Monday, 16 November 2020

Nuclear Photon Rockets: Flashlights to the Stars

In this post, we will have a look at the concept of using a nuclear photon rocket for interstellar travel. They are an old concept that should theoretically be the ultimate form of relativistic propulsion.
However, today they are unknown or unpopular. Why might that be the case?

The image above is by David A. Hardy

The interstellar challenge
The Daedalus starship.
Interstellar travel is on a completely different level than interplanetary travel. The distances involved are orders of magnitudes greater. The shortest distance between stars is measured in trillions of kilometres. To face such distances, high velocities are required. 
A robotic probe might not mind spending several centuries to reach a destination. A human crew would want the trip done in their lifetime. Taking longer than that means running into technical and ethical trouble. The closest star to our Sun is Alpha Centauri A, currently sitting  40 trillion kilometres away, or 4.2 light-years. It would take 4.2 years to reach it when travelling at the speed of light. If we want to complete the trip within 20 years, we would have to travel at 21% of the speed of light. We also want to slow down at the destination. This means that we need a way to accelerate up to 21% of the speed of light, and then slow down back to zero - the deltaV sum is 42% of the speed of light.

So how do we go that fast?
The Falcon 9's Merlin rocket engines.
Rockets are the space propulsion system we are most experienced with. There are many ways to measure a rocket’s performance, but only some are relevant to interstellar travel. Thrust, for example, is much less important when the trip will take many years; taking one month to accelerate instead of ten months is no longer a significant factor. Instead, let’s focus on exhaust velocity. Using the Tsiolkovsky rocket equation, we can work out the ratio between propellant and non-propellant masses of the rocket we are using.
  • Mass Ratio = e^(DeltaV/Exhaust Velocity)
DeltaV in m/s
Exhaust Velocity in m/s

A chemical rocket consuming oxygen and hydrogen propellants has an exhaust velocity of 4,500m/s. We find that for a chemical rocket to achieve a deltaV of 42% of the speed of light, we would need e^28000 kilograms of fuel for each kilogram of equipment, structure, engines and payload. That is a number that lies between 10^8428 and 10^13359. For comparison, the entire mass of the Universe is estimated to be 10^53 kg. Chemical rockets for relativistic travel are beyond impractical. 
The needle array of Enpulsion's IFM nano thruster.
How about a rocket engine with a better exhaust velocity? Something like one of our most efficient ion thrusters? The Ultra-FEEP thruster that accelerates liquid indium to nearly 1,000 km/s is the best we can expect for now. It would still not be enough for relativistic velocities. To achieve a deltaV of 42% of the speed of light, we would need 6*10^55 kg of indium for each kilogram of dry mass. 

If you run the numbers yourself and lower the deltaV target, you would still find ridiculously high mass ratios being required. A deltaV target of just 2% of the speed of light, which would turn the trip to the nearest star an endeavour that spans about half a millennium, would still require a physics-breaking mass ratio of 10^579 from the chemical rocket, and a mass ratio of 453 from the Ultra-FEEP thruster. The lower value for the electric thruster seems much more reasonable, until you consider that indium is found at a concentration of 0.21 ppm in Earth’s crust. At our current output of 700 tons per year, a 1,000 ton dry mass craft would require at least seven centuries of indium production to fill its propellant tanks.
To get away from these extreme figures, a logical decision would be to increase the exhaust velocity all the way to the maximum. The maximum is the speed of light. 

Photon Propulsion
When your exhaust is light itself, the mass ratios required for relativistic velocities become decidedly modest. Light, more specifically photons, can be produced indefinitely ‘out of nothing’. In other words, if you heat up a surface, you can create a photon rocket that spontaneously produces and emits light without ‘running out’ of anything. All that is required is a power source. The more energetic the power source, the more photons that can be produced and the higher the photon rocket’s performance. The theory fits together neatly. 

The concept of using a nuclear reactor to heat up a surface so that it emits enough photons to produce appreciable thrust is at least 50 years old. Nuclear photon rockets could solve our problem of interstellar travel by harnessing the greatest sources of energy and utilizing the exhaust with the highest velocity. All the fuel they would ever need would be loaded up at departure, so they do not have to rely on the existence of any infrastructure at the destination or any assistance along the way. Perhaps they would have enough to return to us without having to refuel!

However, ‘photon starships’ are not a popular idea today. They are not featured in NASA’s NIAC programs, nor are aerospace engineers dreaming up modern designs for them. What ‘catch’ has them relegated to relics of the past?

