With recent missions to Mars, like NASA’s Perseverance, UAE’s Hope, and China’s Tianwen-1 all being smashing successes, you could be forgiven for thinking that getting to Mars is easy. But there’s a big difference between sending a rover or orbiter to the red planet and sending the kind of infrastructure and technology we’ll need to set up a human presence there.
Chemical propulsion may have taken us out into the solar system, but for the next phase of human exploration of space, we’ll need new propulsion technologies to supplement those we’ve been using for the last 50 years. To get the details on what a propulsion for a crewed expedition to Mars might look like, we spoke to Kareem Ahmed, associate professor at the University of Central Florida Department of Mechanical and Aerospace Engineering and an expert in cutting-edge rocket propulsion systems.
This article is part of Life on Mars, a 10-part series that explores the cutting-edge science and technology that will allow humans to occupy Mars
Old reliable: The chemical propulsion systems we use now
To send a rocket flying up through Earth’s atmosphere and out into space beyond, you need a lot of thrust. You need to counteract not only the friction from Earth’s atmosphere but also the significant force of gravity, which pulls objects back down to ground.
Since the 1950s, we’ve used the same basic principle to power rockets, called chemical propulsion. Essentially, you ignite a propellant (a mixture of fuel and an oxidizer), which creates heat. This heat makes the material inside the rocket expand, which is then pushed out of the back of the rocket. This expulsion of propellant creates thrust, which pushes the rocket upward with tremendous force, and this force allows it to overcome the effects of gravity and to escape into the space beyond our planet.
“Chemical-based propulsion is just adding heat to the propellants at really fast rates. That propellant, once you have it at a really high heat, it expands at a very high velocity,” Ahmed explained. “That velocity is a function of how much heat you’re putting in. So think of it as when you have an explosion, you have a massive amount of gas which is moving fast. And that is the velocity.”
This is the big advantage chemical propulsion has over other types of propulsion being considered: Speed. Chemical propulsion helps rockets go really, really fast. But it’s not always the most efficient option.
“Think of it like a Prius versus a Corvette,” Ahmed said. “If you want to get from point A to point B very fast, it’s hard to beat chemical-based propulsion.” When you want to be more efficient, however, other propulsion systems can come into their own. “If you’re trying to get from point A to point B at a reasonable speed but at high efficiency, then chemical-based propulsion might not be the right tool.”
Improving chemical propulsion systems
The principle of chemical propulsion may have stayed the same for the last several decades, but that doesn’t mean there aren’t improvements being made to the technology — such as research into different types of fuel.
The efficiency of fuel types is a matter of energy density — how much energy can be stored by a certain amount of fuel. That’s why it’s difficult to use something like hydrogen as fuel, even though it releases a lot of heat in chemical reactions, because it’s so light and has low density. It’s hard to store a lot of hydrogen in a small amount of space, so it doesn’t make for very efficient fuel.
Current rockets most often use kerosene-based fuels — basically the same thing as jet fuel — but the big area of interest right now is looking at methane- or natural-gas-based fuels. This fuel wouldn’t necessarily be any more effective as a propellant, but it would be considerably cheaper as natural gas is abundant and we already have technology in place for collecting it.
“If SpaceX could use natural gas to fly their Falcon 9, they’d have a lot of savings and therefore accelerate space exploration,” Ahmed said as an example. “If we could reduce the cost of getting out to outer orbit, that makes space more attainable to us.”
Another area of research is into improving the engines themselves. Ahmed’s team is one of several groups working on a system called a rotating detonation rocket engine, which could generate more power from less fuel compared to traditional engines.
By carefully controlling the amount of hydrogen and oxygen being fed into an engine, pressure can be created more effectively. This can reduce the size of a rocket engine by eliminating the need for a very powerful compressor, and it uses fuel more efficiently as well. The technology is on its way to being usable soon: Ahmed says the U.S. Air Force is planning to test such an engine by 2025.
Why chemical propulsion isn’t going anywhere
For taking off from Earth, chemical-based propulsion is essential. “From the ground level, chemical-based propulsion becomes critical because you need that amount of power to drive that weight off from the ground up all the way to higher altitude. To get over the gravitational force,” Ahmed explained.
He brought up the example of SpaceX. When the company launches a rocket, why doesn’t it use an electrical system like that used by Tesla? The two companies are owned by the same person, Elon Musk, so surely they could share technologies. But an electrical propulsion system can’t generate the amount of thrust needed to get a rocket off the ground — it simply doesn’t produce enough power.
So we’ll need to continue to use chemical propulsion for launching rockets for the foreseeable future. But this changes once a rocket is in orbit. Once it has overcome Earth’s gravity and is in space, it’s like using cruise control. Controlling a spacecraft in space requires relatively little thrust, as there is no air friction or downward gravitational pull to deal with. You can even make use of gravitational forces from nearby planets and moons.
So a different propulsion system can take over for more efficient operations.
