# Wouldn't it be better to leave Earth less dramatically? [closed]

I'm not claiming this concept outlined below as a massively original idea. But perhaps the idea of "gentle" access to space is unusual?

What I'm talking about is a High-Altitude Platform (HAP), kept up with a very large amount of balloons, at an altitude of maybe 10 miles?

You could send all the bits of spaceship, and any payloads and people, up there by balloon, assemble the spaceship on the HAP and fly off slowly through the upper atmosphere. Only when you reached space would you start accelerating significantly, heading to the Moon for example. You wouldn't bother going into Earth orbit because that would involve too much acceleration. On the way back you would equally avoid orbit situations... and you would be using rocket motors to prevent you accelerating towards Earth (see below about fuel on the return from the Moon).

The final approach to the HAP on the way back would then involve decelerating through the upper atmosphere until you reach the HAP, where you would dock. Balloons would then be used to keep the spacecraft (and HAP) from falling to Earth.

People seem to be a bit confused about this idea of "escape velocity". The only thing you need to escape the gravitational well of a planet is to keep accelerating upwards with a slightly higher acceleration than the one in the opposite direction due to gravity: the velocity at any point is immaterial and both on the way out and on the way back could be low, low enough to avoid all the problems to do with atmospheric friction, and the imparting of huge acceleration forces to a vessel.

Starting off 10 miles above sea level, and travelling slowly through the atmosphere, would have lots of advantages from the design angle: your spaceship wouldn't have to be terribly aerodynamic in shape (because the velocity would be low and the air very thin), and it wouldn't have to have ablative heat shields. The amount of fuel saved by starting (and returning) slightly higher up might also be quite useful, if not dramatic (I've haven't tried to do any calculations).

Maybe such a HAP, and the spaceships, could even be made partly of wood? I love the idea of partly wooden spaceships, and wood as a material has some advantages over metal.

later
Just to answer the point about the possible gain from launching 10 miles up compared to sea level: JCRM says that space is big. True, but the bottom of a gravitational well is the worst place to start from, or to return to. This objection, if I may call it that, also assumes that there would be some big cost in maintaining this giant HAP 10 miles up in the sky, and in the launch and docking procedures. There would some cost... but not necessarily a prohibitive one.

Re the question of gravitational drag. I was vaguely aware of this. From a related question, as referenced by Hobbes, it would appear that it might be nice to position the HAP on the Equator and maybe to accelerate away with some lateral acceleration component (?). Nevertheless, the answer there with all the maths says that the gravity drag is significantly reduced by being just 5 miles above sea level. Not sure how all this works with returning...

Note about returning from the Moon

I described in a comment to the answer by Anthony X how you would use "juggling" (or "tacking" to use a maritime term) between the lunar and terrestrial gravitational fields to drag the spacecraft into lunar-terrestrial orbit and then land on the Moon. Leaving the Moon would be analogous: you launch into the direction of the orbital path of the Moon and cleverly use the combination of its gravity and that of Earth to slingshot you back behind the Moon, helping to take you out of (lunar-terrestrial) orbit. This makes you like a falling stone 250,000 miles above Earth.

You then need a full tank. With current re-entry, you use the air's friction to extract the energy from the craft's motion in terrestrial orbit, so you need only minimal fuel to return to Earth.

With my idea you need to refuel on the Moon. Impossible? No. There is water on the Moon. Water + electroysis using solar power --> (liquid) hydrogen and oxygen. You'll need pretty much the same amount of fuel to return as to leave Earth. Obviously in longer-term future you might want to take other fuel and chemicals from other parts of the solar system and transport them to the Moon.

