How fast to enter orbit

Paul Reilly, London UK You are right, but in order to "keep going up" on sustained power you'd have to carry immense reserves of fuel to push you up. When Isaac Newton first described the concept of escape velocity he was thinking of a ball shot from a cannon.

And modern rockets are much like that, their fuel tanks can only push them up for a few minutes and from then on they have rely on their own inertia. If they don't reach escape velocity by then, they fall back or remain in orbit. Alejandro Pareja, Madrid Spain True, in theory any speed would do. But in practice the difficulty comes from the trade-off between the rate at which you escape the Earth's gravity and the rate at which you consume the fuel needed to do so - and hence the fuel needed at take-off.

If you ascended at 5mph, but had the typical fuel capacity of the shuttle and would be largely used up in 10 to 15 minutes, you'd only get up a couple of miles. Martin, Manchester UK Escape Velocity is the minimal velocity required to escape earth's gravity without applying any further force.

In this case, when any speed is maintained continuously a force is constantly applied. So the key point is the Force applied. Shravan Singh, Kolkata India An unpowered projectile needs escape velocity to travel arbitrarily high but a continually powered vehicle does not, of course. When the vehicle is in orbit above the atmosphere almost any force will do to push it further out - hence the ability of puny ion drives to push craft into higher orbits. It's getting into orbit that takes all the power.

A grand space elevator is proposed to take the craft out to where the orbital speed is the rotation speed of the Earth's surface, so doing away with the flashy rocketry. Don't hold your breath. However, you are quite right that, if you have means of propulsion, such as a rocket ship, you can travel as slowly away from the surface as you like. Michael Hall, Canberra Australia A stone thrown upwards would need to achieve this speed, however the space shuttle coule go up as slow as required Assuming enough fuel reserves.

The distinction is whether the flight is powered or not. Lee, Leeds UK There is a big difference between an object being aimed vertically upwards and shot out of a cannon and a powered rocket.

The ball shot from the canon receives energy only as it passes through the barrel, from then on it is unpowered and slows down as it climbs through the earths gravitational field.

Escape velocity refers to this case, not a powered rocket. Incidentally the mass of the ball does not effect the escape velocity if there is no atmospheric friction, which means that an elephant and a mouse would both have to be given the same escape velocity if launched from the surface of the moon!

As such an object travels upwards it will of course be slowed by gravity, but at the same time an object that moves upwards from the earth the effect of gravity gradually dimishes. If you begin travelling upwards too slowly gravity will bring you back down to earth. If you start out travelling fast enough, whilst gravity will slow you down it will not be sufficient to bring you back down to earth. The escape velocity is the break point between these two alternatives. The escape velocity is of course dependent upon the distance from the earth or indeed any large body , diminshing as you travel away.

However we attain speeds in excess of 40,kph, we will have to ramp up to and down from them patiently. Rapid acceleration and deceleration can be lethal to the human organism: witness the bodily trauma in car crashes as we go from a mere tens-of-kilometres-per-hour clip to zero in the span of seconds. The reason? A property of the Universe known as inertia, whereby any object with mass resists change to its state of motion.

Pilots are tested in centrifuges such as these to see how many Gs their bodies can withstand Credit: Science Photo Library. About a century ago, the invention of sturdy aircraft that could manoeuvre at speed led to pilots reporting strange symptoms related to speed and directional changes.

These included temporary vision loss and the sensation of either leadenness or weightlessness. The cause is G-forces, otherwise called gravitational forces, or even simply Gs.

These are units of accelerative force upon a mass, such as a human body. G-forces experienced vertically, from head to toe or vice versa, are the ones that can be truly bad news for pilots and passengers. Blood pools in the heads of those undergoing negative Gs, from toe to head, causing an engorged sensation like when we do a handstand. Conversely, when acceleration is positive, from head down to foot, the eyes and brain become starved of oxygen as blood collects in the lower extremities.

Many aviation deaths result from pilots blacking out and crashing. The average person can withstand a sustained force of about five Gs from head to toe before slipping into unconsciousness. Pilots wearing special high-G suits and trained to flex their torso muscles to keep blood from whooshing out of their heads can still operate their aircraft at about nine Gs. If only for mere moments, we humans can tolerate way stronger Gs without grievous injury. He rode a rocket-powered sled backwards in and recorded a pummelling Astronauts, depending on their vehicle, have also experienced fairly high Gs — between three and eight on takeoffs and atmospheric re-entries, respectively.

If you want to slow all the way down to zero—and drop gently into the atmosphere—the fuel requirements multiply your weight by 15 again. These outrageous fuel requirements are why every spacecraft entering an atmosphere has braked using a heat shield instead of rockets—slamming into the air is the most practical way to slow down.

And to answer Brian's question, the Curiosity rover was no exception to this; although it used small rockets to hover when it was near the surface, it first used air-braking to shed the majority of its speed. I think the reason for a lot of confusion about these issues is that when astronauts are in orbit, it doesn't seem like they're moving that fast; they look like they're drifting slowly over a blue marble. When you look at the sky near sunset, you can sometimes see the ISS go past In those 90 minutes, it's circled the entire world.

The ISS moves so quickly that if you fired a rifle bullet from one end of a football field, [7] Either kind. To get a better sense of the pace at which you're traveling, let's use the beat of a song to mark the passage of time. That song is about In the time it took to sing the first line of the chorus, you could walk from the Statue of Liberty all the way to the Bronx:.

This is a neat idea, but so far has only been tried in small scale demos, raising models a hundred feet or so on laser beams. Who knows, it might become the standard way to get into space some time in the future, but it is a long way from achieving that potential right now. If you can send a spacecraft into orbit that way, you can also return it from orbit the same way if you want to.

