The Tsiolkovsky Gap: Mining the Energy-Saving Potential of the Rocket Equation
title: "The Tsiolkovsky Gap: Tapping the Energy-Saving and Speed-Increasing Potential of the Rocket Equation" date: "2026-06-02" category: "space" author: "Zhigeng"
The Tsiolkovsky Gap: Tapping the Energy-Saving and Speed-Increasing Potential of the Rocket Equation
I. Finding Hope in Despair
This publication's Friday (May 29) essay, The Hidden Message of the Tsiolkovsky Equation: Humanity's Century-Long Struggle with Gravity, leaves one silent and heavy-hearted upon reading.
The Tsiolkovsky Rocket Equation, in its exceedingly simple mathematical form:
velocity increment = exhaust velocity times ln(liftoff mass divided by final mass)
Its hidden message is brutal and blunt: over 95% of a rocket's liftoff mass is used to transport fuel and rocket structure. The payload mass that can truly be delivered to space is less than 5% of liftoff mass.
To solve the velocity increment problem, generations of space engineers have had no choice but to use multi-stage rockets to increase the rocket's velocity increment. Rockets fly while burning and discarding stages along the way, hoping that the final residual thrust will push the payload out of the atmosphere and into space.
5% efficiency, 95% cost—given the current state of Earth's resources and environment, and the enormous demands of the long journey of future space development, this is truly an unbearable weight. As the previous essay stated, this problem has shackled human spaceflight for over 120 years.
But if we observe and engage with this equation carefully—not as a well-worn conclusion, but as a living, communicative mathematical tool—perhaps we can ask one question:
In that final mass (the mass after fuel is exhausted), what is the lower bound of final?
II. The Equation Conceals a Gap
Final mass consists of two parts: the payload you want to send up, plus the shell that holds the fuel and payload—the tanks, engines, plumbing, valves... The heaviest part is not the payload; it is the shell.
Here lies a latent possibility, silent for over 120 years, insufficiently interrogated: what if the shell could become infinitely lighter?
Consider a chain: if the rocket structure becomes lighter and lighter, to the point of approaching zero—then the final mass approaches the payload mass.
Liftoff mass has not changed, but final mass has become smaller. Thus, liftoff mass divided by final mass—this ratio—becomes larger. The velocity increment a rocket can achieve depends precisely on the logarithm of this ratio.
When the shell approaches zero, that ratio can become very large—far larger than any practical empirical value at current engineering levels.
This does not break the laws of physics. The laws of physics are the final constraint; they have always been written within the equation. It is just that everyone has been staring at the part that says logarithmic growth is too slow, forgetting to ask: can the denominator become smaller, and then smaller still?
In other words, what most people take as settled—95% is structure, beyond saving—is actually the current state of engineering, not a verdict of physics.
Physics dictates: to reach orbital velocity, you must satisfy that velocity increment requirement.
Engineering dictates: right now, you must burn 95% to do it.
The laws of physics do not change, but engineering can be changed. The tension between engineering and physics can pry open what I call the gap.
III. The Data of the Gap
Theory alone may not be intuitive; let us look at a set of concrete numbers.
Suppose a chemical rocket with a liftoff mass of 100 tons, aiming to send 5 tons of payload into orbit. Now, let us steadily push down the shell's proportion within the final mass from the current level and see how the velocity increment changes:
This table tells a story: liftoff mass unchanged, payload unchanged; only the shell's mass changes. And going from a 95% shell ratio to 50% does not require any fantastical, god-tier materials—replacing aluminum alloy tanks with carbon fiber structural components, simplifying plumbing, and increasing engine thrust-to-weight ratio would suffice. This is precisely what the space industry has been working toward over the past decade-plus.
And this ordinary progress has already yielded some results. In the future, single-stage-to-orbit rockets might even become feasible.
Going further down from 50% requires more fundamental breakthroughs.
IV. The Two Doors That Open the Gap
This gap is not a road that already exists. It is a direction, pointing toward two doors.
Door One: Breakthroughs in Materials
Why is the rocket's shell so heavy? Because it must withstand extremely high thrust and pressure—hundreds of tons of thrust, combustion chamber pressures of hundreds of atmospheres, violent temperature swings—it cannot be made thin.
The direction of this path is clear: replace today's materials with lighter but equally strong ones.
Carbon fiber is already in use—the Falcon 9's interstage sections, payload fairings, and landing legs use carbon fiber composites, about 30% lighter than aluminum alloy of equivalent strength. The next step is materials with even higher specific strength—carbon nanotube-reinforced composites, theoretically tens of times stronger than steel while lighter than aluminum. But carbon nanotubes remain at the laboratory stage; there is still a long road from there to manufacturing a full-scale rocket tank.
