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Raumfahrt - PHYSICS GETS A VOTE: NO STARCRUISERS FOR SPACE FORCE

29.06.2021

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Wars in space will never be like Star Wars. “Starfighters” will not engage in dogfights with unlimited maneuverability and range. An actual conflict in space would be slow and deliberate, requiring prepositioning of weapons and meticulous planning.

Even defense analysts working on the matter understate the physical constraints on warfare in space. In a recent War on the Rocks article, Jeff Becker claims that the era of “starcruisers” — spacecraft whose maneuvering is not principally dictated by orbital mechanics — is closer than people think. While Becker examines some meaningful technological developments, his analysis — like other work on the topic — does not recognize the challenges and physics that would be involved in fighting a conflict in space.

Policymakers and defense planners need to have a realistic understanding of what is physically possible and practical. As we explain in a recent research paper and accompanying video, in any space war physical limitations will constrain both the movement of assets and overall strategy. Space is big and satellites maneuver slowly while following predictable trajectories. Major limitations such as these mean that starcruisers like the Millennium Falcon or Galactica will never exist.

Physical Constraints in Space 

Spacecraft and space weapons cannot defy physics. As they develop their ideas about spacepower, strategists should internalize five major points and use them to attain advantages, rather than working against physical realities.

Satellites move quickly. Those in commonly used circular orbits move at speeds of 6,700 to 18,000 miles per hour (three to eight kilometers per second), depending on their altitude. In comparison, an average bullet only travels about 1,700 miles per hour (0.75 kilometers per second). To an Earth-bound observer, satellite speeds might suggest that science fiction is the right way to imagine space war, but speed is not actually the biggest driver of fighting in space.

Satellites move predictably. Due to orbital dynamics, satellites exhibit a strict relationship between altitude, speed, and orbit shape. Gravity dictates that satellites at lower altitudes always move more quickly than those at higher altitudes. This makes their path predictable — and deviating from an orbit is costly and slow — which means that engagements, whether peaceful or aggressive, can be planned long ahead of time regardless of the speed at which satellites are traveling.

Space is big. The volume of space between low Earth orbit and geostationary orbit is about 50 trillion cubic miles (200 trillion cubic kilometers). That is 190 times bigger than the volume of Earth. Even if satellites and spacecraft are designed to have more energy for maneuverability, distances in space are so big that extensive maneuvering will remain painstakingly slow.

Timing is everything. Within the confines of the Earth’s atmosphere, airplanes, tanks, and ships can move in multiple directions. By comparison, satellites are always moving in a circular or elliptical path due to the gravitational pull of Earth. The nature of conflict often requires two competing weapon systems to get close to each other, in this case “on-orbit.” Therefore, timing is everything.

Satellites maneuver slowly. While satellites move quickly along their orbit, purposeful maneuvers to a different orbit are relatively slow. That is because space is big. Since gravity dictates the motion of a satellite, deviating from the prescribed orbit requires the use of an engine to maneuver. However, those deliberate maneuvers can only happen at certain points in an orbit. Adopting a mindset and a strategy of slow and deliberate movements is needed, rather than thinking about flashy “light speed” jumps.

The X-37B: Not a Forerunner to Starcruisers 

A starcruiser will not exist because it would have to defy these physical constraints, especially the realities that satellites move predictably and maneuver slowly. Becker argues that two current programs — the X-37B and SpaceX’s Starship — show that starcruisers will be here soon enough that analysts need to begin thinking through the implications for spacepower. But, in reality, there’s no prospect of a technological breakthrough that will allow spacecraft to effectively “beat” physics. Neither the X-37B nor the Starship show that the fundamentals of spacepower will change anytime soon. Those programs should not give false hope to those who dream big about space.

The X-37B spacecraft offers real benefits because of its overall reusability and its ability to conduct experiments and then return them to Earth. It is unique in that it operates in low Earth orbit and leverages the atmosphere to maneuver. That allows it to move in unpredictable ways, but it is not a precursor to starcruisers. To understand why, consider the concept of “delta-v,” which is the change in velocity required to alter a spacecraft’s trajectory. Since there is a strict relationship between a satellite’s velocity and its orbit, a change in velocity will necessarily change the orbit. To deviate from the path determined by orbital dynamics, delta-v is required. The amount of delta-v carried by a satellite is known as its “delta-v budget.” This metric is independent of the satellite’s size and propellant type. Thus, contrary to what Becker suggests, there is no concept of delta-v efficiency. What matters is the overall delta-v capacity of a satellite and how quickly it can be applied to accelerate the satellite.

The X-37B has a total delta-v budget of 10,200 feet per second (3,100 meters per second) for a mission. That is not radically different to the delta-v budget of a typical low-thrust geostationary orbit communications satellite. That type of satellite spends 5,900 feet per second (1,800 meters per second) of delta-v just to get to its orbit and then requires about 180 feet per second (55 meters per second) per year to maintain its orbit. If we also include what is needed for re-positioning, disposal, and other maneuvers, a typical geostationary orbit satellite’s delta-v budget approaches 13,100 feet per second (4,000 meters per second). Even more strikingly, the X-37B’s delta-v budget is small compared to the commercially available Boeing 702SP satellite bus, which first flew in 2012 and carries up to 22,300 feet per second (6,800 meters per second) of delta-v. Nobody thinks these satellites are forerunners to starcruisers. Neither is the X-37B.

