Physics Question: Here's something I've never understood: escape velocity. In order to break away from the earth's gravity and go into orbit, a rocket has to achieve an escape velocity...

jerryc41 Loc: Catskill Mts of NY

Here's something I've never understood: escape velocity. In order to break away from the earth's gravity and go into orbit, a rocket has to achieve an escape velocity of about 25,000 MPH. Gravity is strongest at the surface of the earth, and it diminishes as the rocket climbs higher. If a rocket can leave the surface traveling at a relatively low speed, why does it have to go so fast as it goes higher - in the realm of reduced gravity. Why couldn't the rocket just continue pushing until it gets into space?

There was a TV movie, Salvage-1, with Andy Griffith years ago that pushed this slow-but-steady idea. Instead of achieving escape velocity, his ship would just keep going and going until it got into space.

https://www.imdb.com/title/tt0078681/

Longshadow Loc: Audubon, PA, United States

Fuel usage vs. the time required to "escape" at a slower speed. LOTS of fuel.

Country Boy Loc: Beckley, WV

Jerry, While I don't have the knowledge to give the specific correct answer I would guess it has to do with fuel consumption. Similar to a car, driving at slower speeds takes more gas than when you get on the highway and can travel at higher speeds. Once you get the object moving at an increased speed, it takes less to maintain the speed.

jerryc41 Loc: Catskill Mts of NY

Country Boy wrote:

Jerry, While I don't have the knowledge to give the specific correct answer I would guess it has to do with fuel consumption. Similar to a car, driving at slower speeds takes more gas than when you get on the highway and can travel at higher speeds. Once you get the object moving at an increased speed, it takes less to maintain the speed.

That makes some sense, but driving my Honda Fit at 40 MPH on back roads will let me take a weekly thirty-mile trip and get 50 MPG. If I take the Interstate at 65 MPH for part of the trip, I average about 40 MPG. Lower speeds give me considerably better mileage.

jerryc41 Loc: Catskill Mts of NY

Longshadow wrote:

Fuel usage vs. the time required to "escape" at a slower speed. LOTS of fuel.

Good idea.

nospambob Loc: Edmond, Oklahoma

Country Boy wrote:

Jerry, While I don't have the knowledge to give the specific correct answer I would guess it has to do with fuel consumption. Similar to a car, driving at slower speeds takes more gas than when you get on the highway and can travel at higher speeds. Once you get the object moving at an increased speed, it takes less to maintain the speed.

Boy, sure do wish I could drive faster on less gasoline ….

Bob Mevis Loc: Plymouth, Indiana

Fuel consumption.

Country Boy Loc: Beckley, WV

Jerry, Not every car works like your Honda. Many vehicles at a higher rate have cylinders that no longer work saving gas and transmissions with extra high gears to take less fuel. However, the space ship has less resistance (gravity) the higher it goes so they in reality can use less fuel and maintain or gain speed. Your vehicle has constant or increased resistance (wind) as speeds change.

jeweler53

From Wikipedia
"A rocket moving out of a gravity well does not actually need to attain escape velocity to escape, but could achieve the same result (escape) at any speed with a suitable mode of propulsion and sufficient propellant to provide the accelerating force on the object to escape."

https://en.wikipedia.org/wiki/Escape_velocity

IR Jim Loc: St. Louis

At about 400 km (where the ISS orbits) gravity is about 90% (10% lower) than on the surface of earth. Basically the ISS along with all other satellites need to be traveling very fast (around 4 or 5 miles a sec) because in their orbits they are essentially free falling to earth but moving so fast they keep missing it.
When capsules, shuttles, or whatever need to return to earth they turn around and burn thrusters retrograde to slow them down so they fall and hit earth.

Another large factor is the amount of fuel needed to punch through the atmosphere. All that fuel adds weight and compounds the problem.

Just to clarify, there is still plenty of gravity in space where humans and satellites go.

birdman12 Loc: Pinopolis, SC

I think it has to do with the speed the rocket or object is going around the earth. If it is going less than the escape speed, there won't be enough centrifugal force to overcome the pull of gravity.
Barry

johngault007 Loc: Florida Panhandle

birdman12 wrote:

I think it has to do with the speed the rocket or object is going around the earth. If it is going less than the escape speed, there won't be enough centrifugal force to overcome the pull of gravity.
Barry

ELNikkor

Consider at take-off, the most fuel is being used by the boosters, yanking the heavy craft perpendicular to the pull of gravity. (Moves slowly straight up.) Then, as it angles east, speed really increases and the boosters drop off, so the lighter craft, now going at an angle to the pull of gravity spends less fuel, but goes much faster. At that point, what if they went straight for the moon, rather than circling the earth? They'd go back to perpendicular from the pull of gravity. I believe the answer must be in the angle, as circling the earth, high enough to where they are in the vacuum of space (no wind resistance), then increasing speed "slingshots" them out of orbit, and, "To the moon, Alice!"

FredCM Loc: Central Illinois

At the risk of showing my ignorance, how about Einstein explaining gravity as warped space, and time? And gravitons, has anyone found them yet?

johngault007 Loc: Florida Panhandle

ELNikkor wrote:

Consider at take-off, the most fuel is being used ... (show quote)

The angle is a combination of fuel efficiency and maintaining a rate of ascent for a specific initial orbit. As the atmosphere thins (10K feet and below is very thick) the energy required to maintain speed falls off, and with the change in angle, dramatically increases velocity. If the object was propelled straight up it would take an vastly large amount of energy to go straight up. Once the energy is stopped, gravity would pull the object back down, as the apoapsis would be very high, and the periapsis would not be in orbit, but somewhere on the Earth's surface.

The angle allows the object to reach a speed (depending on the orbit height and shape) to maintain what was mentioned before, a sort of free fall back to Earth, but where the arc of the object matches the curvature of the Earth, and therefore the object stays at a specific distance from the surface (like the ISS).

The escape velocity is the ground level relative speed that is required not to be pulled back down to Earth when no energy is released, and is separate from orbital velocity.