But this is not fantasy—the direction is right there; it is only a question of how far the distance is.
Door Two: Breakthroughs in How Energy Is Used
Another efficiency bottleneck for current rockets is that fuel simultaneously plays the roles of both energy source and working mass.
What you need is not the mass of tens of thousands of tons of fuel, but the energy released by their combustion; propulsion, meanwhile, must be realized through working mass. But current rockets cannot separate the two—they can only haul both up together, burning and ejecting as they go.
What if energy and working mass could be separated?
The idea is: carry a relatively lightweight energy device (such as a small nuclear reactor), use it to generate energy, and then use that energy to accelerate a small amount of propellant—in this way, at least you no longer need mountains of fuel tanks.
Nuclear thermal propulsion proposes precisely this path: use a small nuclear reactor to heat liquid hydrogen to extremely high temperatures and eject it, producing thrust. Its specific impulse (a measure of propulsion efficiency) can exceed that of chemical rockets by more than twofold. NASA's NERVA project had already built ground-test prototypes in the 1960s, with thrust meeting crewed-mission requirements. Its sticking point is not physics—it is launch safety and cognitive willingness, and this, in turn, circles back to engineering problems.
This is not to say that engineering problems are not difficult, nor that cognitive willingness is a small issue—it is equally a major problem involving safety, ethics, and more. But engineering problems, and the cognitive-willingness problems built atop engineering safety, have the possibility of being solved—unlike the bottom-line nature of physical constraints.
V. The Shared Logic Behind Both Doors
Whether it is materials breakthroughs or energy breakthroughs, they face the same situation:
The physical equations are already written in stone. The equation tells you: to fly that fast, you must satisfy that velocity increment condition. But the equation does not say the shell must use specific materials, does not say the shell must be this heavy, and does not say energy must be supplied in specific ways.
When Tsiolkovsky wrote down the equation over 120 years ago, the strongest engineering materials on Earth were steel; no one dared imagine shells could be that light. The energy forms of that era were the chemical combustion of the time; the engineering practice of that era could scarcely conceive of other energy modalities.
The equation separates the limits of physics from the circumstances of engineering: physics draws a line; engineering seeks beneath that line to find the operable, realizable point. The gap is the distance from that point to that line.
VI. The Gap Is Not a Road—But Seeing It Causes the Road to Begin Taking Shape
Writing to this point, I am reminded of a story.
In 1970, NASA's Apollo 13, en route to the Moon, suffered an explosion in the service module's oxygen tank. Oxygen and electrical power rapidly drained; three astronauts were stranded in the command module. Their only path to survival was to turn around immediately and return to Earth.
In the process, the astronauts, along with ground control and technical support personnel, overcame countless difficulties. One of the life-threatening problems was this:
The lithium hydroxide canisters used to filter carbon dioxide in the command module had square connectors. The only module with remaining electrical power—the lunar module—had canisters with round connectors. The two had never been designed to connect to each other—a square peg would not fit into a round hole. Carbon dioxide levels were rising; if not resolved, the astronauts would be poisoned.
The engineers on the ground did something that seemed preposterous at the time: they cobbled together plastic bags, cardboard, duct tape, and a hose from a spacesuit into an adapter—cardboard bracing the four corners of the square canister to fit the round opening, a plastic bag wrapping the entire interface to prevent air leaks, and duct tape securing every seam. A crude device that would make you wryly smile, yet perfectly fitted—born within just a few hours.
No one had anticipated such a problem would arise; the design had never contemplated the need for this. But the items in the toolbox were only those—if they did not cobble something together, death was certain.
This adapter saved the three lives aboard Apollo 13.
Humanity's situation facing the Tsiolkovsky Equation is the same.
The equation is dead—it clearly writes out the conditions, immovable. But engineering is alive—can the shell be made a little lighter? Can energy be supplied in a different way?—just like those engineers looking at a pile of theoretically mismatched parts and piecing them together, cobbling together a path to survival.
The Tsiolkovsky Rocket Equation reveals the relationships among various quantities; it does not lock down each quantity itself. The 95% curse is the optimal solution found by previous generations of engineers under the materials and energy conditions available to them. As later thinkers, perhaps we can, in theory, ask: can this optimal solution be optimized once more?
The gap is not a road—but when enough people see it, it begins to become one.
The gap may be narrow, but it is where the light gets in.