Elena Voss
Aerospace Propulsion Engineer
Designs the engines that get us off this rock — and keep us going once we're out there.
Day in the Life
6:40 AM, propulsion test stand 3. Elena Voss is already in her flight suit, headset on, watching technicians purge the feed lines on a brand-new ion thruster. In four minutes, this engine — smaller than a dinner plate — fires for the first time at full power.
Every number it produces decides whether it flies on next year’s cargo run to the Gateway station in lunar orbit.
“Ignition in ten… five… three, two, one.”
A thin violet-blue plume of ionized xenon streaks out the back of the thruster, silent in the vacuum chamber except for the hum of the magnets. No fire, no thunder — just a quiet, relentless push that will run for months once it’s in space.
She’s not watching a number. She’s watching the difference between a six-month Mars transit and a nine-month one.
By mid-morning she’s in a different kind of meeting — whiteboards instead of control panels. Mission planning needs a trajectory: when do they leave Earth, how much fuel do they carry, how long is the crew stuck up there before they can come home? Elena sketches a transfer orbit and makes her point again: “more thrust” isn’t always the answer. A slower, more efficient engine wins, because every kilogram of propellant you don’t carry is a kilogram of food, water, or science instrument you can.
She ends the day back at the test stand, signing off on tomorrow’s higher-power run. Out the window, the evening’s first stars are coming out — and somewhere up there, in a few years, something she helped build will be quietly, efficiently, pushing a crew toward Mars.
The Science
Newton’s Third Law — the only thing that actually moves a rocket
Every rocket engine, from a tiny ion thruster to the biggest chemical booster, works on the same principle: for every action, there is an equal and opposite reaction. The engine throws mass (propellant) out the back at high speed, and the rocket gets pushed forward with exactly that much momentum in the opposite direction. There’s no “pushing against the air” — rockets work better in the vacuum of space, because there’s nothing in the way.
Specific impulse — the engine’s “fuel economy”
Engineers compare engines using specific impulse (I_sp), which is essentially how much push (impulse) you get per unit weight of propellant burned. A high I_sp means the engine is very efficient — it needs less fuel to produce the same change in velocity. Chemical rockets typically have an I_sp around 300–450 seconds; the ion thruster Elena tests has an I_sp closer to 3,000 seconds — about 10 times more efficient, at the cost of much lower thrust (it pushes gently, but for a very long time).
The rocket equation
The relationship between how much a spacecraft’s velocity can change (Δv, “delta-v”), how fast the exhaust leaves the engine (v_e), and how much of the spacecraft’s mass is propellant is captured in the Tsiolkovsky rocket equation:
$$ \Delta v = v_e \ln\left(\frac{m_0}{m_f}\right) $$
where $m_0$ is the spacecraft’s total starting mass (including fuel) and $m_f$ is its mass after the fuel is used. That natural logarithm is the reason rockets are mostly fuel by mass — and the reason a more efficient engine (higher $v_e$) has an outsized effect on how much payload you can carry.
From thrust to travel time
Once Elena’s team knows how much Δv an engine can provide, the mission planners can work out trajectories — and that’s exactly where Problem A picks up: given a transfer distance and a cruise velocity (or a more realistic velocity that changes over time), how long does the trip actually take, and what does the spacecraft’s velocity and acceleration look like along the way? Try Problem A →
Specific impulse: chemical rocket vs. ion thruster
Higher specific impulse = more push per kilogram of fuel burned. This is why Elena’s ion thruster can run for months on a tank chemical engines would empty in minutes.