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Just wanted to check the scientific feasibility for a method of moving a shuttlecraft from the surface of an earth-like planet into space. This more for a space opera style setting, rather than strict hard sci-fi.

In many works of science fiction, ascending into space in a small craft without the aid of a rocket loaded with several tonnes of fuel is made to seem trivially easy. Could one way to achieve this, without having to rely on any sort of hand-wavey "anti-gravity" technology, be to have the small craft equipped with a force field generator that can repel air molecules, thus creating a bubble of vacuum around the craft? Would this vacuum bubble surrounding a shuttlecraft float up to the top of the atmosphere once a mass of air greater than the mass of the craft has been repelled? Basically what I'm imagining is a craft that takes off from the ground in a way similar to modern planes, but then once it's at a certain distance off the ground, it starts projecting this force field and growing its radius until the effect of gravity on craft begins to weaken and then neutralise, allowing the engines to only need to engage once the craft "floats" out of the atmosphere.

Is there any obvious problem with this idea that I'm missing?

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    $\begingroup$ A rocket uses relatively little fuel to "climb up". Most of the fuel is spent accelerating the rocket to orbital speed. You might want to calculate what would actually be the savings offered by this method. In the immortal words of Randall Munroe, "getting to space is easy; the problem is staying there". $\endgroup$
    – AlexP
    Jul 7, 2022 at 23:54
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    $\begingroup$ Isn't this just a balloon, deriving lift from buoyancy? It's well-established that you can't balloon into space. $\endgroup$
    – Cadence
    Jul 8, 2022 at 0:59
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    $\begingroup$ The question I would ask as a reader, if the tech exists to repulse the air with a force field, is why don't they just use it as an in-atmosphere thruster? $\endgroup$
    – Jedediah
    Jul 8, 2022 at 3:35
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    $\begingroup$ If your force fields can displace air to a hard vacuum, perhaps they can also manipulate air with high fidelity. You could maybe create some kind of specialized system like a variable-geometry force field ramjet to accelerate to high velocities, as well as gain altitude. Instead of heating the air with combusting fuel, maybe the force fields themselves compress and heat the air, and/or microwaves do the heavy-heating. $\endgroup$
    – BMF
    Jul 8, 2022 at 3:58
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    $\begingroup$ With the force fields any shape and size you want, you can get very streamlined and aerodynamically optimal based on speeds and atmospheric conditions, all the way up to near-orbit. At high altitudes with almost negligible drag, efficient ion engines can carry you the rest of the way to true orbital trajectories, somewhat similar to JP Aerospace's "Airship to Orbit" concept, while the specialized geometry deflects you off whatever's left of the atmosphere below you. $\endgroup$
    – BMF
    Jul 8, 2022 at 3:59

3 Answers 3

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Yes, there are problems

First, getting out of the atmosphere is the comparatively easy part of going into orbit. Getting a rocket to travel a hundred or so km straight up so that it is effectively out of the atmosphere is not that difficult, getting it to travel "sideways" at the 7.8 km/s required to achieve low earth orbit (LEO) requires enormously more delta v. If you look at the old Space Shuttle launch profile, the Shuttle was outside the denser sections of the atmosphere (altitude approximately 50 km) at the point of solid rocket booster (SRB) separation but "only" travelling at 1279 m/s. It needed to accelerate for a further six-and-a-half minutes to (almost) achieve LEO. (A final correction burn was required in order to achieve a stable orbit.)

(Note that while the SRBs made it to an altitude of 50 km under thrust and continued to a peak altitude of 67 km on inertia, they then fell back to Earth and were recovered for recovery. Getting to altitude did not get them into orbit.)

The two SRBs combined massed 1,180,000 kg and were more than half the mass of the shuttle at launch. So can you equate your forcefield generator to the SRBs and say it does the same job? Unfortunately, no - the SRBs also provided structural support to the fully-fueled orbiter + external tank and provided most of the thrust to not only clear the lower atmosphere but to get the shuttle up to almost a sixth of the necessary speed to make orbit. The rocket equation is painful - you need more fuel at the start to accelerate not only the payload but also the fuel you will need later. Unlike the SRBs, the forcefield generator is presumably retained as part of the shuttle, which means that more fuel will be required in order to accelerate the shuttle later in its flight profile, offsetting any savings in the early part of the launch. So unless the forcefield generator has a negligible mass, it probably isn't going to help much.

A second problem is structural. While the lander was designed to glide and land like a conventional aircraft, the combined structure at launch could only withstand very limited lateral forces. You cannot simply weld a big ring onto the top of the shuttle and lift the combined mass from that point, the craft would disintegrate. In order to make the structure "liftable" from some arbitrary point where the forcefield generator is mounted, a huge amount of structural reinforcement will be required, which will increase the total mass of the craft, which will increase the fuel required to accelerate it, which will increase the mass further... This is one of the main reasons why various real-world proposals for piggybacking launch vehicles into the upper atmosphere using balloons or conventional aircraft have been abandoned as infeasible.

