A modern, nuclear-armed submarine is capable of launching its missiles while submerged, which travel thousands of miles to the target. This raises the question of why orbital rockets cannot be launched underwater.
Although theoretically possible, orbital rockets do not launch from underwater primarily due to the challenges presented by water’s density, which greatly increases pressure and drag on a rocket engine and vehicle’s structure. The infrastructure needed for this type of launch adds to the challenge.
Before delving into the question of why orbital rockets don’t launch from underwater, the reason why a ballistic missile submarine has the ability to launch its nuclear-armed missiles while submerged needs to be explained.
Before launch, a submarine first has to rise to less than 50 meters (164 feet) below the ocean’s surface. A tank of water is then vaporized into steam by an explosive charge, which drives the missile out of the tube with enough momentum to clear the water’s surface.
Once clear from the water’s surface, motion sensors detect the launch vehicle slowing down, which ignites the rocket engines & sends the missile on its planned trajectory to its target. It is important to note that the missile does not ignite its engines while still submerged.
There is a good reason why a submarine-launched ballistic missile does not fire its engines while submerged, which brings us to why orbital rockets are not launched from underwater, as the following section will illustrate.
Why Rockets Don’t Launch From Underwater
The question or idea of launching an orbital launch vehicle from underwater is not a new one. In fact, during the height of the Space Race between the United States and the former Soviet Union, one such idea was actually proposed.
It was called the Sea Dragon, a gigantic 150-meter (492 feet) long rocket that would have dwarfed the 110-meter-long Saturn V rocket that carried astronauts to the Moon in the 1960s. It was proposed by Robert Truax from a company called Aerojet in 1962.
The proposed launch vehicle would have been a two-stage launch vehicle constructed at a dry dock, towed out to sea, and then partially submerged by filling a ballast stage with water, putting the rocket in a vertical position for launch.
The concept & design drew initial interest from large agencies like NASA, but already having a launch vehicle capable of reaching the Moon and enough surplus Intercontinental Ballistic Missiles to use for orbital launches, such a huge, expensive project was not pursued.
Although a lack of demand for an additional heavy-lift launch vehicle, as well as budget cutbacks, were cited as reasons for not pursuing the development of such a vehicle, there are a number of significant problems in building and launching a water-based orbital rocket.
Although theoretically not impossible to solve, the following factors provide the biggest challenges to a successful underwater orbital rocket launch:
- Water Pressure
- Water Density And Drag
- Vehicle Size And Weight
- Water Proofing
To better understand how each of these factors impacts a submerged launch, one needs to look at each one individually.
1) Water Pressure
For a rocket engine to work efficiently and produce the maximum amount of thrust, the pressure of the hot gases exiting the nozzle must be equal to the pressure of the medium surrounding it.
At sea level, the air pressure is around 1 000 millibars. It quickly decreases with altitude to 100 millibars at 12 km and only one millibar at 50 km. This makes it impossible for one single nozzle design to maintain maximum efficiency throughout a launch vehicle’s ascent.
For this very reason, sea level and vacuum-optimized engines have different-sized engine nozzles to optimize performance close to the planet’s surface and the upper atmosphere.
(Learn more about why the nozzles of vacuum-optimized rocket engines are that much bigger level than those found on sea-level optimized engines in this article.)
Air pressure is also directly related to density, which explains why the air pressure close to the planet’s surface is also much denser than the thin air in the upper atmosphere.
However, the difference in density between the air at sea level and the upper atmosphere is almost inconsequential when comparing the difference between the density of air and water.
To illustrate this stark difference, one only needs to look at the density of water, which is 997 kg/m³, while the air at sea level is 1.225 kg/m³. This means water is approximately 814 times denser than air, or put another way, air is only 0.12% the density of water.
This creates a huge challenge for the rocket thrusters of a large orbital rocket trying to ignite while submerged. The amount of water pressure “pushing” against the hot gases trying to exit will create a tremendous amount of pressure within the combustion chamber & nozzle.
Not only does the internal structure of a rocket engine have to be strengthened significantly to cope with the additional pressure buildup within the combustion chamber & nozzle, but a substantial amount of additional thrust will need to be created to displace the dense water.
Even if this were practically possible, it would create a huge problem as soon as the rocket’s engine nozzles leave the water’s surface. Instantly, the 814-fold pressure difference between the water and air will instantaneously make the nozzles underexpanded.
