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By Dan Kotlyar, Georgia Institute of Technology 

NASA plans to send crewed missions to Mars over the next decade – but the 140 million-mile (225 million-kilometer) journey to the red planet could take several months to years round trip.

This relatively long transit time is a result of the use of traditional chemical rocket fuel. An alternative technology to the chemically propelled rockets the agency develops now is called nuclear thermal propulsion, which uses nuclear fission and could one day power a rocket that makes the trip in just half the time.

Nuclear fission involves harvesting the incredible amount of energy released when an atom is split by a neutron. This reaction is known as a fission reaction. Fission technology is well established in power generation and nuclear-powered submarines, and its application to drive or power a rocket could one day give NASA a faster, more powerful alternative to chemically driven rockets.

NASA and the Defense Advanced Research Projects Agency are jointly developing NTP technology. They plan to deploy and demonstrate the capabilities of a prototype system in space in 2027 – potentially making it one of the first of its kind to be built and operated by the U.S.

Nuclear thermal propulsion could also one day power maneuverable space platforms that would protect American satellites in and beyond Earth’s orbit. But the technology is still in development.

I am an associate professor of nuclear engineering at the Georgia Institute of Technology whose research group builds models and simulations to improve and optimize designs for nuclear thermal propulsion systems. My hope and passion is to assist in designing the nuclear thermal propulsion engine that will take a crewed mission to Mars.

Nuclear-powered rockets could one day enable faster space travel.
NASA

Nuclear versus chemical propulsion

Conventional chemical propulsion systems use a chemical reaction involving a light propellant, such as hydrogen, and an oxidizer. When mixed together, these two ignite, which results in propellant exiting the nozzle very quickly to propel the rocket.

These systems do not require any sort of ignition system, so they’re reliable. But these rockets must carry oxygen with them into space, which can weigh them down. Unlike chemical propulsion systems, nuclear thermal propulsion systems rely on nuclear fission reactions to heat the propellant that is then expelled from the nozzle to create the driving force or thrust.

In many fission reactions, researchers send a neutron toward a lighter isotope of uranium, uranium-235. The uranium absorbs the neutron, creating uranium-236. The uranium-236 then splits into two fragments – the fission products – and the reaction emits some assorted particles.

More than 400 nuclear power reactors in operation around the world currently use nuclear fission technology. The majority of these nuclear power reactors in operation are light water reactors. These fission reactors use water to slow down the neutrons and to absorb and transfer heat. The water can create steam directly in the core or in a steam generator, which drives a turbine to produce electricity.

Nuclear thermal propulsion systems operate in a similar way, but they use a different nuclear fuel that has more uranium-235. They also operate at a much higher temperature, which makes them extremely powerful and compact. Nuclear thermal propulsion systems have about 10 times more power density than a traditional light water reactor.

Nuclear propulsion could have a leg up on chemical propulsion for a few reasons.

Nuclear propulsion would expel propellant from the engine’s nozzle very quickly, generating high thrust. This high thrust allows the rocket to accelerate faster.

These systems also have a high specific impulse. Specific impulse measures how efficiently the propellant is used to generate thrust. Nuclear thermal propulsion systems have roughly twice the specific impulse of chemical rockets, which means they could cut the travel time by a factor of 2.

Nuclear thermal propulsion history

For decades, the U.S. government has funded the development of nuclear thermal propulsion technology. Between 1955 and 1973, programs at NASA, General Electric and Argonne National Laboratories produced and ground-tested 20 nuclear thermal propulsion engines.

But these pre-1973 designs relied on highly enriched uranium fuel. This fuel is no longer used because of its proliferation dangers, or dangers that have to do with the spread of nuclear material and technology.

The Global Threat Reduction Initiative, launched by the Department of Energy and National Nuclear Security Administration, aims to convert many of the research reactors employing highly enriched uranium fuel to high-assay, low-enriched uranium, or HALEU, fuel.

High-assay, low- enriched uranium fuel has less material capable of undergoing a fission reaction, compared with highly enriched uranium fuel. So, the rockets needs to have more HALEU fuel loaded on, which makes the engine heavier. To solve this issue, researchers are looking into special materials that would use fuel more efficiently in these reactors.

NASA and the DARPA’s Demonstration Rocket for Agile Cislunar Operations, or DRACO, program intends to use this high-assay, low-enriched uranium fuel in its nuclear thermal propulsion engine. The program plans to launch its rocket in 2027.

As part of the DRACO program, the aerospace company Lockheed Martin has partnered with BWX Technologies to develop the reactor and fuel designs.

The nuclear thermal propulsion engines in development by these groups will need to comply with specific performance and safety standards. They’ll need to have a core that can operate for the duration of the mission and perform the necessary maneuvers for a fast trip to Mars.

Ideally, the engine should be able to produce high specific impulse, while also satisfying the high thrust and low engine mass requirements.

Ongoing research

Before engineers can design an engine that satisfies all these standards, they need to start with models and simulations. These models help researchers, such as those in my group, understand how the engine would handle starting up and shutting down. These are operations that require quick, massive temperature and pressure changes.

The nuclear thermal propulsion engine will differ from all existing fission power systems, so engineers will need to build software tools that work with this new engine.

My group designs and analyzes nuclear thermal propulsion reactors using models. We model these complex reactor systems to see how things such as temperature changes may affect the reactor and the rocket’s safety. But simulating these effects can take a lot of expensive computing power.

We’ve been working to develop new computational tools that model how these reactors act while they’re starting up and operated without using as much computing power.

My colleagues and I hope this research can one day help develop models that could autonomously control the rocket.

About the Author:

Dan Kotlyar, Associate Professor of Nuclear and Radiological Engineering, Georgia Institute of Technology

This article is republished from The Conversation under a Creative Commons license. Read the original article.

