# Inertia and Rocket Propulsion

We were taught in school that the law of inertia indicates that an object tend to stay the way it is, so if you throw something in space it will tend to go on forever and ever. The reason an object falls down when you throw it on Earth is because of gravity and air resistance. If that's the case, why don't rockets and spaceships need just enough fuel to escape the atmosphere plus the single thrust to push the craft in the right direction and let inertia drift it away?

• Because gravity is stil acting on the rocket (its magnitude is proportional to $\frac{1}{r^2}$). – fibonatic Dec 1 '13 at 22:24

That's exactly the case. If you look at the trajectory of any given spacecraft, you will see that it has a few burns of the rocket engines punctuating very long periods just coasting along in orbit around some other body.

For example, the flight path of Apollo 8 has something like eight different rocket burns: launch, translunar and transearth injection (to get out of orbit and go towards the other body), three course correction burns, lunar orbit insertion to catch up with the moon, and one orbit correction burn on the moon.

Image source: Wikipedia

The rocket engines spend most of their time turned off, and carry just enough fuel for all of this plus a little extra for safety. This still means that the initial rocket needs to be huge, because the translunar injection requires quite a bit of fuel and that fuel needs a huge other load of fuel to get into orbit.

• (I might also mention that ion thruster-driven spacecraft constitute the one (very rare!) exception to this rule.) – Emilio Pisanty Dec 1 '13 at 22:23
• Aha, I always thought that they were burning fuel all the way. Thanks. – Aelgawad Dec 1 '13 at 22:33
• You should watch "Apollo 13" the movie, or "The Right Stuff" which dramatizes the above scenario. – ja72 Dec 2 '13 at 0:24
• +1 for the simple pleasure of finding a 26 year old deftly work in a much older image of an Apollo moon mission. I can earnestly say that I do wish you were of my age (9) then and could have shared the wonder of this 9 year old when all of this was going on. – Alfred Centauri Dec 2 '13 at 3:50
• @AlfredCentauri For sure. I did very much enjoy Tom Hank's From the Earth to the Moon, and particularly Apollos 9 and 15, though. (nudge nudge.) – Emilio Pisanty Dec 2 '13 at 10:01

I suspect that you may be under the mistaken impression that there is no gravity in space. This is a common belief since we all can see the astronauts floating in "zero g" when on the ISS are some other spacecraft.

However, we can easily dispense with this misconception by asking "what keeps the ISS in orbit around the Earth if there is no gravity?". Of course there is gravity; Earth's gravity keeps the ISS circling 'round the Earth.

The reason the astronauts float and experience "zero g" is because they are in free-fall.

In fact, when the Apollo astronauts left Earth orbit at about 25,000 mph, their spacecraft was slowing down and, by the time the spacecraft begin falling towards the Moon rather than the Earth, their speed had fallen to, if I recall, around 2000 mph (I'll verify and update later if necessary).

So, you see, it isn't the case that, once outside the atmosphere, a spacecraft will maintain its speed, relative to the Earth, without firing the rocket engine. That would only be the case if it were in space far away from any other gravitating body.

• Fun exercise for the reader: Imagine a tower that is as tall as the average altitude of the ISS. What would you weigh if you stood on top of the tower and waved at the astronauts as the ISS whizzed past? (Hint: Use Newton's Universal Law of Gravitation) The answer might surprise you (or not, if you are a physicist or a space flight engineer.) And please be careful! The ISS will be moving about 12x as fast as the bullet from a high-powered hunting rifle when it goes by. You don't want to stand in the way of that. – Solomon Slow Jun 10 '16 at 20:56

The escape velocity from earth (the speed required for an object to leave earth completely, i.e. travel infinitely far away) is 11.2 km/s. If the object has a smaller velocity it will return eventually.

Unless an object is launched straight up, it also has a sideways velocity. This means that it will not fall back directly on top of the launcher. If it moves horizontally at a speed of 7 km/s or more, it will keep "falling" beyond the edge of the earth and stay in orbit. Of course, you need to do this high enough so that you are out of the atmosphere.

Spacecraft initially travel vertically to escape the atmosphere. When they are high enough they start changing direction so they eventually travel about 7 km/s "horizontally". At this point they can stay in orbit indefinitely. To travel to other planets, a further boost changes their orbit so that it becomes large enough to eventually reach the other planet. En route it may get more boosts, either from on-board rockets, or from other planets.

This is a wonderful question and filled with interesting albeit incomplete or inaccurate answers.

Launching an "orbital body" concerns two issues and two issues ONLY namely mass and intertia.

So before we "launch" anything we must first understand THE EARTH IS MASSIVE...which works AGAINST the "inertial reality" of a conical shaped item which albeit is very MASSIVE (weight, size, volume, etc) is still "just sifting there."

Does one need to compute the Earth's Mass? No, not at all. Simply the knowledge that the Earth is truly massive AND IS AN ORBITAL ITSELF is sufficient to become an expert in orbital mechanics.

In other words you need no understanding of physics AT ALL to launch a rocket.

What you do need to know is the Earth wants to "throw" this object so to paraphrase a famous term "the more massive the better."

What "the physicists" get wrong is that getting a rocket off the ground has nothing to do with physics at all but merely "dumping the mass of fuel on the ground." Since the mass of the fuel is far larger than the mass of the object...the object is physically "lifted off the ground."

Everything that comes after said "lift" is just a function of performance and working against the natural forces having lifted the object simply wanting to fall.

The United States struggled MIGHTILY with this problem in the 1950's I might add...something the German Scientists solved quite easily.

This involved "controlling the dump" (to dramatic effect with the Saturn V) as it were...but literally nothing more.

Performance enhancements have been an interesting sidebar however.

• You can launch a rocket in space with nothing to push on. The mass of the Earth isn't what makes the rocket take off. – Brandon Enright Jun 10 '16 at 19:57
• What do you mean? If I launch from the equator I get far better efficiency. There is not a subject for "debating" but merely grasping the reality that the Earth is not only launching your rocket but in fact is about the ONLY thing. Yes, true...we must ignite the fuel...but we don't do that at the top like a candle now do we. – Doctor Zhivago Jun 10 '16 at 21:50
• Bah, gibberish! – M. Enns Jun 10 '16 at 22:25
• You do understand the Earth revolves around the Sun not the other way around, yes? – Doctor Zhivago Jun 10 '16 at 22:27