Because the hot gases had to produce enough pressure to power through the dense water, they will generate pressures exiting the nozzles far exceeding that of the surrounding air, making the engine performance very inefficient and creating several additional problems.
The high-pressure problem that the dense water presents also presents another significant problem for a large launch vehicle trying to launch from underwater, as the following section will illustrate.
2) Water Density And Drag
Before launch, more than 85% of an orbital rocket’s mass consists of its fuel. And the vast majority of this fuel is spent just getting the launch vehicle out of the Earth’s atmosphere.
The two primary reasons why so much energy is spent this early during the deployment of a rocket are the Earth’s strong gravitational pull as well as its dense atmosphere. It is the latter of the two that adds to the complexity of a potential underwater orbital rocket launch.
The planet’s strong gravity means the air particles are packed closely together (even though it is invisible to the naked eye most of the time), making the air in our atmosphere very dense. These particles cause drag and friction as an object moves through them at speed.
One experiences drag whenever the wind blows or we travel at high speed in an open vehicle. A large spacecraft traveling at supersonic speeds experiences huge amounts of drag as it has to “push” through the air particles, using a large amount of energy in the process.
But as stated in the previous section, water is approximately 814 times denser than air. This will make it extremely difficult for a large launch vehicle to push through this dense medium, especially while trying to accelerate to orbital velocity while still submerged.
The Saturn V rocket, the largest rocket to reach orbit, would have struggled to move through the drag provided by the dense water.
(A rocket needs to accelerate to a speed of approximately 28 000 km/h or 17 500 mph to obtain orbital velocity. Learn more about how fast rockets must travel to reach space and beyond in this article.)
Although the concept of the Sea Dragon proposed in the 1960s was never further developed, it at least solved this problem in theory by having the vehicle only partially submerged, with only the first stage section of the rocket below the waterline.
3) Vehicle Size And Weight
Another factor that must be addressed is the sheer size & mass of an orbital rocket. A modern submarine-launched ballistic missile like the Trident used by the US and British Armed Forces is approximately 13.5 meters (44 feet) and is carried inside a submarine.
An orbital rocket, however, has an average height of 58 meters (190 feet) with a weight of 1063 metric tons or 2.34 million pounds. No submarine or any other form of infrastructure exists that can carry a vehicle of this size while submerged.
(The average height and weight were determined by calculating the average length and mass of the 30 biggest and most widely used launch vehicles ever produced, past and present. Learn more by reading the original article here.)
This means a whole new type of infrastructure will need to be developed to support a fully submerged launch, which will not only be extremely costly but also complicated and challenging due to the characteristics of seawater and ocean currents.
One alternative would be to follow the original proposal of the Sea Dragon and build the rocket in a drydock before towing it out to sea and submerging the vehicle using ballasts.
4) Water Proofing
It may look like a solid object standing on a launchpad, but a rocket is far from watertight. Apart from components like fuel tanks and the capsules of crewed flights (like the Apollo Command Module & SpaceX Crew Dragon), most of the vehicle is not airtight/waterproof.
The area around the engine and nozzle section of a launch vehicle is particularly exposed to potential water penetration, but there are numerous gaps and openings around its surface that are used for ventilation, alternative purposes, or simply don’t require any waterproofing.
Completely sealing a vehicle as large as an orbital rocket just to make it waterproof for the first few seconds of its launch will be a tedious and complicated task. Components and processes that require ventilation will also need to be reworked, and alternatives developed.
Smaller launch vehicles like the Trident ballistic missile used in submarines are pressurized with nitrogen to prevent any seawater from entering the vehicle for the short period it is exposed, but this is not a realistic and practical solution for a large multistage orbital rocket.
Conclusion
Launching a rocket from underwater, far away from any populated area where it can cause no damage and without the need for a support structure, seems like a good idea in theory. In practice, though, it poses a host of problems, as this article clearly illustrated.
Also, although this is theoretically possible, as the concept of the Sea Dragon illustrated and has already been implemented on a much smaller scale as submarine-launched ballistic missiles, completely submerging and launching a full-scale orbital rocket is not realistic.
This article was originally published on headedforspace.com. If it is now published on any other site, it was done without permission from the copyright owner.