Fission Photon Rocket
A nuclear photon rocket from Boeing's PARSECS study.
Let us start with the most familiar of nuclear energy sources: the fission reactor. 

A fission reaction produces about 80 TeraJoules for each kilogram of maximally enriched fuel. 95% of this energy is in the form of gamma rays or fission fragments; they can be blocked by a thick wall and converted into heat. About 5% leaks out in the form of neutrinos. This reduces the ‘useful’ energy density of fission fuel to 76 TJ/kg.

In a typical reactor, the fuel is in solid form. Only a fraction of its potential 76 TJ/kg can be extracted in one fuel cycle. The products of fission, such as xenon-135 and samarium-149, remain trapped next to the fuel. These isotopes have a high neutron cross-section, which means that they trap and absorb the neutrons needed to sustain a fission reaction. Nuclear engineers consider these products to be ‘poisons’. If enough poisons accumulate in the fuel, the fission reaction cannot be sustained. 

The result is that a single fuel cycle achieves very low burnup of the fuel, which is the percentage of fissile fuel that has undergone fission. Typically, this is 1% to 5% of the total fuel load inside a reactor. On Earth, nuclear engineers deal with this problem by shutting down a reactor, extracting the slightly used fuel and sending it off for reprocessing. This involves removing the poisons, mixing in a small quantity of fresh fuel, and then returning it all to the reactor. 

A spaceship does not have the luxury of regularly halting its reactor while also lugging around a nuclear fuel reprocessing facility. 
Instead, we need to use a type of reactor that grants high burnup with no reprocessing necessary. The best option seems to be a gas-core nuclear reactor. In this high temperature design, the fuel and poisons are in a gas phase. It becomes easy to filter out the poisons as they are chemically very different from the fuel. We can have the fuel  circulate within the core for as long as needed to achieve near 100% burnup.

With the burnup problem solved, we can convert those 76 TJ/kg into heat. 

From a physics perspective, only about 0.77 grams of matter in a kilogram of fissile fuel becomes energy. This leaves us with 999.23 grams of waste after consuming the fuel. With no further use for it, we eject it to lighten the spacecraft. 

Imagine a nuclear starship designed specifically to make our next calculations easier.

It consumes 1 kg of fuel per second. The average power output is 76 Terawatts. 
  • Photon Thrust = Power/ Exhaust Velocity
Thrust will be given in Newtons
Power is in Watts
Exhaust Velocity in m/s

Those 76 Terawatts should result in 253.3 kiloNewtons of thrust. With a 95% efficient photon emitter, we gain a real thrust of 240.7 kN. 

After producing this thrust, we eject 999.25 grams of waste. 
  • Effective Exhaust Velocity = Thrust / Mass Rate
Effective Exhaust Velocity will be given in m/s
Thrust is in Newtons
Mass Rate is in kg/s

The ‘effective exhaust velocity’ based on this thrust and the amount of matter being ejected is actually 240.8 km/s. The critical point we make here is that while the thrust comes from photons travelling at the speed of light, exhaust velocity calculations must take into account all the masses being ejected.

So what can a fission photon rocket do with an effective exhaust velocity of 240.8 km/s?

It certainly cannot reach our desired deltaV. Achieving 42% of the speed of light would require a mass ratio of 10^523. Unless we have access to multiple Universes filled with highly enriched fissile fuel, this is impractical.

Even with an extraordinary feat of engineering so that we could load a starship with 100 kg of nuclear fuel for each 1 kg of dry mass (and not have it immediately go critical), the achievable deltaV is only 1,108 km/s or 0.37% of the speed of light.

Fusion Photon Rocket
What if we used the better nuclear rocket: the fusion rocket?

There are many different fusion reactions involving different fuels, but we are interested in those that provide the highest energy density.

Proton-proton fusion provides a whopping 664 TJ/kg. However, it is very slow, taking thousands of years to complete, and it is not realistic to ever expect to take place outside of stellar cores. Next down the list is Deuterium-Helium3. About 353 TJ/kg is on tap.

We won’t dive into the details of the various reactor designs that could be used, but suffice to say that near-complete burnup of fusion fuels is possible, and all the energy released can be converted into heat.

If we compare the mass of the Deuterium and Helium 3 before fusing, with the mass of the helium and proton particles after fusion, we notice that 0.39% of the mass is missing. That is the percentage of mass converted into pure energy. It is a much greater percentage than nuclear fissions’ 0.077%.
The list of particles involved in fusion reactions, with their exact masses.