• Space is big. Your HAP gives you very little advantage over launching from sea level when it comes to going to the moon.in a straight line. The fuel needed to get there with 'gentle' acceleration (and decelerate on the way down to the surface) would be astronomical.
– user20636
Feb 4 '18 at 19:03
• @mikerodent - Re your comment "Obviously the amount of fuel on the way out would be the same as if you applied big acceleration" -- Obviously not. Feb 4 '18 at 19:11
• @mikerodent 'People seem to be a bit confused about this idea of "escape velocity"'. Yes. and you are one of them. Feb 4 '18 at 19:15
• GRAVITY drag is the cost of having to hold the rocket up in a gravity field, and has nothing to do with atmospheric drag. The longer you spend flying at slower than orbital speed, the more energy you lose due to gravity drag. Feb 4 '18 at 19:53
• I'm wondering who finds this question "well researched", "useful" or "clear" let alone all three.
– user20636
Feb 5 '18 at 10:43

The further up you go, the thinner the atmosphere and therefore the less it can support your hypothetical ascent vehicle, regardless of whether we are talking about buoyancy (balloons) or aerodynamic lift (wings). There is also less stuff you can use as propellant/oxidizer (what a jet engine processes with fuel to make thrust). What that leaves you with is the thrust of a rocket engine to both support you against gravity and accelerate you upwards (you need to accelerate to accumulate any speed to get anywhere).

If your rocket engine maintains exactly 1g thrust, it exactly counterbalances gravity, and your vehicle goes nowhere.

If it produces 1.1g thrust, you get a nice gentle 0.1g upward acceleration. The problem with this is, most of the power of your rocket - most of the fuel it is consuming is simply offsetting gravity; only about 9% is actually getting you somewhere.

If your rocket produces, say, 3g thrust, now 1/3 of your fuel is fighting gravity while the other 2/3 - most of it - is taking you somewhere.

As for going to Earth orbit...

If your destination is the Moon, you are still in Earth orbit; much higher than we usually think of for things orbiting the Earth, but the Moon is still in an Earth orbit, travelling about 1km/s in its path around the Earth. Now, you may be thinking: LEO orbits are like 7.8km/s, way faster than the Moon, so it must take way more energy to go from Earth surface to LEO than it would to go from Earth surface to the Moon...

WRONG!

If we keep this discussion simple by talking about circular orbits, for any given orbital altitude there is an associated "energy state"; the higher the orbit, the higher the energy state, even though the orbital velocity actually decreases the higher you go. This is because the higher orbit has more potential energy and gravity is weaker the farther away you are. This can all be shown with high school math and physics.

To go from a 7.8km/s low-Earth-orbit to a lunar equivalent orbit, you still have to accelerate to a higher velocity. Doing this will cause your vehicle to ascend on an elliptical path outward, but as it travels outward, it will slow down. If you added the right amount of speed when you were in LEO, you'd arrive at lunar distance travelling at pretty much lunar orbital velocity. At this point, a small tweak and you are orbiting at the Moon's distance.

As for Earth's "gravity well"... 10km is a long way up in human terms - the air is too thin to breath, but it is insignificant as far as climbing out of Earth's gravity well. We have 1g at Earth's surface - 6370km from its center. Gravity falls off with square of distance, so going from 6370 to 6380km gravity is still more than 0.99g.

To sum up: rockets do what they do (accelerate fast and go into orbit) because (1) it's most efficient, and (2) building up high velocity is pretty much unavoidable.

For perspective:

Apollo made it to the Moon with a vehicle just big enough/powerful enough to do the job. NASA scientists and engineers sought out the most efficient flight profile. It wasn't your "straight line". Apollo accelerated to LEO (about 17,000 mph) and then to about 25,000 mph, at which point the CSM/LM gently coasted for about 3 days, arriving at the Moon such that it needed the least possible fuel burn (delta-V) to put it into lunar orbit. Apollo launch mass was about 3,000 tons (more than 90% of it propellant) to set the CSM/LM of about 44 tons on its way to the Moon, arriving, as I said, with the minimum delta-V necessary to establish lunar orbit. It would not be better to take a path that would require even more propellant (perhaps an order of magnitude?) to do the same job, just "slower and gentler".

• Comments are not for extended discussion; this conversation has been moved to chat. Feb 5 '18 at 13:05

You could send all the bits of spaceship, and any payloads and people, up there by balloon, assemble the spaceship on the HAP

Balloons are big, and the cargo they carry is small. And wind is going to push them in subtly different directions unless the ground and surface winds are perfect and all balloons launched simultaneously.

Getting the bits of payload together would be... a heck of a lot more complicated than assembling it on the ground.

• Comments are not for extended discussion; this conversation has been moved to chat. Feb 5 '18 at 13:05