About Stephanie Thomas

Ms. Thomas is vice president of Princeton Satellite Systems. She is the Principal Investigator for the NASA NIAC grant supporting Direct Fusion Drive. Ms. Thomas has been with PSS since her first internship as an MIT undergraduate in 1996!

Doing the Mars run with fusion propulsion at 1 G

We received a comment on LinkedIn about how fast the “Mars run” could be achieved with a sustained 1 G acceleration. The reader suggested this could be done in 40 hours. What engine parameters would be required to make that happen?

Using a simple constant-acceleration, straight-line analysis, you can indeed compute that the trip should take only a couple of days. Assuming a Mars conjunction, the straight distance is about 0.5 AU. At this speed you can ignore the gravitational effects of the sun and so the distance is a simple integral of the acceleration: d = 1/2 at2. The ship accelerates for half the time then decelerates, and the change in velocity is ΔV = at. Combining the two halves of the trip, at an acceleration of 9.8 m/s2, the trip takes about 2.1 days.

% straight line: distance s = 0.5*at^2
acc = 9.8;             % accel, m/s^2
aU  = Constant('au');  % km
dF  = 0.5*aU*1000;     % distance, m
t   = sqrt(4*dF/acc);  % time for dF, s
dV  = t*acc/1000; % km/s
fprintf('\nAccel: %g m/s^2\n',acc)
fprintf('Time: %g days\n',t/86400)
fprintf('Delta-V: %g km/s\n',dV)

Accel: 9.8 m/s^2
Time: 2.02232 days
Delta-V: 1712.34 km/s

Now, your ship mass includes your payload, your engine, your fuel tanks and your fuel. Assume we want to move a payload of 50,000 kg, somewhat larger than the NASA Deep Space Habitat. The engine mass is computed using a parameter called the specific power, in units of W/kg. The fuel tank mass is scaled from the fuel mass, typically adding another 10%. When we run the numbers, we find that the engine needs to have a specific power of about 1×108 W/kg, and an exhaust velocity of about 5000 km/s results in the maximum payload fraction. We can compute the fuel mass and trajectory using our MassFuelElectricConstantUE and StraightLineConstantAccel toolbox functions:

d = StraightLineDataStructure;
d.dF = 0.5*aU;   % 0.5 AU
d.tF = 2*86400;  % 2.1 days
d.uE = 5000;     % km/s
d.eta = 1;       % jet power sigma
d.sigma = 1e8;   % W/kg
d.mP = 50000;    % kg
dOut = MassFuelElectricConstantUE( d )
fprintf('Acceleration: %g m/s^2\n',dOut.a*1e3)
fprintf('Exhaust velocity: %g km/s\n',d.uE)
fprintf('Initial power: %g GW\n',dOut.p(1)*1e-9)
fprintf('Engine mass: %g kg\n',dOut.mE)
fprintf('Payload fraction: %g\n',dOut.mP/dOut.m0)

Acceleration: 10.02 m/s^2
Exhaust velocity: 5000 km/s
Specific Power: 1e+08 W/kg
Initial power: 2832.61 GW
Engine mass: 28326.1 kg
Payload fraction: 0.442172

This produces the following plots:

The power needed is… over 2.8 terawatts! That’s about equal to the total power output of the entire Earth, which had an installed power capacity of 2.8 terawatts in 2020. And the engine would need to weigh less than 30 tons, about the size of a loaded tractor-trailer truck. For comparison, we estimate a Direct Fusion Drive would produce about 1 MW per ton, which is a specific power of 1×103 W/kg. So, this is why you see us trying to design an engine that can do the Mars transfer in 90 days and not 3 days!

Now, there is another consideration here. Namely, constant acceleration at 1 G is not the optimal solution by any means. The optimal solution for a fast, light transfer is actually a linear acceleration profile. This knowledge goes way back: 1961! Here’s a reference:

Leitmann, George. "Minimum Transfer Time for a Power-Limited Rocket." Journal of Applied Mechanics 28, no. 2 (June 1, 1961): 171-78. https://doi.org/10.1115/1.3641648.

This would mean that the engine changes its exhaust velocity during trip, passing through infinity at the switch point. We compute this in our “straight-line, power-limited” or SLPL function series. While this can’t be done physically, even an approximation of this with a variable impulse thruster will one day be more efficient than constant acceleration or thrust. How much better? The power needed is nearly 1/2 the constant acceleration solution, 1.5 TW, and the specific power needed is reduced by half, to 5.6×107 W/kg. However, those are still insane numbers!

mD = 80000;  % dry mass: engine, tanks, payload
m0 = 1.5*mD; % wet mass: with fuel
tF = 3*86400;
vF = 0;