Let’s repeat the previous calculation for the effective exhaust velocity of a nuclear photon rocket.

1 kg/s of fusion fuels are consumed, for a power output of 353 TW. This produces 1,117.8 kN of thrust out of a 95% efficient emitter. We expel 996.1 grams per second of waste, so the effective exhaust velocity is 1,122.2 km/s.

This is nearly five times than a fission photon rocket’s effective exhaust velocity. However, this is still not enough. 

Our desired deltaV of 42% of the speed of light comes at the cost of a mass ratio of e^112. While we could gather enough galaxies together to fuel our fusion photon rocket, we want something more practical.

The reality is that a plausible fusion photon rocket with a mass ratio of 100 would only have a deltaV of 5,167 km/s or 1.7% of the speed of light. Barely enough for a multi-century generation ship to cross the stars and certainly not enough for travel within a lifetime. Staging the fusion rocket will not help very much. 

Also notable is the fact that an effective exhaust velocity of 1.7% of the speed of light is actually lower than the exhaust velocity of direct drive fusion propulsion, where charged particles are directly released into space through a magnetic nozzle. DHe3 releases a 3.6 MeV helium ion and a 14.7 MeV proton. Their averaged velocity is 7% of the speed of light. A photon rocket is a very inefficient use of fusion energy.

Antimatter Photon Rocket
The ultimate fuel should give the ultimate performance. Nothing beats antimatter!

There are many types of antimatter. There are antielectrons, antiprotons, antineutrons and their combined form, anti-atoms like antihydrogen. Antielectrons annihilate with regular electrons in a ‘clean’ annihilation reaction that produces high energy gamma rays and nothing else. They are however the hardest type to store. Antiprotons are much easier to store, especially in the form of frozen antihydrogen ice. The downside is that their annihilation is ‘messy’, as it releases a plethora of products. With solid shielding, enough of the energy of those multiple products can be absorbed and converted into heat. We set the efficiency at 85%

Each kilogram of antimatter contains a potential for 90,000 Terajoules of energy. It must be matched by another kilogram of regular matter, so the average energy density is halved to 45,000 TJ/kg. As we only capture 85% of that amount, the useful energy density is 38,250 TJ/kg.

If we consume one kilogram of antimatter/matter mix per second, we would have a drive power of 38,250 TW. A realistic emitter would convert this into 121,125 kN of photon thrust. The effective exhaust velocity is 121,125 km/s or 40.3% of the speed of light.

With such a high exhaust velocity, an antimatter photon rocket would be able to achieve the relativistic velocities we desire. 

A deltaV of 42% of the speed of light would only require a mass ratio of 2.85. That’s 1.83 kg of antimatter/matter mix for each 1 kg of rocket dry mass. We might even be able to go much faster with high mass ratios; travel times to the stars in single-digit years seems possible.

However, antimatter is exceedingly difficult to collect or create. A mass ratio that seems acceptable for a conventional rocket would actually imply an unreasonable amount of antimatter. Existing accelerator facilities, if tasked with solely producing antimatter, would require about 3.6 ZettaJoules to produce 1 kilogram of antimatter. That’s 3,600,000,000 TeraJoules, equivalent to 286 times the total yield of all nuclear bombs today (1.25*10^19 J), or the total output of the United States’ electrical grid (1.5*10^19 J) for the next 240 years. 
If we were very serious about producing large quantities of antimatter, we could design a superbly optimized antimatter production facility, with very efficient antimatter capture mechanisms. Production efficiency can be increased to 0.025%. This means that 1 kg of antimatter would require ‘only’ 360,000 TJ to manufacture. An antimatter photon starship would ‘just’ need the combined output of all humanity (8*10^19 J/yr) for the next couple of millennia to fill it up.
An antimatter production facility.
In practice, the awesome performance of antimatter propulsion would be reserved for civilizations higher up the Kardashev Scale. 

Verdict and Consequences
All the calculations so far have assumed nearly perfect use of the energy released by fission, fusion or antimatter reactions. We have also ignored the massive complications that arise from trying to handle the power of those reactions. Despite this best case scenario, nuclear photon rockets do not seem to be up to the task of rapid interstellar travel. 

Fission and fusion power are just not energy dense enough. Antimatter is far too difficult to produce in huge quantities. The ‘catch’ is that physics is not kind to photon propulsion. For this reason, this sort of starship will remain a bottom-drawer concept for the foreseeable future. 

What effect does that conclusion have?