The Starship: Evolutionary Technology 

SpaceX’s Starship is very exciting, but for the purposes of space warfare it is evolutionary not revolutionary. Starship is a huge, reusable rocket and spacecraft. It is able to refuel on-orbit, which provides it with a delta-v budget of 22,600 feet per second (6,900 meters per second) and the maneuverability that comes with it. If Starship was fully topped off outside of the Earth’s atmosphere, this uniquely large spacecraft would be able to perform more maneuvers and enjoy a longer lifetime. While these gains are real, they do not eliminate the constraints on maneuvering due to orbital mechanics. Making decisions about the number and location of fueling depots would also be a complex challenge, precisely because space is so big.

We are all for gaining refueling capabilities on-orbit, but for different reasons than one might think. Once the Space Force can have hundreds of millions of pounds of propellant in space waiting to refuel spacecraft, it will have the means to greatly improve freedom of movement and prolong the lifetime of satellites. But that will not make space operations analogous to air operations. Even Starship’s large delta-v budget is too small to enable it to match the orbit of many satellites. We calculated that if a fully loaded Starship is in an equatorial orbit (zero-degree inclination) at an altitude of 250 miles (400 kilometers), it would barely have enough delta-v to match the International Space Station’s orbit, at 51 degrees inclination. It definitely could not match satellites in sun-synchronous orbits, which have an inclination of about 98 degrees and are a common orbit for Earth observation satellites.

Among its major capabilities, the Space Force needs the ability to quickly launch payloads into their desired orbits and Starship could play an important role in performing that mission. But that need can also be met better by other systems. Recently, the Space Force demonstrated its ability to rapidly put satellites into orbit by using the air-launched Pegasus rocket. That rocket has a critical advantage over Starship: While Starship can currently only launch from two locations — Boca Chica, Texas, and Cape Canaveral, Florida — Pegasus can already be launched from a number of locations worldwide.

Science-fiction movies have conditioned people to believe that advanced propulsion systems enable travel in a straight line through space, unconstrained by orbital mechanics. However, our calculations show that in order to travel in a straight line from an altitude of 250 miles (400 kilometers) to 620 miles (1,000 kilometers), a spacecraft would need at least 130,000 feet per second (40,000 meters per second) of delta-v split into two equal, instantaneous bursts about 30 seconds apart — one to start the maneuver and one to stop it. To go from 250 miles (400 kilometers) to geostationary orbit — an altitude of 22,236 miles (35,786 kilometers) — a spacecraft would need at least 7,900,000 feet per second (2,400,000 meters per second) of delta-v split into two equal, instantaneous bursts about 30 seconds apart. The spacecraft would also need to survive the immense forces generated by accelerating that fast. This is not happening in our lifetime. Nor is it clear why this would be desirable. At the very least, the Starship, just like the X-37B, is nowhere close to achieving this type of maneuver. Even if they were, supporting infrastructure would be required to fight in space. Currently, the Space Force does not have the communications, navigation, and other capabilities necessary for the effective use of a starcruiser or other advanced spacecraft. That should not alarm American policymakers, precisely because starcruisers will not exist.

Starcruisers: The Stuff of Science Fiction 

Without having delta-v budgets orders of magnitude greater than that currently being assumed for Starship, orbital mechanics bound what spacecraft can do. As we have written elsewhere, large applications of delta-v by a spacecraft in a low Earth orbit can move it into orbits that either hit Earth or that enter the Van Allen belts, a region of charged particles that can reduce the spacecraft’s lifetime or permanently damage it. By trying to be fast, a starcruiser would be forced to make many large corrective burns to avoid these fates, destroying any “efficiency” provided by the large delta-v load.

Discussions focused on delta-v overshadow another key component of conflict in space: time. Since space is big, maneuvering in space is slow, even with large delta-v budgets. Maneuvering to rendezvous with another satellite, whether to refuel or attack, requires the approaching satellite to match orbital planes and phasing with the other one. Both satellites will need to end up in the same spot, at the same time, with no velocity difference. Larger delta-v budgets certainly enable faster transfers to different orbits, but it may still take days for a satellite to reach its target. As the Space Force’s capstone doctrine emphasizes:

Constrained freedom of maneuver is a defining attribute of space operations. … Due to extreme velocities, the amount of energy required to reach a different orbit may be significant enough to render the option unfeasible or impractical.

By embracing the constrained nature of space movement, the Space Force and other space operators can use Starship-like capabilities to enhance their warfighting abilities, rather than fight needless battles against natural forces that will inevitably win.

Looking Ahead 

Emerging space technologies are exciting. They promise to create more responsive and reusable launch vehicles and to provide refueling and repair opportunities on-orbit. However, contrary to the hopes of some defense analysts, they will not allow the Space Force to defy physics. Policy planning and strategizing should be based on reality and built upon an understanding of astrodynamics. Having spacecraft with massive amounts of fuel will certainly be beneficial to space operations, but that will not take space warfare into the realm of science fiction. Any conflict in space would be a slow and deliberate affair. An effective spacepower strategy should be developed with physics-based constraints in mind, not based on dreams of starcruisers.

Quelle: WAR ON THE ROCKS

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