A more efficient option: Electrical propulsion
Once a rocket is in orbit, it will often need to make trajectory changes — small adjustments to tweak its velocity and ensure it’s heading in the right direction. This requires a thrust system. “You need thousands of newtons just to fly a vehicle, to get off the zero-velocity state and to get it up and get over the gravitational force of the weight that you’re carrying. That’s why you need a big, big rocket system. But in outer orbit, you don’t have gravitational forces influencing you any more, you just have your terminal velocity you’re trying to overcome,” Ahmed explained.
And there are plenty of ways to generate the force needed to adjust the course of a spacecraft. “Thrust is thrust,” he said. “You’re injecting mass. You’re throwing away mass, therefore it moves you in the opposite direction. It’s the amount of mass, and how fast you’re exhausting that mass.”
A technology often used in small satellites, or smallsats, is electric propulsion. They use electric power (often collected using solar panels) to ionize a gas propellant. This ionized gas is then forced out of the back of the satellite using an electronic or magnetic field, creating thrust that moves the spacecraft.
This is an extremely efficient system that can use up to 90% less fuel than chemical propulsion.
“For electric propulsion, your mass is very small and you don’t really need a lot of velocity to give you the thrust,” Ahmed said. And electronic propulsion systems can ionize virtually any material, so they can work with whatever is available.
The elephant in the room: Nuclear propulsion
People are often uncomfortable with the idea of nuclear power in space. And there certainly are safety concerns that have to be considered when using nuclear power, especially for crewed missions. But nuclear propulsion might just be the ace that allows us to visit distant planets.
“Nuclear is actually highly efficient,” Ahmed explained. A nuclear propulsion system works through a reactor which generates heat, which is then used to heat a propellant which is expelled to create thrust. It uses this propellant far more efficiently than chemical-based propulsion.
NASA’s goal is to minimize the time the crew travels between Earth and Mars to as close to two years as is practical.
And it’s sustainable, which is its big benefit. “A chemical-based system, you’re burning propellant and exhausting it, and you no longer have it any more,” Ahmed said. “You released that energy and you lost it. Versus a nuclear-based system, the uranium or plutonium that you’re going to use is there and it’s not going to go away. It’s sustainable as you maintain your core reactor.”
Even though this reaction is sustainable, however, the heat it generates still needs to be channeled into a mass. You wouldn’t want to exhaust the uranium or plutonium being used in the reaction. The helpful thing is that the material being heated can be practically any gas or solid, though gas is preferable since it responds better to heat.
In space, there’s no gases to use, so you’d still need to bring some along with you. But on a planet with an atmosphere, like Mars, you could theoretically use readily available gases like carbon dioxide as the propellant.
NASA is currently looking into nuclear propulsion systems for missions to Mars specifically. “NASA’s goal is to minimize the time the crew travels between Earth and Mars to as close to two years as is practical. Space nuclear propulsion systems could enable shorter total mission times and provide enhanced flexibility and efficiency for mission designers,” the agency wrote about nuclear systems. But there haven’t been any firm decisions made yet. “It’s too soon to say what propulsion system will take the first astronauts to Mars, as there remains significant development required for each approach.”
It’s not one or the other; it’s all of the above
We’re still very much in the early planning stages of a crewed mission to Mars. We need to consider practical requirements as well as factors like cost when it comes to planning out our next steps.
Ahmed doesn’t think that one propulsion system is going to prove itself massively superior to the others. Instead, he envisions a combination of different systems used according to specific mission needs.
“I would say all three systems are going to be needed,” he explained. “You don’t have a perfect propulsion system which fits all your missions.” While it is possible to use chemical propulsion for any mission, it’s not always appropriate — he compared this to getting to a building next door using a Ferrari and wasting a bunch of fuel when you could just walk.
For crewed missions to Mars, “you’re going to have to use nuclear, you’re going to have to use electrical, and the chemical-based you can’t get away without,” he said. For example, you might use an electrical propulsion system for delivering cargo like habitats, use nuclear propulsion to set up a reliable relay system between Earth and Mars, and then send your astronauts using a chemical propulsion system. That’s because humans are, essentially, hefty pieces of hardware. “Our mass is not light!” he said. “We’re a significant amount of mass, even for just a few personnel. Therefore you need that chemical-based propulsion.”
Are we ready for Mars?
There are many complexities about arranging a crewed mission to Mars. But when it comes to propulsion systems, we have the technology to send a mission there tomorrow.
“The traditional ’50s-based rocket motors will get you there,” Ahmed said. The limiting factor turns out to be something more prosaic. “The question is how much it is going to cost you.”
Sending rockets to Mars using chemical-based propulsion systems is simply very, very expensive. And while there is both a public and an academic appetite for more exploration of Mars, the amount of money available for such a mission is not endless. Therefore, we’re going to need to develop and exploit technologies like electric or nuclear propulsion systems to make exploration more affordable.
Even in the realm of chemical-based propulsion, developments in the technology, like rotation detonation engines or new fuels, can help to reduce costs, which will promote more exploration. “The challenge is developing engineering systems which are more economical than current rocket systems,” he said. “The ’50s technology will get you to Mars without an issue. It’s just super, super expensive. And nobody’s going to want to pay for it. But the technology is there.”
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