If we want to use rockets, we must accept that interstellar travel will be slow. Other techniques or technologies have to be employed to make crossings that last centuries. Cryogenic hibernation, life extension or digitizing the mind can enable the original crew to survive that long; generation ships or embryo seeding can allow another group of people to arrive at the destination. 
Robert L. Forward's Laser-propelled lightsails.
If we instead want interstellar travel done quickly, we cannot rely on rockets. All the popular methods for interstellar travel depend on non-rocket propulsion, such as Robert L. Forward’s massive laser-propelled sails or the ‘bomb-tracks’ discussed in a previous post. The energy cost of relativistic travel is no longer derived from a fuel carried onboard a starship, but from an external source. This external source takes the form of large infrastructure projects and preparations that require many years to complete; we trade away the flexibility and autonomy of rockets to gain huge speed, efficiency and cost advantages. 

A consequence of non-rocket propulsion is that interstellar travel cannot be a whimsical affair. It has to be planned a long time in advance (which has implications for the stability of the civilization organizing it all) and it would be evident to all observers at the departure and destination what is going on. No ‘secret’ missions to other stars!

Of course, a scifi writer might not like the sound of that. Their options lie in more exotic types of rockets, more advanced civilizations or speculative science. 

Examples of exotic rockets include a starship powered by a rotating black hole, where matter is converted into energy at 42% efficiency (an effective exhaust velocity of up to 252,000 km/s or 84% of the speed of light) or a Ram-Augmented Interstellar Ramjet, where the thin interstellar medium is added to the exhaust of a fusion reactor for a greatly improved effective exhaust velocity. More advanced civilizations handle enough energy to be able to produce large quantities of antimatter, overcoming the main difficulty with this fantastic fuel. 
Speculative science opens up the possibility of using ‘quark nuggets’ to rapidly and easily create antimatter, as well as wormholes and Alcubierre warp drives. 

Though, we must warn you, that these different options might be more troublesome than photon rockets!


  1. When you use up a fuel, you get a certain amount of energy for a given amount of mass. That mass of the spent fuel is now undesirable - it makes your spacecraft slower to accelerate and gives you no benefit. You might as well get rid of it. You are also using the energy you create from the fuel to impart momentum to your spacecraft - this is what gives you the thrust or delta-V. You always get more momentum for the same energy by using that energy to accelerate more mass. So compared to not accelerating any mass and trying to use pure electromagnetic radiation pressure, you will always get better performance by accelerating your spent fuel with that energy instead. The only exception is for reactions (like electron positron annihilation) where the spent fuel is purely electromagnetic radiation - but even here you're still using the spent fuel directly for your momentum. Hence, using anything other than complete annihilation to photons to produce light for a photon drive is inefficient and you would be better served by using that energy to accelerate the reaction products for thrust.

    1. Unfortunately nucleon annihilation produces a good fraction of pions which decay first to muon / anti-muon pairs and then electron / positron pairs. Each decay produces neutrinos, the energy of which is essentially useless for thrust.

      Efficiency may be high but it is not 100%

    2. @Luke:
      You are correct. There might be a complication though. While it is straightforward to design a a reactor that absorbs the total output of nuclear reactions within and radiates away that energy as photons, it is much harder to transfer that power into the spent fuel, especially when the temperatures involved are in the millions of kelvin. For example, how would you add energy to fission fragments without slowing them down?

      That's right, but from this paper that I cited ( it seems that solid shielding can effectively absorb nearly all the annihilation products before they escape as anti/neutrinos.

    3. If we are separating out the fission products (as was supposed in the article), you can run the reactor as a generator rather than a rocket, and electrically accelerate the fission products (for example). There will be inefficiencies, but the net result should still be orders of magnitude better than using the radiation pressure of the heat of the reaction.

    4. @Matter Beam, @Luke:

      The energy of fission reactions manifest themselves mainly as the kinetic energy of fission fragments already, so if these are allowed to simply bounce off against a magnetic field then no acceleration of these fragments is required.

      In standard fission reactors this is impossible because most of the fragments impact the fuel rod atomic lattice and decelerate, giving off their energy as low-grade heat. But this need not be the case - in fission fragment rockets, the fuel pellets are smaller than the mean free path of the fission fragments, and as a result many of the fragments simply escape the pellets and are collimated into a very high velocity exhaust stream. With fission-fragment rockets you can get close to the theoretical maximum specific impulse of fission-powered rockets, or about 5% light speed.