By Bruce Huber, University of Notre Dame 

Fossil fuels are the leading driver of climate change, yet they are still heavily subsidized by governments around the world.

Although many countries have explicitly promised to reduce fossil fuel subsidies to combat climate change, this has proven difficult to accomplish. As a result, fossil fuels remain relatively inexpensive, and their use and greenhouse gas emissions continue to grow.

I work in environmental and energy law and have studied the fossil fuel sector for years. Here’s how fossil fuel subsidies work and why they’re so stubborn.

What is a subsidy?

A subsidy is a financial benefit given by a government to an entity or industry. Some subsidies are relatively obvious, such as publicly funded crop insurance or research grants to help pharmaceutical companies develop new drugs.

Others are less visible. A tariff on an imported product, for example, can subsidize domestic manufacturers of that product. More controversially, some would argue that when a government fails to make an industry pay for damage it causes, such as air or water pollution, that also amounts to a subsidy.

Subsidies, especially in this broader sense, are widespread throughout the global economy. Many industries receive benefits through public policies that are denied to other industries in the same jurisdiction, such as tax breaks, relaxed regulations or trade supports.

Governments employ subsidies for political and practical reasons. Politically, subsidies are useful for striking bargains or shoring up political support. In democracies, they can mollify constituencies otherwise unwilling to agree to a policy change. The 2022 Inflation Reduction Act, for example, squeaked through Congress by subsidizing both renewable energy and oil and gas production.

Practically, subsidies can boost a promising young industry such as electric vehicles, attract business to a community or help a mature sector survive an economic downturn, as the auto industry bailout did in 2008. Of course, policies can outlive their original purpose; some of today’s petroleum subsidies can be traced to the Great Depression.

How are fossil fuels subsidized?

Fossil fuel subsidies take many forms around the world. For example:

• In Saudi Arabia, fuel prices are set by the government rather than the market; price ceilings subsidize the price citizens pay for gasoline. The cost to state-owned oil producers there is offset by oil exports, which dwarf domestic consumption.
• Indonesia also caps energy prices, then compensates state-owned energy companies for the losses they bear.
• In the United States, oil companies can take a tax deduction for a large portion of their drilling costs.

Other subsidies are less direct, such as when governments underprice permits to mine or drill for fossil fuels or fail to collect all the taxes owed by fossil fuel producers.

Estimates of the total value of global fossil fuel subsidies vary considerably depending on whether analysts use a broad or narrow definition. The Organization for Economic Cooperation and Development, or OECD, calculated the annual total to be about US$1.5 trillion in 2022. Tche International Monetary Fund reported a number over four times higher, about $7 trillion.

Why do estimates of fossil fuel subsidies vary so dramatically?

Analysts disagree about whether subsidy tabulations should include environmental damage from the extraction and use of fossil fuels that is not incorporated into the fuel’s price. The IMF treats the costs of global warming, local air pollution and even traffic congestion and road damage as implicit subsidies because fossil fuel companies don’t pay to remedy these problems. The OECD omits these implicit benefits.

But whichever definition is applied, the combined effect of national policies on fossil fuel prices paid by consumers is dramatic.

Oil, for example, is traded on a global market, but the price per gallon of petrol varies enormously around the world, from about 10 cents in Iran, Libya and Venezuela – where it is heavily subsidized – to over $7 in Hong Kong, the Netherlands and much of Scandinavia, where fuel taxes counteract subsidies.

What is the world doing about fossil fuel subsidies?

Global leaders have acknowledged that subsidies for fossil fuels undermine efforts to address climate change because they make fossil fuels cheaper than they would be otherwise.

In 2009, the heads of the G20, which includes many of the world’s largest economies, issued a statement resolving to “rationalize and phase out over the medium term inefficient fossil fuel subsidies that encourage wasteful consumption.” Later that same year, the governments of the Asia-Pacific Economic Cooperation forum, or APEC, made an identical pledge.

In 2010, 10 other countries, including the Netherlands and New Zealand, formed the Friends of Fossil Fuel Subsidy Reform group to “build political consensus on the importance of fossil fuel subsidy reform.”

Yet these commitments have scarcely moved the needle. A major study of 157 countries between 2003 and 2015 found that governments “collectively made little or no progress” toward reducing subsidies. In fact, the OECD found that total global subsidies nearly doubled in both 2021 and 2022.

So why are fossil fuel subsidies hard to eliminate?

There are various reasons fossil fuel subsidies are hard to eliminate. Many subsidies directly affect the costs that fossil fuel producers face, so reducing subsidies tends to increase prices for consumers. Because fossil fuels touch nearly every economic sector, rising fuel costs elevate prices for countless goods and services.

Subsidy reform tends to be broadly felt and pervasively inflationary. And unless carefully designed, subsidy reductions can be regressive, forcing low-income residents to spend a larger percentage of their income on energy.

So, even in countries where there is widespread support for robust climate policies, reducing subsidies can be deeply unpopular and may even cause public unrest.

The 2021-22 spike in fossil fuel subsidies is illustrative. After Russia’s invasion of Ukraine, energy prices surged throughout Europe. Governments were quick to provide aid for their citizens, resulting in their largest fossil fuel subsidies ever. Forced to choose between climate goals and affordable energy, Europe overwhelmingly chose the latter.

Of course, economists note that increasing the price of fossil fuels can lower demand, reducing emissions that are driving climate change and harming the environment and human health. Seen in that light, price spikes present an opportunity for reform. As the IMF noted, when prices recede after a surge, it “provide[s] an opportune time to lock in pricing of carbon and local air pollution emissions without necessarily raising energy prices above recently experienced levels.”

About the Author:

Bruce Huber, Professor of Law, University of Notre Dame

This article is republished from The Conversation under a Creative Commons license. Read the original article.