# Can more thrust come from launching parallel to water?

Would having the exhaust of a rocket or air breathing engine in an early stage in close proximity to water on a horizontal launch provide more thrust? Could a rocket launch off water horizontally in stages us using water and air for lift to save fuel?

Side Question: Could a cushion of air between a rocket and water after a certain speed to create lift like this plane without wings?

– Uwe
Mar 31, 2019 at 22:11
• Apr 1, 2019 at 1:10
• a vertical launch looses 1g of thrust to fight gravity. A horizontal launch, therefore, has $g\frac{drag}{lift}$ "more" thrust. However, as speed increases without increasing altitude air resistance becomes more and more of an issue. In general, the only cases in which a horizontal launch is worthwhile is when you are using the atmosphere as propellant.
– user20636
Apr 1, 2019 at 8:16
• Ground effect works only on wings. Rockets do not have wings, they can not benefit from ground effect. Apr 1, 2019 at 10:43

If you launch horizontally, you'll build up speed while you are in the thickest part of the atmosphere. Pretty soon (around Mach 1, ~300 m/s), you have to start gaining altitude to prevent overheating due to atmospheric drag.

You'll also experience severe buffeting; aircraft that fly fast at low altitude need design features to reduce the vibration to acceptable levels.

• I have added an air breating stage into the question. Thank You
– Muze
Apr 1, 2019 at 17:59

# What is thrust ?

Thrust is a force applied to an object that change its kinetic energy. In other words, it's the propulsive force. Usually measured in Newtons.

With a rocket engine, thrust goes up when pressure goes down because the differential pressure at the mouth of the nozzle is higher.

So no, thrust would in fact be lower than if the rocket gained altitude.

Obviously, this would not be the case of an air breathing engine. But that's not a rocket anymore.

• The nozzle needs to be designed for a higher pressure. What would a nozzle look like to be efficient to push off water?
– Muze
Apr 1, 2019 at 2:12
• @Muze A sea level nozzle is shorter. But that doesn't change the fact that it would be less efficient. A nozzle designed for sea level will still have better performances in vacuum than at sea level. Apr 1, 2019 at 2:21
• I see now thanks
– Muze
Apr 1, 2019 at 2:37
• So having a water wall to push off of would not increase thrust? Maybe a lower velocity nozzle in a stage of its own?
– Muze
Apr 1, 2019 at 2:41
• All else being equal; higher velocity => higher thrust. Thrust = Mass * Velocity. Apr 1, 2019 at 3:12

Having the exhaust moving horizontally over water will not do anything useful for the rocket, the exhaust has pretty much done all the 'work' it is going to do for the rocket by the point it leaves the nozzle. There would probably be some fascinating shock diamond like effects in the air water interface but rocket is gone by that point so the physics details do not matter.

Going vertically the ground effect can have impact for rockets by making them partial ground effect vehicles, but it as per the ground effect wiki page it tapers off quickly based on your pressure generating area, which in a rocket is your engine diameter. So once you are two bell diameters clear of the ground any effect is gone. For serious liquid rockets the back pressure generated from the ground effect causes instability and other problems and is explicitly avoided by having flame trenches/deflectors to produce lots of clear space for the exhaust to move in.

For small (solid) rockets it is possibly to launch from a sealed tube for a slight performance boost, but generally only meaningful when time spent launching is a sizeable amount of the total flight time (aka short range weapons).