  2. Ah well. Only goes to show that humans, in our current state, are unsuitable for space travel. We operate on way too fast of a timescale to appreciate the Universe. I have often wished I could observe the lifecycle of a planet, from planetary coalescence through geologic ages and the evolution of life, and the eventual descent into a steady state or consumption by the aging parents star. Hard to do that in only 3.6e18 transitions of H.

    1. Perhaps then it is our idea of travel that must be adapted. We are used to physically moving our living bodies to a destination, but we also know that many other forms of travel are possible!

      Imagine how easy it would be to transmit our consciousness if it were digitized? It would feel instantaneous.

    2. Not to mention the handy side-effect of digital immortality such a technology would enable! And the not-so-handy side effect of, if it can be digitized and transmitted, it can be COPIED. I can imagine the ethical implications of such a technology are so vast we can't even comprehend how easily it could be abused.

  3. What about tha Annihilation Drive that has been popping up recently, what category would you file it under?

    Link to article:

    1. The exhaust is supposed to be mesons. Mesons decay very very quickly into electrons and neutrinos. Since these are not photons, it is not a photon rocket. However, it could be modified into a photon rocket by absorbing the energy released from this annihilation-like reaction as heat, and then radiating it away.

    2. Would that make it more efficient )thanks for the reply!)?

    3. It would be hard to say without more details.
      If the shielding managed to absorb the mesons very quickly, then an annihilation photon rocket would be more efficient.
      If the shielding was not effective enough, and the meson had time to decay into an electron and a neutrino, then it would be less efficient as the neutrino would slip away and take energy with it.

  4. At first glance, I'm not keen on that idea. If you make the antimatter on board ship, you are introducing inefficiencies and adding extra waste heat to the process, all of which does not contribute to thrust. Better to make the antimatter at home, perhaps using the same method, then carry it along as a kind of energy storage medium.

    1. Agreed. I would also note that there is no practical method of creating antimatter onboard a starship. Whatever the original form of the energy used to create antimatter, it will be less dense than antimatter itself, so it would be a net loss to try to transport it instead of just antimatter.

    2. Regarding onboard vs. offboard creation of antimatter, this reminds me of a concept I came up with years ago. One of the biggest disadvantages of antimatter creation is the huge power cost. Could you do an article on the practicality of solar powered antimatter generators located on the sunward side of Mercury? How big of a setup would be required to create antimatter in reasonable volumes needed for interplanetary travel given current (or near-future) antimatter generation technologies?

  5. Two other related ideas to examine -

    Photonic laser propulsion as developed by Young Bae is an interesting riff on laser sails, where the beam is reflected from the laser sail, but then reflected back to the sail by a high quality optical mirror. One can imagine a laser battery on the moon illuminating the sail, and a field of mirrors gathering the reflected laser light and refocusing it back on the sail.

    If you go to, there is a poster named William Mook who has written fairly extensively on this, although he extrapolates to some rather extreme energy levels (solar lasers driven by the solar photosphere) to postulate vehicles moving at very large fractions of the speed of light.

    A demonstration here:

    A separate idea uses antimatter, but not in the way most proposals do. Positrons are generated and collected from decaying radioactive material and used to drive fusion reactions in duterium - essentially an antimatter driven fusion drive. While the company seems to have closed shop, the idea is interesting:

  6. Thrust = 2 * power / exhaust velocity is not the correct equation. That is a Newtonian equation and a photon is as far from Newtonian as you can get. The actual equation for a photon rocket is Thrust = power / c, twice as much. And even after that the assumed 95% efficiency doesn't even have a theoretical basis as far as I'm aware.

    Or just derive it from the equations for photon momentum and photon energy.

    1. More realistically you might get 60% efficiency in power generation, 95% in power transfer, 80% in photon generation, and 95% in thrust directivity, for a total fuel to thrust efficiency of just over 43%. Still dramatically better than chemical rockets, but the mass penalties for power generation and thermal control are enormous.

    2. I was looking at an idealized 'best case' scenario for photon rockets. Even with those assumptions, they fail to perform well. If we start including all the conversion losses and inefficiencies, they will look even poorer!