[Pj,A,tau] = SLPLFindPower( aU, tF, vF, mD, m0 );

mTank = 0.05*(m0-mD); % tanks, scale with fuel
mLeft = mD-mTank;
mEngine = mLeft - mPayload;

disp('Straight-line Power-limited (linear accel)')
fprintf('Engine power is %g GW\n',Pj*1e-9);
fprintf('Engine mass is %g kg\n',mEngine);
fprintf('Payload mass is %g kg\n',mPayload);
fprintf('sigma is %g W/kg\n',Pj/mEngine);

SLPLTrajectory( A, tau, Pj, m0, tF )

Straight-line Power-limited (linear accel)
Engine power is 1573.26 GW
Engine mass is 28000 kg
Payload fraction is 0.416667
sigma is 5.6188e+07 W/kg

The trajectory and engine output are plotted below. The linear acceleration results in a curved velocity plot, while in the constant acceleration case, we saw a linear velocity plot. You can see the spike in exhaust velocity at the switch point, which occurs exactly at the halfway point.

After all, who needs 1G gravity when the trip only takes 2 days?

Foe even more fun though, we computed a planar trajectory to Mars using the parameters we found – just to confirm the straight-line analysis is in fact a good approximation. This figure shows the paths the optimization takes:

Earth to Mars Trajectory, 2.1 days, 0.5 AU traversed

It is in fact approximately a straight line!

In reality though, these power system numbers are not even remotely plausible with any technology we are aware of today. That’s why we are designing engines to reduce the Mars trip time to 90 days from 8 or 9 months – still a big improvement!

Our Crowdfunding Campaign for Fusion Propulsion is Testing the Waters!

Our prelaunch campaign is now live on the Spaced Ventures crowdfunding portal! We will be raising money for our new DOE INFUSE awards, to support PFRC-2 experimental operations with new diagnostics, and to design a superconducting PFRC-3!

Potential investors can go to the site, create an account and indicate interest in our raise. This is called “testing the waters!” Those who sign up now will be the first to know when our raise goes live.

Line drawing schematic of DFD
Direct Fusion Drive Schematic

Thank you to the Out of This World Design graphics team and the Spaced Ventures team for their support in putting together the pitch! The beautiful new spacecraft render is now on our homepage. The team also made really cool line drawings that show how DFD works!

#fusionenergy #rocketscience

ARPA-E Energy Innovation Summit 2022

We will be at the 2022 ARPA-E Summit in Denver, CO next week – May 23 to 25 – representing our two ARPA-E programs, WIDE BAND GAP SEMICONDUCTOR AMPLIFIERS FOR PLASMA HEATING AND CONTROL and Next-Generation PFRC. The post on our Princeton Fusion Systems website has links to our marketing and technical documents. More information of the Princeton Fusion Systems-GAMOW project can be found here.

We will have booths for each program at the Technology Showcase. Here is our OPEN 2018 booth.

OPEN 2018 Booth Featuring PFRC

In the picture below, we are registering at the registration desk at the ARPA-E Innovation Summit at Denver. More pictures of the event can be seen on the ARPA-E Summit website.

Registration Desk of ARPA-E Innovation Summit, Denver, Co

Our ARPA-E funding has allowed us to increase the magnetic field and RF power in the PFRC-2 experiment in pursuit of hotter plasma, a key precursor to demonstrating the conditions needed for Direct Fusion Drive!

eBook Textbook now available on Barnes & Noble

Our aerospace theory textbook, Spacecraft Attitude and Orbit Control, has been included with purchases of the Spacecraft Control Toolbox for years and available for purchase as a standalone PDF. We have now compiled our book as an eBook and it is available from Barnes and Noble for Nook:


The companion tutorial software for the book (Chapter 2) is available for download from our website.

IAEA Nuclear Systems for Space Exploration Webinar: Recordings now Available

The recordings of this webinar from February 15-16, 2022, are now available on YouTube. Each segment is two hours long. Ms. Thomas’ presentation is in Part 2 at about 30:30.

Organized by the International Atomic Energy Agency (IAEA), this webinar focuses on nuclear systems for space exploration. It gives an overview and historical perspective on the status of development in this area and showcases the ways in which nuclear systems can be used for space exploration, as well as discuss possible future innovations in the field.

IAEAvideo, YouTube

Part 1 Agenda:

  • Progress towards space nuclear power objectives | Mr Vivek Lall (General Atomics Global Corporation)
  • Developing the VASIMR® Engine Historical Perspective, Present Status and Future Plans | Mr Franklin R. Chang Díaz (Ad Astra Rocket Company)
  • Application of Space Nuclear Power Sources in Moon and Deep Space Exploration Missions in China | Mr Hui Du (Beijing Institute of Spacecraft System Engineering)
  • Q&A
Part 1, February 15, 2022

Part 2 Agenda:

  • Promises and Challenges of Nuclear Propulsion for Space Travel | Mr William J Emrich (NASA)
  • Fusion Propulsion and Power for Advanced Space Missions | Ms Stephanie Thomas (Princeton Satellite Systems) – at time 30:30
  • NASA Investments in Space Nuclear Fission Technology | Mr Anthony Calomino (NASA)
  • Q&A
Part 2, February 16, 2022

IAEA Atoms for Space: Nuclear Systems for Space Exploration

This webinar hosted by the IAEA, the International Atomic Energy Agency, is coming up this week, Feb. 15-16, 2022.

