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?
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
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.
(replace "spambait" with "merlinus" to respond directly to me)
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.
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
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!
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.
Bob Kaplow NAR # 18L >>> To reply, there's no internet on Mars (yet)! <<<
Kaplow Klips & Baffle: http://nira-rocketry.org/Document/MayJun00.pdf
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!
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
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?!
"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!!!!!
Registered Linux user #328317 - SlackWare 10.2 (2.6.13)
On 1 May 2006 12:56:41 -0700, " email@example.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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