# How fast would this gravity engine let planes fly?

My advanced civilization has finally managed to get gravitational engines to a smaller and more effective state. With this new technology, planes will now fly by 'falling' in the direction they are going. Steering is easily taken care of by changing the direction of gravity and can be done quickly. The pilots are also able to benefit from this, as the machines put the cockpit of the plane close to a 0-G environment, removing most of the G forces that would pancake the pilot in quick maneuvers and almost negating the human squishieness variable.

How fast would this technology allow a plane to fly/fall?

• I think you are confusing the inertial forces due to change in the direction of motion (which are expressed in G to give a practical reference to the laymen) with actual gravitational forces
– L.Dutch
Sep 15, 2020 at 10:20
• @L.Dutch-ReinstateMonica, probably. im pretty dumb most of the time. Sep 15, 2020 at 10:25
• @Ceramicmrno0b inexperienced* Sep 15, 2020 at 10:38
• @Trioxidane, ive heard it both ways and they both fit the description. dumb is quicker to say. Sep 15, 2020 at 10:48
• "...as the machines put the cockpit of the plane close to a 0-G environment", thank you! It annoys me a writers overlook that a gravity drive like this doesn't need to compensate for inertial forces on the pilot/ship (well, depending on the shear). In fact, if you can create a uniform gravity field, such a vehicle can be subjected to near-infinite acceleration and will never notice, because every atom is simultaneously subject to the same force. It's only when you have to transmit acceleration mechanically that you run into limits. Sep 15, 2020 at 13:05

There is a special term for this situation : terminal velocity.

For most jet planes it is less than speed of sound at sea level but may grow far greater at high altitudes. So jets would prefer to keep lower heights, unlike they do now. For highspeeders (like super-and hyper- sonic "divers") runaway effect is possible due to curvature of the Earth - at some point (about 10 speeds of sound) you will need hard steering to keep up with it.

When moving on terminal velocity equipment and pilots would expirience about 1g acceleration. And their "down" would point forward.

• "So jets would prefer to keep lower heights, unlike they do now" ...I think you have that backwards? The problem with flying at high altitude (besides the lack of air pressure) is that it's hard to get up there, and hard to stay aloft once you do. It sounds like the proposed drive would eliminate those issues, and since there's less air, you can go faster before getting the air out of your way becomes an issue. (Also, I wonder what effect the drive acting also on the air around you would have...) Good point about "down = forward" though. Sep 15, 2020 at 13:17
• @Matthew, going faster than you construction can hold is a very bad idea. Most (if not all) modern comerce jets would fall apart if they reach speed of sound. Since braking is the major problem for such type of a craft, pilot would like to have some "terminal speed reserve" in case of emergency (like they do with altitude now) and thus - keep his craft lower. Sep 16, 2020 at 5:34

### No theoretical upper bound on maximum speed except speed of light.

Your gravity engine can create as much acceleration as it needs, the limits are:

• Pilot survivability (See No-rules Nascar for a relevant discussion). If your pilot moves around the craft in a special seat, and is liquid breathing, you can maintain a constant 10G acceleration. A normal seat your probably limited to about 3-5G, depending on how much puke you want to hose off the windshield.
• Air resistance - plane implies in air. Air heats up plane. Your maximum speed will be determined by how hot your craft can get before it breaks apart or melts.
• Manoeuvrability. If you're travelling very very fast, you can't turn very quickly without doing damage to yourself. You may want your plane to be able to turn 15 degrees within a few km, so this would limit your max speed.

Note that with a gravity engine like your describing, you don't need big wings. Just turn the gravity drive "upwards" to climb. If you only have one gravity drive per plane, you will need some traditional flight controls on small wing-like things to pitch, roll, and yaw, however. If you have 3 gravity drives, you can do away with all wings, tail, etc.

• If you have a gravity drive that works in a field you can get around air resistance by accelerating the air in front of you in a cone as well. Sep 15, 2020 at 12:45
• @JoeBloggs -it would make things worse, since it would increase effective cross-sectional area Sep 15, 2020 at 12:58
• @ksbes: it would increase the area and heat generated, but all the problems would be in front/to the sides of you. You’d basically be accelerating a heat shield made of plasma just ahead of your hull. If you got clever you could even just accelerate all the air out of a tube in front of you and not have to worry about air resistance at all. Sep 15, 2020 at 13:07
• A space probe can break apart or melt upon reentry because it's moving very fast when it hits the atmosphere. A skydiver, on the other hand, will never burst into flames due to air resistance, since their terminal velocity is far too low. Long before air resistance begins to melt a plane, it will simply slow the plane down - I can't imagine a craft that would burst into flame before reaching terminal velocity. Sep 15, 2020 at 13:11
• @NuclearWang The SR-71 Blackbird always leaked fuel on the ground because the various joints in the metal were designed to have room to expand when the air resistance heated them up. That plane was pretty much all engine though, so... Sep 15, 2020 at 13:36