  7. It seems we long way from doing star travel.
    It seems people rather than government will be star traveling.
    And we need more markets other than just Earth orbital satellite market.
    Maybe a photon rocket could used to go from Earth to Mars more quickly- so the thrust has to exceed to force gravity of Sun between Earth and Mars and travel a short distance of less than 100 million km distance.
    And we can develop a photon rocket that does that the best.
    Or at moment we are at 200 years ago, trying to imagine a car going 100 mph or the airline business.
    It seems all we need at moment is chemical rockets and cheap chemical rocket fuel in Space.
    And it seems with chemical rockets and cheap rocket fuel {say made on our moon} one could get to Mars from Earth in less than 3 months. Whereas photon rocket might do it, in 1 month and use less rocket propellent.
    What is obvious to get to Mars in less than 3 months, requires a trajectory which is not hohmann transfer- which roughly require going from one side of sun {in some solar orbit} to opposite side of the sun.
    It seems one could do non hohmann transfer from Earth to Mars with a chemical rocket- though we never done it before.
    We have done non hohmann transfers. One say gravity assists involve them, and ion rocket which spiral up a gravity well are doing a non hohmann.
    Now to do non hohmann, you would changing the vector- gravity assist are largely used to change vector [but it's robbed from orbital body}.
    So with photon rocket to Mars in 1 month, one paying for the vector change, but delta-v needed is cheap with the photon rocket.
    And it seems for intersteller travel {with whatever non chemical rockets] one probably going need a lot of stealing orbital energy or other energy. And/or use most efficient way {as we are mostly using most efficient transfer of hohmann.
    Plus since humans copy things like birds flying, observation of space aliens might give human something to copy. Or it seems one say large telescopes are "like" star travel, but with large telescope we might discover how other creatures are star traveling.

    It seems main problem {other than not close to able to do star travel] is we could have wrong kind of star. And finding the right star {or gravity well}, could be needed.
    Or it seems Trappist solar system is designed to allow star travel.
    Though being 40 lightyear away from us, makes it not useful to us.
    And somehow making our solar system into something that, does not seem possible.
    But it seems might find stuff within lightyear or 2 of us, which could be useful, perhaps.

    1. Hi!

      Distances like 100 million km might seem huge to us, but they are rather small on the interplanetary scale. They are in fact too small for photon rockets to be useful for crossing them, because their acceleration is too low. Any other rocket, even 'weak' ion engines, will cross that distance in far less time.

      You mention non-Hohmann transfers. It is only possible to make a 'near-straight' line from one planet to another if your acceleration greatly exceeds the solar gravity in the space between them. At 1 AU from the Sun, where our Earth orbits, the solar gravity is 6 mm/s^2. For a photon rocket to get that sort of acceleration, it would have to have an engine that has a power density of over 3 MW/kg. That is beyond the capabilities of even the most powerful nuclear thermal rockets. In fact, it is most likely going to be in the handful of kW/kg, a thousand times less than our requirement!

  8. Well, what if chemical rocket could get it a 1/3 of the way there.
    So chemical rocket have reached Moon distance in 9 hour {New Horizon did it, as did others, I think Ranger probes impacting moon did this. So chemical rocket finished it's burn while still within LEO distance, will throw its payload so it passes by a full Moon within say 6 hours {or say, +10 km/sec added to Earth orbital velocity] and chemical stage brakes and stays within Earth's gravity well. But starting from LEO distance one is adding to velocity with second stage photon rocket. So photon rocket is reducing gravity loss from Sun for third of the way, and Sun gravity weakens as get closer to Mars.
    1.2 Squared: 1.44, and 1.42 AU is 1/2 of Sun gravity.
    But if not, perhaps one do it with the ion or a NERVA type nuclear thermal rocket- though, with much longer burn with less thrust.

  9. It seems if spacecraft can't provide enough thrust that it's can't accelerate more than 10 km/sec in a month, it's not spaceship that can travel to stars, though if using something like solar sails which can get a lot acceleration near a sun/star- say about 100 km/sec in week- it might work.
    Can photon rocket use solar energy for propulsion?

    It seems important aspect related to star travel is getting "free energy" from gravity well- robbing orbital velocity and getting a Oberth effect- with planet Earth one can add or remove it's orbital energy and can get an Oberth effect with chemical rockets.
    Jupiter better than Earth. But if Jupiter was colder, it would work better.
    Jupiter core temperature may "about 24,000 degrees Celsius" what if it were 1/2 as hot- core temperature cooled to about 12,000 C? Plus if it had less lethal radiation it would be more useful.
    And question is how many Jupiter or greater mass planets are between us and our nearest stars.
    And when will solar systems become closer to us.
    If there was Jupiter mass .1 lightyears from us and in direction one wanted to go, can it be used.
    It seems if there was Jupiter mass .1 lightyears from us, it would have a good chance of being much colder and not have lethal radiation.