For the entire space age, going back to the late 1950s, the vast majority
of satellites have been launched into space with virtually no propellants. The extent of their ability to move has been small movements with small
thrusters - primarily station keeping, that is maintaining their position and
orientation.
It has often been said that once a satellite achieves its desired orbit, it's
not going anywhere. That's because the energy needed to make significant
changes to one's orbit is very high.
In space today, the current choices of on-orbit maneuverability are not
optimal. There are conventional rocket-powered thrusters that require an
extraordinary amount of propellant to move around. There are ion thrusters,
which are significantly more fuel-efficient, but cannot make changes
quickly. And that's really about it.
You've long heard the saying, "space is hard" and this is a shining example of
that. Changing velocity, usually referred to by the more or less mathematical
shorthand "delta-v," is the way orbits get changed. More than that, it's how
one can get around the solar system, or from Earth orbit to a different Solar
orbit.
But a new company, Portal Space Systems, emerged from stealth on Tuesday
with an alternative: solar-thermal propulsion. The company's founder, Jeff
Thornburg, said Portal Space will focus on mobility in orbit.
"All propulsion has historically been designed for station keeping for
satellites, not maneuverability," he said in an interview. "And too many
commercial and military customers are struggling with how to spend the very
little delta-v they have before they end the life of their asset."
"Delta-v" is how the space industry measures the change in velocity that a
spacecraft is capable of—more precisely, it is a measure of the
impulse per unit of spacecraft mass. So if your spacecraft has a
delta-v capability of 500 meters per second (1,120 mph), and starts at a
velocity of zero, then after it burns all of its propellant it would be
traveling at 500 m/s.
You know the saying that the first step is the hardest and it applies in
extreme to getting around in space.
It requires an extraordinary amount of delta-v to go from the surface of our
planet into low-Earth orbit (LEO)—very nearly 10,000 m/s. This is why
powerful rockets are needed to launch satellites. After reaching LEO, the
relative delta-v costs to go places from there are lower, but still high.
For example, it costs another 6,000 m/s to reach the surface of the Moon
from LEO.
Note that most satellites are capable of 500 m/s delta-v, or less, compared to
that 6,000 to get to the moon.
This is where Portal Space believes it has a solution. Thornburg says the
company is developing a spacecraft built around the concept of solar thermal
propulsion, which uses solar energy to heat propellant and produce thrust.
Such engines have been studied for decades, but have never been developed
for practical purposes. The company has not disclosed its propellant of
choice, but Thornburg said it is storable on orbit, and not toxic like
hydrazine. (It might be something like ammonia.)
For most of the three years
since the company was founded in 2021, the small crew of just Thornburg and his cofounders Ian Vorbach and
Prashaanth Ravindran, has been working on the engineering of the engines they
envision. Turning the vision into hardware. There are ~25 employees
supporting Portal now, per Thornburg, and the company’s plans for growth over
the next year are aggressive.
He envisions a fleet of refuelable Supernova vehicles at medium-Earth and
geostationary orbit capable of swooping down to various orbits and providing
services such as propellant delivery, mobility, and observation for
commercial and military satellites. His vision is to provide real-time,
responsive capability for existing satellites. If one needs to make an
emergency maneuver, a Supernova vehicle could be there within a couple of
hours.
"If we’re going to have a true space economy, that means logistics and
supply services," he said.
Developing a spacecraft with a novel propulsion system and an enormous
amount of delta-v capability may sound ambitious, especially for a small
startup. However, Thornburg has some credible experience, having worked in
the military, for NASA, and at various space companies, including SpaceX,
where he was a vice president of propulsion and a lead designer of the
Raptor rocket engine.
To Thornburg's statement that a true space economy means logistics and supply
services, I would add the ability to repair satellites on orbit and refuel the
ones that are exhausting their maneuvering fuel. A long way of saying maintenance.
Concept drawing of a possible Supernova spacecraft design. Image credit:
Portal Space Systems
Portal Space announced Thursday that it has received $3 million in funding
from the US Space Force to support development of the Supernova satellite
bus. Thornburg said the company plans to launch its first satellite toward
the end of 2025 or in early 2026. It will likely go to medium-Earth orbit
and, at a minimum, demonstrate its large delta-v capability.
...
“Our DoD customers have said they have real needs that really need to get
met by 2026,” Thornburg said. “So…how can we accelerate this capability that
the warfighter says they want as fast as possible to meet some of the needs
that they’re looking at here in the next couple of years?”