Solar System Travel With Advanced Propulsion

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If we achieve advanced nuclear, antimatter propulsion or other advance propulsion it would be possible to achieve near constant acceleration. This would enable travel times to Mars in the 3-8 day range depending upon where Mars was in its orbit relative to Earth. This would be with constant 0.33G acceleration and deceleration.

What exactly is a Brachistochrone Orbital Transer?

A Hohmann orbit is the maximum transit time / minimum deltaV mission. Weak spacecraft use this because they do not have a lot of deltaV. All current space probes use Hohmann because currently there ain’t no such thing as a strong propulsion system.

A “Brachistochrone” is a minimum transit time / maximum deltaV mission. Torchships use this because they have lots of deltaV to spare.

A torchship is a spacecraft with more than 300 km/s total delta V and an acceleration greater than 0.01 g.

There is an online site for determining solar system travel times with advanced propulsion.

The most probable torchship designs proposed are nuclear pulse propulsion (e.g., Project Orion) and nuclear salt water rockets (NSWR). These leverage high thrust and exhaust velocity to achieve >300 km/s delta V and >0.01 g acceleration, enabling Brachistochrone transfers. Among nuclear rockets, those using nuclear fission, particularly Project Orion, are the most achievable with current technology due to their development in the 1960s and reliance on known physics. The NSWR is also fission-based and promising but less mature. Fusion-based propulsion (e.g., Daedalus) offers superior potential but awaits technological breakthroughs. Beamed power to ion drives or laser thermal rockets could work but demands advanced infrastructure, reducing near-term feasibility.

1. Nuclear Pulse Propulsion (e.g., Project Orion)
Description: This concept, developed in the 1950s and 1960s, uses nuclear explosions (typically fission bombs) detonated behind a spacecraft to propel it. A pusher plate absorbs the blast, transferring momentum to the vehicle.

Performance:
Exhaust velocity: Approximately 20–60 km/s, depending on the design (e.g., specific impulse of ~6,000 seconds yields ~59 km/s).

Thrust: Extremely high due to the explosive force, enabling accelerations well above 0.01 g (potentially up to 1 g or more, though typically lower for crew comfort).

Delta V: For interplanetary missions (e.g., Mars in 125 days), delta V is in the tens of km/s, but interstellar designs (e.g., Freeman Dyson’s analysis) suggest velocities up to 10% of light speed (30,000 km/s). With a mass ratio of ~160, a delta V of 300 km/s is feasible, though it requires significant propellant.

Fit for Torchship: Meets both criteria handily, supporting Brachistochrone trajectories (continuous acceleration to midpoint, then deceleration).

Status: Feasible with 1960s technology, making it one of the most developed proposals.

2. Nuclear Salt Water Rocket (NSWR)
Description: Proposed by Robert Zubrin, this system uses a solution of fissionable material (e.g., uranium salts) in water as propellant. The mixture undergoes a continuous nuclear reaction in a nozzle, producing a high-thrust exhaust.

Performance:
Exhaust velocity: Up to 4,725 km/s (based on theoretical estimates), far exceeding most fission systems.

Thrust: High, due to the continuous explosion-like reaction, supporting accelerations >0.01 g.

Delta V: With an exhaust velocity of 4,725 km/s, a delta V of 300 km/s requires a mass ratio of just ~1.065, meaning very little propellant is needed, easily surpassing the 300 km/s threshold.

Fit for Torchship: Exceptional efficiency and thrust make it a strong candidate for Brachistochrone missions.

Status: Conceptually sound but untested; engineering challenges (e.g., containment) remain.

Fusion-Based Propulsion (e.g., Project Daedalus)
Description: Uses fusion reactions, such as inertial confinement fusion (ICF), where small pellets of fusion fuel (e.g., deuterium-helium-3) are ignited by lasers or electron beams. Project Daedalus aimed for an interstellar mission to Barnard’s Star in 50 years.

Performance:
Exhaust velocity: Potentially 10,000 km/s or higher.

Thrust: High enough for significant acceleration, though dependent on pellet ignition rates.

Delta V: Could exceed 300 km/s with moderate mass ratios, aiming for velocities up to 12% of light speed (36,000 km/s).

Fit for Torchship: Meets the criteria, though interstellar focus might exceed typical interplanetary needs.

Status: Requires fusion technology breakthroughs, making it less achievable currently.

3. Beamed Power Propulsion (e.g., Laser Thermal Rocket)
Description: An external power source (e.g., a laser or microwave beam) provides energy to the spacecraft, either heating propellant (laser thermal) or powering ion thrusters (beamed ion drives).