Third, in order for any decent size shuttle to become buoyant, it needs to be displacing a truly huge quantity of air. If we continue to use the old space shuttle as an example it had a mass of over 2,000,000 kg at launch. Given that a cubic metre of air at sea level has a mass of 1.29 kg, this means that the force field needs to push the air out of around 1,550,000 cubic metres of space at sea level.

As it ascends and the atmosphere becomes thinner the forcefield needs to keep increasing in size to compensate. At a height of 50 km, the atmosphere is approximately 1/650th as dense as at sea level, so it will need to maintain around 1,000,000,000 cubic metres of vacuum.

Assuming that the forcefield is spherical and centred on the shuttle - what happens when the shuttle is sitting on the ground and the forcefield is turned on, with half the sphere underground? What happens if solid objects impinge on the forcefield? (Maybe the bottom of the sphere should always be just above the shuttle, just an idea.) A significant issue if the shuttle is inside the sphere, as described, is that it is totally at the mercy of the weather during a long ascent. If it fires its engines at all it is effectively just redistributing its mass within the sphere rather than actually changing its vector. Along the same lines - very important to turn the forcefield off before firing the main engines!

In summary - even if this technology did exist, unless it had negligible weight and power requirements it would probably cause more problems than it solved in launching a shuttle. There are plenty of potentially great applications of this technology, but that is outside the scope of the question.

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    $\begingroup$ As a note, space ship one is a modern spaceship which "launches" from higher up in an attempt to circumvent a lot of atmosphere. en.wikipedia.org/wiki/SpaceShipOne $\endgroup$
    – PipperChip
    Jul 8, 2022 at 2:37
  • $\begingroup$ @PipperChip - Space Ship 1 "goes to space" exactly the same way the SRB boosters do: a sub-orbital trajectory. It's.... not a useful for anything other than rich people tourism. $\endgroup$
    – codeMonkey
    Jul 8, 2022 at 13:11
  • $\begingroup$ Actually if the force field is a kind of Maxwell Daemon, thrust from inside the bubble would work. $\endgroup$
    – toolforger
    Jul 8, 2022 at 18:55
  • $\begingroup$ Note that the 1,000,000,000m^3 corresponds to a cube of 1km diameter. 50km above ground that may not be so bad. Of course, your other points still stand. $\endgroup$
    – quarague
    Jul 9, 2022 at 6:30
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While the vacuum bubble is indeed an important part of this drive's operation, it's actually not the most important application of the force field for the process of escaping a planet's gravity well. The main goal of the relatively passive vacuum bubble phase is just to get high enough into the atmosphere that it's safe to extend the fields and start pulling atmosphere to gain thrust in a way akin to ancient jet engines, but with the ability to gather much more air and accelerate it to much higher speeds. In the process it creates a vacuum bubble in front of the accelerating ship, clearing the way for rapid acceleration despite a relatively thin atmosphere. In principle the drive's jet function can be operated in the denser lower atmosphere, but all vacuum jet drives are legally required to have safeguards to prevent this, as the consequences can be extremely destructive.

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I see two problems with your theory, but don't let these stop you as you have already said it is a space opera story. My concern is always the suspension of disbelief.

Problem #1: Your forcefield can push the air molecules away from your craft. This is fine, but as your craft moves, it has to push the forcefield that is pushing the air, so you have effectively increased the air resistance as presumably the forcefield is larger than the hull of the craft.

Problem #2: You still have to fight gravity which is a bigger problem than the air. Your craft will not just float up because of your vacuum bubble. The air on earth applies an average of 15 lbs per square inch under STP (Standard Temperature and Pressure) at sea level. Of course your forcefield is still having to repel that air so it is still effecting the craft generating the forcefield. Unless your forcefield also acts as an anti-gravity field, you still have to provide enough thrust to reach escape velocity.

I agree with KerrAvon2055 that your forcefield could have other applications, but outside the scope of your question.

Remember when writing your story that you don't want to necessarily allow facts and science to screw up a perfectly good story. As mentioned at the beginning, you have to guage how far you can push the suspension of disbelief without breaking it. Once that is broken, the story will suffer in the eyes of the readers because they will get hung up on the "fantasy" aspect you have introduced.

Good luck and have fun with your writing !!!

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    $\begingroup$ Problem #2 isn't a problem. A vacuum bubble is less dense than air, and is buoyant in the atmosphere. A large enough vacuum bubble with a buoyant force higher than the payload weight will indeed spontaneously rise through the atmosphere - it's basically a hot air balloon. The issue is that this only gains altitude, but 90% of the energy needed to get to orbit is spent gaining speed, not height. Even though the forcefield spontaneously rises, you still need thrust to gain speed. $\endgroup$ Jul 8, 2022 at 13:50

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