I haven't run the numbers, but he's right: it's pretty hard to get something that can carry itself to orbit. It has to climb out of the atmosphere directly against gravitational force and atmospheric drag, and then as early as possible turn sideways and start accelerating to around 28000kph while making sure the rocket is pointed in the right direction and holding together. You need a lot of propellant to get to that state, which means you have to carry it around until you need it, which means the machine needs to be much bigger than just the cargo, which means that all of your challenges just magnified by many orders of magnitude.

More payload means more fuel. More fuel itself means more fuel.

Maybe this is obvious, but two rockets can get twice as much payload to orbit using twice as much fuel. You can that up linearly all day long. (It holds for "twice as much", but the same argument doesn't work for "twice as fast" or other metrics.)

If X is 500t and Y is 5t, you now need 51,000t of fuel to achieve 2 * Z speed.

But yes, you still require more energy (exactly sqrt(2) times as much) to "escape" Earth's gravity entirely than to orbit it with a circular orbit.

So if you can get off the planet you can get anywhere... but that doesn't mean cost-effectively. If your planet is such that it takes a Saturn V to launch a cubesat, it may simply cost too much to make it to the Moon.

mass of the rocket with fuel = mass of the payload * exp(escape velocity / engine exaust velocity)

That is because the faster you need to go, the more fuel you need to use, but now, you also need even more fuel to accelerate the fuel that you just added. So, it becomes exponential.

So, if the earth was bigger, its escape velocity would increase, and the amount of fuel needed to power a space rocket would increase (exponentially) so much that it would become unpractical. Vice versa, with a smaller planet it would require exponentially less fuel to reach orbit.

The only solution would be to have an engine with a higher exaust velocity. With the current technology, ion engines have very high exaust velocity but low thrust are very energy hungry. The thermal nuclear engines have both high exaust velocity and high thrust, working prototypes have been built, but all projects were boxed 60 years ago because of the fear of an accident.

A fully fueled and loaded Saturn V weighs about 3,000,000 Kg, and has a payload of about 40,000 Kg to the moon.

So that means once you've put all the bits of the rocket together that collectively give you sufficient thrust to get you to the moon, you've only got about 1.3% left for actual payload.

S if earth's gravity had been ~1.3% stronger, then the weight of the "getting us there" bits of the rocket would have left no room for payload. Any more gravity, and we wouldn't even be getting to the moon.

Equally, if the earth's gravity had been 1.3% weaker, we could have doubled our payload to the moon (or done things way easier).

Contrast that with a Boeing 747 which has 50% of total mass as payload, and so is relatively insensitive to changes in gravity.

Someone else posted somewhere that if the Earth was about 3x heavier, we wouldn't be able to use chemical rockets to escape.

Yeah, but they wouldn't be, because the extra mass would have influenced their evolution.

So basically in your example, if the Earth gravity was 1.3% stronger, payload would be ~37.8t instead of 40.

I think the Jovian Moon Io is very low, look up an eclipse photo from Juno; it would be key to exploring low Jovian orbit, just like Venus kicks PSP and Jupiter kicked Voyagers.

Voyager 1/2 and New Horizons are at escape velocity and leaving the solar system.

The problem is not how do we get up high, but rather how do we get going really, really, really fast.

Unlike the more modern ship launched SM3 that is part of the Ageis system, the older ASM-135 https://en.wikipedia.org/wiki/ASM-135_ASAT was designed to be air launched in a supersonic zoom climb to get maximum speed and altitude from the aircraft in order to maximise its capabilities.

For an even better comparison, you can compare air launches ASAT missiles with sounding rockets, they have a greater similarity in their trajectories/launch profiles, so it’s easier to see what the air launch “gains” over the ground launch.

> If the radius of our planet were larger, there could be a point at which an Earth escaping rocket could not be built. Let us assume that building a rocket at 96% propellant (4% rocket), currently the limit for just the Shuttle External Tank, is the practical limit for launch vehicle engineering. Let us also choose hydrogen-oxygen, the most energetic chemical propellant known and currently capable of use in a human rated rocket engine. By plugging these numbers into the rocket equation, we can transform the calculated escape velocity into its equivalent planetary radius. That radius would be about 9680 kilometers (Earth is 6670 km). If our planet was 50% larger in diameter, we would not be able to venture into space, at least using rockets for transport.

1 - https://space.stackexchange.com/questions/19852/where-is-the...

I can't find the video I saw, but here's a general link on the topic:

https://en.wikipedia.org/wiki/Laser_propulsion#Laser_energiz...

This comes with many problems, but it does bypass the rocket equation, while not requiring Project Orion levels of trouble.

[1] https://www.youtube.com/watch?v=XhUasBcoj-Q

Imagine how hard space exploration would be if the only practical rocket were a Project Orion.

This is all assuming yields and fuels we have now. If we lived on a more massive earth and we were trying to escape its gravity, I'm sure we'd be using more exotic and dangerous fuels (like all those fun fluorine and boron fuels Dr. Clark mentions in Ignition![0]) to do the job. We just happened to have the capability in the middle of the century to use a fuel we were already making (refined petroleum) for jet engines.

[0] https://www.amazon.com/Ignition-Informal-Propellants-Univers...

I think a dynamic structure such as a launch loop https://en.wikipedia.org/wiki/Launch_loop would work.

Also inflatable towers for launch assist https://en.wikipedia.org/wiki/ThothX_Tower

For even kookier options we've got lasers! https://en.wikipedia.org/wiki/Laser_propulsion https://en.wikipedia.org/wiki/Beam-powered_propulsion

(Consider the difference in difficulty of riding a bike at 15mph vs 16mph and 25mph vs 26mph... 25->26 requires a hell of a lot more effort, all due to drag.)

Or you could fly a high-altitude airplane into a skyhook: https://en.wikipedia.org/wiki/Skyhook_%28structure%29

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

He didn't actually say that. He was talking about "rapidly reusable orbital rockets" and "fully reusable orbital rocket[s]" being the "critical breakthrough" that's necessary, the "holy grail of space" and the "fundamental thing that's required".

So fully and rapidly reusable.

https://youtu.be/sOpMrVnjYeY?t=1234

Very interesting that the only intelligent species we are aware of is on a planet that we can leave.

I wonder to what extent higher gravitational planets could sustain love that couldn't leave it. An interesting answer to Fermi's paradox.

This diagram has some exoplanets on it for size comparison- https://en.wikipedia.org/wiki/Circumstellar_habitable_zone#/...

What if it’s just like fossil fuels and there’s enough energy there for us to smear ourselves out? Oh cool, kinda like fission then!

Spaceflight is about velocity and energy.

It's also about gravity, since gravity is what you are fighting against.