Photonic Sailbeam

[Bill Phil]

Just today I had a thought about space propulsion. This is an attempt to make that thought coherent and to make the case for "Photonic Sailbeam Propulsion."

There are numerous challenges to travelling in space. Not least of which is the problem that the rocket equation brings in. It has been said that "every gram counts," and for good reason. Rockets are governed by the Tsiolkovsky rocket equation. The consequence of this equation is that, for a given specific impulse, doubling the delta-v of a rocket requires squaring the mass ratio. Of course this also means increasing the specific impulse will improve the delta-v as well - but this has consequences. Rockets carry their own energy - and the higher the specific impulse the lower the thrust for a given power output (this is not specific to rockets). These limitations make just reaching Earth orbit difficult. Perhaps the best example of the tyranny of the rocket equation is the Saturn V (a fairly inefficient rocket by modern standards, however the point stands). The first stage of the Saturn V - the S-IC - represented most of the mass of the rocket. The mass of just this first stage was over 2200 tonnes - while the total vehicle mass was around 3000 tonnes. Despite its immense size the S-IC only delivered its payload (the remaining stages and CSM/LM among other hardware) to an altitude of about 67 km and a velocity of only 2.76 km/s. Of course better performance could be found with modern rocket engines but the problem remains - rockets are inherently inefficient at imparting kinetic energy to payloads (however their efficiency for the entire rocket is fairly high, but this also includes the dry mass of the vehicle as well as payload). Overcoming the tyranny of the rocket equation would make space travel far more practical.

So how do we overcome the rocket equation?

The primary method of overcoming the rocket equation is by separating the vehicle from the energy and propellant source. Generally this is accomplished with beamed power.

The beamed power can take one of many forms, of which there are two categories with a third subcategory that warrants attention. These two categories are photon beam propulsion and mass beam propulsion - a subcategory of mass beam propulsion is pellet stream propulsion. Photon beam propulsion is fairly widely known and was made somewhat famous by Robert Forward and his many proposals regarding the concept. Mass beam propulsion is usually envisioned with neutral particle beams but pellet streams are another possibility. All forms derive thrust from momentum exchange against a "sail", the main difference being the source and mass of the propellant.

Photon beam propulsion has been studied to a fairly large extent. However it is known to have intrinsic limitations that limit its efficiency and capability. These limitations come from the fact that photon beams (lasers are an example of a photon beam and are fairly well studied for propulsion) are made up of photons, massless particles that can not exchange energy by slowing down but only by changing wavelength. Depending on how much the wavelength changes after reflecting off of the photon sail very little energy may be imparted to the sail, despite the immense amount of energy spent to create and emit the photon. Essentially this is an inelastic collision process. Photon beams also suffer from diffraction, leading to large space structures (such as lenses) to overcome this, and of course the high energy requirement for relatively little impulse leads to massive power requirements - about 150 megawatts per newton. For a photon rocket the required energy would be twice as high (ignoring losses), however the sail requires half as much due to the reflection of the photons - doubling the momentum exchanged (or at least very close to double). There are other limitations as well such as the heat capacity of the sail material - which limits force per unit area. These problems all combine to make laser propulsion fairly inefficient, though the vehicle does not need large mass ratios for large mission delta-vs.

The alternative to photon beam propulsion is mass beam propulsion - where the propellant is composed of discrete units of mass that are converted to some easily manipulated form (plasma is often considered) and then the vehicle exchanges momentum with the propellant (for example, by forcing against plasma with a magnetic field). These have the advantage of potentially being more efficient than photon beams. This is because the particles/pellets can be decelerated - allowing for more elastic collisions than photon beams. But there are limitations and other issues with mass beams as well. Particle beams have dispersion that limits their range, and this is not as "easy" to fix as using large optics as a laser system could use. Pellets also have their own issues as well - to launch a pellet stream with sufficient momentum would require a massive accelerator facility. It appears that mass beams have not been scrutinized to the same degree as photon beams for propulsion purposes. Even so it appears from the current research that mass beams may be a promising form of propulsion. 

An interesting proposal was made by Jordin Kare - what if both photon beam propulsion and mass beam propulsion were to be combined? His proposal for doing so involves using a laser facility to accelerate a stream of small objects (a pellet stream) to a mass beam vehicle. This is essentially a pellet stream propulsion system but the primary issue with accelerating the pellets - large infrastructure to accelerate the pellets - is massively reduced in scale. Kare also compares such a system to a photon beam propulsion system - the total energy and laser power do not change (at least not by much) but the infrastructure for collimating the beam can be much smaller. According to Kare's analysis it is possible to accelerate the pellets at 32 million gees, though this may not be achievable or desirable. This system is called a "sailbeam", because the mass beam is made up of photon sails. However, because it uses photon beam propulsion the efficiency is fairly low. Though collimation/dispersion may not be an issue as the pellets, or "microsails" in Kare's terminology, may have a capacity for self guidance. This may be difficult to implement, however.

In recent years a new concept for photon beam propulsion has been studied - photonic laser thrusters. This is relatively new and also not very widely known. The concept is derived from the improved momentum exchange when a photon is reflected off of a sail as opposed to emitted by a photon rocket. What if the photons could be "recycled"? That is, what if the photons could be reflected multiple times? This takes advantage of the excess energy/momentum photons retain after reflecting off the sail. They still contain quite a large amount of energy and momentum after they provide impulse to the vehicle. So a photonic laser thruster can be much more efficient than a photon laser sail. This proposal also takes advantage of optical cavities, and the technology has been demonstrated in a laboratory setting. However this propulsion system may be limited in range.

My idea is this: what if the sailbeam and the photonic laser thruster were combined? Of course this can introduce new issues, but this would allow for a less powerful laser propelling the sailbeam - improving system efficiency tremendously, and potentially allowing for an achievable pellet stream propulsion system. The potentially limited range of photonic laser thrusters is not nearly as much of an issue when the photonic laser thruster is not directly accelerating the vehicle, and pellets may not suffer from dispersion/diffraction or related issues nearly as much (and there is potential for self guided pellets). I decided to call this system concept "Photonic Sailbeam Propulsion." Essentially this could allow for much higher sailbeam performance.

Kare's Presentation Slides:

http://www.niac.usra.edu/files/library/meetings/fellows/oct01/597Kare.pdf

Lab demonstration of photonic laser thrusters:

 

(Sorry for the wall of text - I felt that I had to explain the underlying concepts and the thought process. Photonic laser propulsion is quite new and not many are aware of it, so I felt that I had to explain it.)

Any thoughts?

Edited August 29, 2019 by Bill Phil

[farmerben]

Catching electrons is a lightweight solution.  A giant helmholtz coil can concentrate electron flux from several square kilometers.  The same hardware can catch protons too.  To travel long distances a hydrogen beam would hold up better than pure charges.

We could potentially use the atmospheres of planets, moons, and stars as sources of massive particles stimulated by remote lasers.  

[Bill Phil]

10 hours ago, farmerben said:

Catching electrons is a lightweight solution.  A giant helmholtz coil can concentrate electron flux from several square kilometers.  The same hardware can catch protons too.  To travel long distances a hydrogen beam would hold up better than pure charges.

We could potentially use the atmospheres of planets, moons, and stars as sources of massive particles stimulated by remote lasers.  

A neutral particle beam would go much farther than a charged beam, but there is still beam divergence. 

However a concept in recent years can potentially increase the range by coupling lasers and particle beams.