This is the way it is done today: to use the upper atmosphere as a brake, then slowly parachute to the surface or glide down in the lower atmosphere. How easy that is to do depends on the spacecraft. If it is a heavy one like the Space Shuttle now retired of course then it can only slow down deep in the upper atmosphere, where it is dense. So it gets very hot. It will be able to fly to orbit from a conventional runway though reinforced to carry the extra weight of all the fuel , return back to Earth, and then take off again within a couple of days with a crew of to assist.

Its design is much lower in density than the space shuttle, once it has used up its fuel to get into orbit. So it slows down in the atmosphere at higher altitudes on the way down.

What really matters is the mass per cross sectional area it presents to the atmosphere or more exactly, its ballistic coefficient. Skylon could slow down even higher in the atmosphere if it presented a large blunt face like an aeroshell, but it has to be streamlined for the other stages of its flight.

However, it is also able to compensate for that to some extent by steering during the early part of the flight to slow down more quickly.

It flies to orbit from a normal length runway, reinforced to take the weight of fuel on lift off and may fly in the s. It is heavy when it takes off, but during the landing, having used up most of its fuel, it is low density and so slows down much higher in the atmosphere than the Space Shuttle.

As a result, it will reach lower temperatures than the Space Shuttle on re-entry though higher than a supersonic jet at Mach 3. Here are a few figures for skin temperatures for comparison, hottest first.

These are the figures for the hottest parts of the spacecraft or plane:. But the Skylon uses a structure much more like a zeppelin or a small plane. It has aluminium propellant tanks suspended inside it. Covering that, it has a thin outer aeroshell of a high temperature silicon carbide fibre reinforced glass ceramic material. For details see page 2 of this report. This ceramic outer skin is black, which is why Skylon is shown that color in most of the artist renderings.

This is an animation to show the concept for a mission to orbit and back by Reaction Engines, who developed the idea.

Re-entry starts about seven minutes into the video. This approach of reducing the density of the spacecraft to lower its re-entry temperature is taken much further with the plans of JP Aerospace. Their kilometer scale orbital airship is filled mainly with hydrogen. It only operates above , feet and is balanced for the upper atmosphere.

It also has a huge cross section which it presents to the atmosphere. This spaceship design consists of a near vacuum of hydrogen floating in a near vacuum of normal air. If they succeed in building it, then it will be able to slow down just through friction in the very tenuous upper atmosphere.

It would be a leisurely journey, as you would get there slowly over several days. Although it may not look it, its huge V shape is designed to be aerodynamic at hypersonic speeds in the near vacuum upper atmosphere.

They have done modeling, calculations, and wind tunnel tests with scale models to test this. So on the way up, it gradually accelerates to supersonic speeds, then to hypersonic speeds by which time it is already in a near vacuum. It has solar panels over its vast upper surface to generate power, and uses these to power ion thrusters.

These let you accelerate with a very high exhaust velocity, and so, with a small total amount of fuel, so long as you have plenty of power. It would have no shortage of power with such a large area of solar panels. It has no internal girders. Its outer shell covers an interior of many large bags of hydrogen to give it rigidity and to stop the gas bunching up at its nose. It also has inflatable trusses, with nitrogen filling the gaps in between these components. The nitrogen is vented if necessary and then replaced from liquid nitrogen tanks.

It is balanced to float at , feet altitude in the atmosphere. But since it is aerodynamic, it also behaves like a glider on the way down. It doesn't look much like a glider to our eyes perhaps, but that big voluminous V shape makes a great glider in the very tenuous upper atmosphere during re-entry. So what keeps it up is partly aerodynamic lift and partly buoyancy.

The aerodynamic effects keep it higher in the atmosphere for longer, and so keep it cooler on the way down. On page they say: "By losing velocity before it reaches the lower thicker atmosphere, the reentry temperatures are radically lower This makes reentry as safe as the climb to orbit.

Instead, every stage along the way pays for itself. At present they pay for the tests through pongsats and other ways to lift material to the edge of space. Their tests involve high altitude balloons and V-shaped airships rated for the lower atmosphere.

They have also tested a high altitude balloon-based airship design. JP Aerospace holds the altitude record for an airship , propeller driven, remotely controlled from the ground, and flying at a height of 95, feet above sea level.

It gets the name because at that height the sky will be dark even in daytime, as for the Moon. Next, they plan small airships doing test hypersonic glides back to Earth. Finally, they do test flights to orbit with smaller airships, then the first human pilots to orbit, and then huge orbital airships with passengers and cargo. The idea started off as a US Air Force contract for a near space reconnaissance airship. It was only rated as sturdy enough for launch in a 2 mph wind at the time an airship is particularly vulnerable in the short time it takes to launch it from the ground.

They did this with some reluctance - and it blew apart in the strong winds, causing some minor injuries. The inventor himself sustained three broken ribs.

That was enough for the US Air Force to cancel the contract. JP Aerospace have now solved the problem and can launch their lower atmosphere V-shaped airships in any wind conditions. You can read their account of this story here. You might wonder what happens if the airship is hit by a meteorite or orbital debris. From page of the book:. A balloon pops because the inside is at a higher pressure than the air on the outside. The inner cells of the airship are "zero pressure balloons".

There is no difference in pressure to create a bursting force. All a meteorite would do is to make a hole. The gas would leak out staggeringly slowly The JP Aerospace orbital airships are so lightweight they could never survive at ground level. The slightest wind would tear them apart. If you want to fly all the way down to ground level on Earth in one go, then you need a more massive airship.