK, they call it 'architectural elements') here in chicago www.salvageone.com. It's a neat place for a Bob Vila like person to wander. When we built our house in 1989, we wandered the place looking for an antique fireplace mantle. We ended up finding one elsewhere, dating back to about 1900. Then we found someone to repair and refinish the thing whose become a friend ever since.
Actually, the Saturn V at least started off as the low acceleration for a long time flight profile. 1.1:1 Thrust:weight ratio at launch. The Shuttle at 3:1 leaps off the pad in comparison.
And the DC-X was in this low thrust realm as well.
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So there is a REAL salvage one junk yard .. that is great ... I wonder if they accepted or were offended by the TV show using their name?
Yes on the acceleration rates ... it just depends on what you are trying to accomplish ... it is intersting that for decades, UFOs were observed to be slow in flight ... then as we progressed with our aviation technology, UFOs suddenly started accelerating to Mach 6/7 and beyond in order to avoid our fighters and missiles.
I loved the DC-X idea ... but with gas prices climbing (not that the DC-X uses petrol) using a DC-X system would be quite expensive indeed!
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The TV show predates the company. I don't know if they licensed the name or what.
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Kaplow Klips & Baffle: http://nira-rocketry.org/Document/MayJun00.pdf
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Greg Heilers wrote:

OMFG ... I am not the only one who remembers Salvage One! That show was on for what ... 5 or 6 episodes?! They would cover their rocket with a tarp, and they always had a government FBI/NSA guy spying on them to stop their rocketry! It would have been a fantastic show if they had Don Knotts teamed up with Andy on the Salvage One crew! The Reluctant Astronaut meets Get Smart!
Yes, I remember the race track demonstration ... the car crept along, but after an hour it was ZOOOOOOMING around the track.
Ya know, NASA has an ION drive space craft that has been doing the SAME thing for the last couple of years now!
Where have all the GOOD shows gone; Cliff Hangers, Man from Atlantis, The Invaders, The Prisoner, Police Squad, oh man ... too much time wasted!
What was that show that had the intergalactic garbage man (comedy and NOT British!) ... Gort or something like that ... did I just DREAM that show up?!
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snipped-for-privacy@juno.com wrote:

Nope, you're not. I still have an old video tape of the pilot movie that I occasionally watch during long sanding sessions.

Are you talking about Quark, starring Richard Benjamin? It had a wise-ass computer parodying HAL, I believe, too. I loved that show, short-lived as it was.
...Rick
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Lots of us remember it we're just not going to admit it. ; )
Seems I also remember another show from that time period called Quark that introduced Judy Landers to the world.
Randy www.vernarockets.com
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On 2nd thought maybe she was in Salvage I too.
Randy www.vernarockets.com

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MMMMMMMMMMMMMMMMM.....................Joooooooooooooooooooody!
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On Mon, 01 May 2006 17:59:46 -0500, Randy wrote:

"Quark" starred Richard Benjamin; and featured the late, great Hans Conreid as the voice of "The Source". Did Judy Landers make her introduction here? I remember her on Robert Urich's "Vegas"...but she was completely overshadowed by the most incredibly sexy "older woman" of all time: Phyllis Davis!!!!!
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Right on all counts! Phyllis Davis was incredible.
Randy www.vernarockets.com
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On Mon, 01 May 2006 14:22:31 -0700, snipped-for-privacy@juno.com wrote:

Actually, I believe it made it into its second season.
They would cover their rocket

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Quark
snipped-for-privacy@juno.com wrote:

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On 1 May 2006 12:56:41 -0700, " snipped-for-privacy@juno.com"

You need thrust greater than the equivalent of g, but your acceleration will be thrust/mass-g--so a thrust equivalent to, say, 1.1g would produce acceleration of .1g. As long as your acceleration is positive, you'll go up (or as long as it's 0, you'll keep going up if you were already headed that direction).
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snipped-for-privacy@juno.com wrote:

Here's a tricky photo of a slow moving rocket:
http://spaceflightnow.com/atlas/av008/launch/a09.jpg
We *all* know that this rocket is pinwheeling like mad... :-)
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