We are in the space age. Rockets are launched into space almost every day. Orbital space stations have now housed humans continuously for decades. The sky is teeming with satellites and space telescopes. People have been to the moon – and are going back. And robots are scattered across the solar system and poking around on the surface of Mars.
All this incredible innovation is indebted to a modest experiment that took place 100 years ago: on March 16, 1926, American physicist and engineer (and sometimes Scientific American contributor) Robert H. Goddard launched an 11-foot-tall rocket prototype nicknamed “Nell” from a cabbage patch in Auburn, Mass. Nell was airborne for only a few seconds, but the flight was a milestone – the first ever liftoff of a liquid-fueled rocket.
Before that moment among the cabbages, solid fuel was used in all previous rockets, going all the way back to the gunpowder-filled “fire arrows” used to fight invading Mongols in China in the 13th century. Liquid fuel gave rockets a more powerful thrust and, thanks to their variable flow, also offered more control – just what would be needed for any serious attempt at space travel. Other early visionaries—Russia’s Konstantin Tsiolkovsky and Germany’s Hermann Oberth—had also recognized the transformative potential of liquid rockets, but Goddard was the first to prove it.
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The rest, as they say, is history. To commemorate the centenary of Goddard’s flight and understand what the future holds for rockets, Scientific American spoke with two NASA experts – Kurt Polzin, chief engineer for the Space Nuclear Propulsion Project at NASA’s Marshall Space Flight Center, and David Manzella, senior space propulsion technologist at NASA’s Glenn Research Center.
(An edited transcript of the interview follows.)
Given how modest Goddard’s “Nell” prototype was compared to today’s rockets, do you think it’s fair to say that Nell’s flight a century ago marks the beginning of “modern rockets?”
KURT POLZIN: Robert Goddard was a pioneering figure who moved rockets beyond their early roots in solid propellant systems, like powder-packed canisters. His scientific and analytical approach established a framework for the systematic development and improvement of rocket components, a methodology that is still followed today.
Goddard’s milestone flight laid the foundation for the development of various space propulsion systems, including chemical rockets, nuclear-thermal rockets, and both solar and nuclear-electric propulsion. Despite their differences, these systems share a common principle: converting an energy source—whether chemical bonds, nuclear reactions, or solar energy—into a high-speed stream of gas or particles that produces thrust.
In particular, Goddard’s insights extended to electric propulsion. In his notes, he recognized the potential of accelerating charged particles, such as electrons, for propulsion—a concept that anticipated the ion thrusters now used in modern spacecraft.
Space launches are now so common that they are hardly considered newsworthy. One might have the impression that we have reached the limit of what Goddard-inspired chemical rockets can do. What do you see as the remaining limits?
POLZIN: Chemical rockets, often associated with Goddard’s pioneering work but now encompassing a century of collective innovation, have been the backbone of space exploration. Traditional propellant combinations such as liquid oxygen-liquid hydrogen, liquid oxygen-kerosene, and various solid rocket engine propellants have been extensively refined. Recent developments by “new space” companies have introduced alternatives such as methane and hybrid propellants, which can offer additional advantages in terms of reliability, cost and operational flexibility.
Innovative approaches like boost-stage propulsion landings (used by SpaceX’s Falcon 9 and Blue Origin’s New Glenn rockets, for example) have reduced launch costs and increased launch frequency, making space more accessible than ever before. Chemical rockets are likely to remain the primary means of reaching orbit for the foreseeable future, but it is important to remember that no “ultimate” rocket can truly exist—different missions require different solutions, and no single rocket design can serve all purposes.
Looking ahead, several limits remain for chemical rockets. Advances in cryogenic fluid handling may enable long-duration missions using chemical propellants by preventing deboiling, while continued work on nuclear propulsion and the proliferation of miniature propulsion systems for “CubeSats” and “SmallSats” promise to further expand the landscape. And we haven’t even scratched the surface of use cases like flying rockets on other planets, either to change location or to boost payloads or astronauts from the surface.
David, this question is for you. Propulsion systems in space are quite different from rockets used to launch payloads from planets. What excites you about where the rocket ride is headed?
DAVID MANZELLA: Right, so I personally work on propulsion systems in space, which are the technologies used to propel spacecraft once they reach orbit. For these systems, the fundamental challenge is not to have a thrust-to-mass ratio of more than 1; that is, they usually produce less thrust than is required to lift a payload into orbit. But you need propulsion in space because things placed in orbit are valuable and you’ll usually use them for many years.
At the moment, that means that when you launch something, right from the start, you have to take all the fuel you need and use it over the life of the spacecraft. The technology we’re working on tries to solve this problem by creating extremely fuel-efficient rocket engines—what we usually call thrusters—and one of the best ways to do that is to boost your fuel by adding electrical energy to it. And that energy is generated in space.
Today it is done with the help of solar panels. However, remember that the more powerful the electrical systems are, the more power these electrical propulsion thrusters can provide and the bigger things we can push into space.
My favorite poster child for this is NASA’s Power and Propulsion Element under development, which has a 60-kilowatt power system that the onboard propulsion system can use to push an 18,000-kilogram spacecraft to the moon using less than 3,000 kg of propellant. Quite a contrast to launch vehicles, where 90 percent of the mass is propellant, right?
It is impressive. And I know that the power and propulsion element has not yet flown in space – you and your colleagues turned it on for the first time everactually in a test last year. What are you looking forward to in the future?
MANZELLA: The exciting part of the future is that even higher power systems can be developed, and solar panels may one day be replaced by nuclear systems that generate orders of magnitude more electricity. NASA is developing the technology to enable this for things that include human exploration of Mars today. That’s what excites me!
POLZIN: Let me jump in on this as well. What excites me most about the future of rocketry is the expanded horizons of both performance and use. Rockets, at their core, are tools—indispensable for enabling the exploration and exploitation of space, but not the instruments of discovery themselves. Their true value lies in their capacity to deliver the technologies and payloads that drive scientific investigation, exploration and, increasingly, the establishment of a permanent human presence beyond Earth.
On the performance front, innovation continues to push boundaries. Advances in propulsion systems promise greater efficiency, reliability and range. This is realized through incremental improvements in chemical rockets, experimentation with new propellant combinations such as methane or hybrids or the pursuit of systems such as solar energy and nuclear propulsion. This development is crucial for tackling ambitious missions, such as crewed trips to Mars or test missions in deep space. And a diverse range of propulsion systems is essential to meet a wide range of scientific, commercial and exploratory objectives.
From the application perspective, the most exciting developments involve going beyond exploration to expansion and exploitation. We begin to think boldly about questions such as: How do we deliver and return humans safely from Mars? How can we collect and return samples from distant bodies in the solar system? What infrastructure is needed to go from initial exploration to establishing a permanent presence in space? The vision extends further and involves harnessing resources and capabilities gained from space expansion through NASA’s Artemis program, enabling sustainable operations and new opportunities for science, industry and even daily life beyond our planet.
Ultimately, the future of rocketry is about enabling new opportunities. As end users gain access to a growing range of launch options, they are better equipped to carry out different missions: advancing scientific knowledge, developing commercial enterprises, or building the foundation for a permanent spacefaring civilization. The field thrives on bold thinking and inventive solutions, and I am most excited to see how these will shape the next era of space exploration and development.
MANZELLA: We are truly entering a new age in human history where each and every one of us can be affected by space-based systems on a daily basis. I think that trend will only accelerate in the future. It is clear that space is becoming an increasingly large part of our lifestyle as technology continues to advance. And yes, much of this progress traces back to Robert Goddard’s first flight a century ago.
