Did you know there is technology that is being development that is outside of Rockets. The Rocket Equation is as follows:

This equation describes the motion of vehicles that expel some of their mass to generate thrust. This can be represented mathematically as:

Δv = ve ln (m0 ÷ mf) = Ispgo ln (m0 ÷ mf)

Where delta (Δ) is the change in velocity (v), ve is the exhaust velocity, ln is the natural logarithm, Isp is the efficiency of the thrust generated (specific impulse), g0 is the force of gravity, m0 is the initial mass (including propellant), and mf is the “dry” mass (no propellant). This equation has been used for almost seventy years to calculate a rocket's dry mass and the fuel needed to send payloads into space.

A Vicious Circle
To put it simply, there is a major downside to this equation. Since the beginning of the Space Age, every rocket ever made has been mostly propellant in mass.

To illustrate, consider the most powerful rockets in the world - NASA’s Space Launch System (SLS) and SpaceX’s Starship & Super Heavy.

Whereas the SLS weighs 188,000 lbs (85,275 kg) when it’s not fueled up, its mass increases to 5.75 million lbs (2.6 million kg) once it's fueled and ready for launch. Meanwhile, the Starship launch system has a dry mass of 170,000 lbs (77,110 kg) versus a “wet mass” of 11 million lbs (~5 million kg).

Doing the math, this means that over 96% of the SLS’ full weight consists of propellant. Even worse, the weight of a fully-fueled and stacked Starship & Super Heavy is close to 98.5% propellant.

Now consider the payloads these launch systems can send to Low Earth Orbit (LEO). For the SLS, that’s 209,439 lbs (95,000 kg), and 200,000 to 300,000 lbs (90,718.5 to 136,078 kg) for the Starship. Comparing this to their fully-fueled weight, we see that the SLS and Starship can deploy just 3.65% and 2.72% of their mass to LEO (respectively).

And keep in mind that this is merely for sending payloads to LEO. To send cargo and crew to the Moon, Mars, or anywhere else in the Solar System, rockets need to generate even more thrust, which means payloads must be smaller. So for missions beyond Earth, an even smaller fraction of a rocket’s overall mass can be payload.

In essence, the bigger the payload, the bigger the rocket needs to be. The bigger the rocket, the more massive it becomes. The more massive it becomes, the more propellant it needs to make it to space. The more propellant it needs, the more massive it becomes. It’s a vicious circle and none too efficient, either.

And this applies to missions once they reach space as well. To ensure that a spacecraft’s mass isn’t too great, spacecraft designers and mission planners limit the amount of chemical propellants used.

More often than not, spacecraft thrusters will rely on solid chemical propellants. These can provide lots of thrusts, but their limited supply means they must be used sparingly and only for course corrections and maneuvers.

Luckily, there are alternatives, some of which are on the drawing board right now. Here is a list of alternate possibilities:

Nuclear Propulsion
Fusion Propulsion
Directed Energy
Antimatter

Many of the above are still theory and some like the Nuclear Propulsion is becoming reality. Nuclear Propulsion is a bit more along as noted below:

These reactors were intended to become part of a Nuclear-Thermal Propulsion (NTP) system, where heat generated by the slow decay of radioactive isotopes is used to heat liquid hydrogen or deuterium fuel. This causes the fuel to expand, which is directed through nozzles to generate thrust. Between 1972 and today, several concepts for NTP have been proposed, and the technology remains the most commonly-researched application.

In 2017, NASA renewed its attempts to create an NTP system through its Game Changing Development Program. In 2023, NASA and the Defense Advanced Research Projects Agency (DARPA) announced a joint effort to develop an NTP concept called the Demonstration Rocket for Agile Cislunar Operations (DRACO). This will culminate with a demonstration of the DRACO in orbit, which is expected to occur by early 2027.

Since the turn of the century, there have also been proposals for Nuclear-Electric Propulsion (NEP). This method consists of a nuclear reactor generating electrical power for a Hall-Effect Thruster or ion engine, which ionizes an inert gas (like xenon) and directs the charged particles through nozzles to produce thrust.

Efforts to realize a NEP system included Project Prometheus, launched by NASA in 2003. This project yielded the Jupiter Icy Moons Orbiter (JIMO), a proposal for an uncrewed NEP spacecraft that would explore three of Jupiter's largest moons, Europa, Ganymede, and Callisto. The proposal was passed over in 2005 in favor of the Constellation Program.

Throughout this same period, proposals have been made for “bimodal concepts,” which rely on both NTP and NEP systems. Both NTP and NEP offer multiple advantages over traditional chemical rockets. Among them, there’s higher energy density, where a nuclear reactor can extract far more energy per pound than chemical propellants.

The other forms of propulsion is still theory and many are looking into advancing these from theory to prototypes. Perhaps within this century some of these other propulsion will become reality. What do you think?

Reference:

https://interestingengineering.com/science/ad-astra-the-future-of-propulsion-technology
https://interestingengineering.com/science/ad-astra-the-future-of-propulsion-technology-part-ii