The exploration of space requires power at many stages, not only for the initial launch of the space vehicle, but also for various house loads such as instrumentation and controls, communication systems, maintaining the operating environment for the space mission’s essential hardware, etc. Nuclear can provide long-term electrical power in space. Nuclear systems can be configured in several ways for use in space exploration.

Atoms for Space: Nuclear Systems for Space Exploration

PSS VP Stephanie Thomas will give a talk during this webinar, Fusion Propulsion and Power for Advanced Space Missions.

Register here: https://iaea.webex.com/iaea/onstage/g.php?PRID=a626af96640b6b59dbee10fcc4910e15

A recording of the webinar will be available! The full agenda:

  • Progress towards space nuclear power objectives | Mr Vivek Lall (General Atomics Global Corporation)
  • Developing the VASIMR® Engine Historical Perspective, Present Status and Future Plans | Mr Franklin R. Chang Díaz (Ad Astra Rocket Company)
  • Application of Space Nuclear Power Sources in Moon and Deep Space Exploration Missions in China | Mr Hui Du (Beijing Institute of Spacecraft System Engineering)
  • Promises and Challenges of Nuclear Propulsion for Space Travel | Mr William J Emrich (NASA)
  • Fusion Propulsion and Power for Advanced Space Missions | Ms Stephanie Thomas (Princeton Satellite Systems)
  • NASA Investments in Space Nuclear Fission Technology | Mr Anthony Calomino (NASA)

Here is the article posted on the webinar:


The Space Show appearance

It was my pleasure to appear on David Livingston’s “The Space Show” radio program last night, now available as a podcast:


People from all over the country called and emailed in their questions about fusion and fusion-propelled spaceflight, and we had a great discussion! David has been running this educational program for 20 years and there are almost 4000 archived episodes covering a wide range of space topics. Author David Brin, whom I met during my NASA NIAC fellowship, is going to be on next week!

So have listen and add to the conversation on The Space Show website!

Funding Options for Fusion Propulsion

The Space subcommittee of the Fusion Industry Association, of which we are a member, has prepared a new white paper recommending government funding for a dedicated fusion propulsion development program, styled similarly to ARPA-E and DARPA.


The next space race is not simply into orbit; it is to the Moon, Mars, and beyond. The global competition is fierce, and the stakes are high—from landing the first humans on Mars to harvesting the limitless wealth of asteroids, and much more. Fusion propulsion is the best path to winning this “Deep Space Race.”

Fusion Energy for Space Propulsion, FIA, June 2021

The goal is to provide funding not just for “paper studies,” but enough funding to build real hardware and start to test fusion propulsion concepts. We want the US to remain competitive in the upcoming Deep Space Race – building a human presence on the Moon, and then Mars, and beyond. Direct Fusion Drive is directly applicable to near-term, modestly sized fusion propulsion!

If you want to express your support for government funding of fusion propulsion, contact your Representatives and Senators!



FISO Talk: Fusion Drive for Rapid Deep Space Propulsion

On May 29, 2019, Ms. Thomas gave an invited talk to the Future In-Space Operations working group on Direct Fusion Drive (DFD) for deep space propulsion. The slides and talk audio are available from FISO’s online archive here. The group hosts weekly telecon seminars to discuss upcoming technologies and their potential impact on space operations.

Our talk introduces Direct Fusion Drive, explains how it is based on the Princeton Field Reversed Configuration (PFRC), and reviews some potential missions. There are summaries of the key physics points enabling the PFRC and the computational and modeling tools we apply. We conclude with the roadmap to spaceflight, including the supporting technologies that will be required for successful space engines, like lightweight space radiators.

We hope you enjoy this talk on DFD!

Job Opening for a Plasma Physicist

We are looking for a plasma physicist to join our staff in support our new ARPA-E contract on the Princeton Field Reversed Configuration (PFRC) experiment.

Candidates should be interested in both theoretical and experimental work in plasma physics related to nuclear fusion power generation. Familiarity with low- and high-temperature plasma diagnostics is desirable. Background on any magnetic fusion device is also desirable. The position includes:

  • Help run experiments on the PFRC-2 (located at the Princeton Plasma Physics Laboratory) and analyze data.
  • Analytical and numerical work, including MHD simulations and PiC simulations.
  • Numerical modeling of plasmas.
  • Work in other areas at PSS including control, estimation, machine learning and orbit dynamics.
  • Programming in MATLAB, Python and C/C++.
  • Write proposals and come up with new topics for proposals including SBIR and STTR proposals.

Requirements include:

  • Ph.D in plasma physics (may be a recent or 2019 grad)
  • Must be a US citizen.

If you are interested, send your resumé to info@psatellite.com