Performance:
Laser Thermal: Exhaust velocity ~10–20 km/s; thrust can be massive (e.g., 700 MN per engine in speculative designs, yielding >2 m/s² for large ships).

Beamed Ion: High specific impulse (5,000–10,000 s, or 50–100 km/s exhaust velocity), but thrust is typically low unless scaled massively.

Delta V: Laser thermal could achieve >300 km/s; beamed ion might struggle with acceleration unless paired with enormous power input.

Fit for Torchship: Laser thermal fits well; traditional ion drives may fall short on acceleration.

Status: Requires advanced infrastructure (e.g., gigawatt-scale lasers – arrays of smaller lasers in the 100kw to megawatt class).

There is a NASA NIAC proposal to beam power to ship (tens of megawatts) and then use that power light weight lithium ion thrusters at 58000 ISP.

4. Plasma Magnet Ships (Dynamic Soaring Against Solar Wind)
Description: Uses magnetic fields to interact with the solar wind or interstellar medium, gaining velocity via dynamic soaring (exploiting velocity gradients, akin to how birds soar). You noted a potential 2% of light speed (6,000 km/s) using interstellar shock waves.

Performance:
Velocity: Could reach 6,000 km/s over time, but acceleration is likely low due to the diffuse nature of solar wind (thrust scales with particle density and field strength).

Delta V: Potentially high, but built gradually.

Acceleration: Estimated at <<0.01 g (e.g., ion thrusters achieve ~0.0001 m/s²), unless a novel high-thrust variant exists. Fit for Torchship: High velocity is promising, but low acceleration may disqualify it unless enhanced designs exist. Status: Speculative; lacks detailed engineering proposals for torchship-level thrust.

14 thoughts on “Solar System Travel With Advanced Propulsion”

  1. Maybe I have a super cheap nuclear rocket idea. I could never get any feedback from someone who knows more than me, not a difficult hurdle to overcome. It uses a linear accelerator sub-critical fission reactor (energy amplifier). The key is long beams can be made light in space and the linear accelerator means it could be easily controlled. Having the astronauts way away from the reactor on the beam means less shielding. I got the idea from a space drive that Brian talked about here that used a aerogel like disk of nuclear material that is moved into a moderator section(Fission Fragment Rocket). My thoughts on that were that it would be scary as hell to control this thing but an accelerator driven reactor would be easy to control. I talked about it here and drew a picture.

    https://www.nextbigfuture.com/2023/01/nasa-and-darpa-will-demo-a-nuclear-thermal-rocket-engine-in-space-by-2027.html#comment-175627

    “If” something like this would work the immense amount of energy in nuclear fuels would mean that you could have a way more mass per drive unit. A glitch might be keeping the nuclear fuel in the pool. Not having it sputter out without being reacted. My idea was to spin it because the velocity of the split atoms is far more energetic than the centrifugal force holding the fission material in the tank. I guess that it would depend on when it fissioned. Deep in the pool, bad, on the surface, good. Though it would be far heavier you could also use this to heat fuel and rocket it out but if just fission could be made to work the advantages would be obvious. If this is a stupid idea say so Be nice if I got and explanation why but just saying it’s wrong or not is better than nothing.

    And BTW what happened to the electromagnetic field drive that used the solar wind to drag the craft through the solar wind at super speeds?

    • Thanks for reminding us of NASA’s planned Draco spaceship by 2027, though I fear that DOGE’s & Trump’s arbitrary and highly damaging (and mostly needless) cuts in spending may doom the 2027 date. Also, Musk’s extreme conflicts-of-interest and desperate need to prove the value of Starship – going up with every test that results in an explosion (8 so far) – may mean twisting government support for a terrible concept vehicle to Mars instead of one that has decades of experience, needs some work, but would be ultimately FAR better. Actually, I believe Starship will NEVER reach Mars, based on:
      1. the current failure record of the largest chemical rocket ever made and possibility that there is something systemically limiting to stability and safety of chemical rockets past a certain size, perhaps something as simple as massive vibration requiring a more reinforced rocket design, that then pushes the fuel requirements so high that there is little left for payload.
      2. The complete untested ability to refuel a spaceship in space repeatedly and safely 10, 20, or even more times (and refuel the refuelers, etc.).
      3. Starship Elon Musk’s projected productive lifetime at this point. Musk is 53. Jeff Bezos, Bill Gates and other roughly comparable tech geniuses peaked and withdrew from active tech-based careers that had a whole lot less stress than Musk’s (who’s also more into politics now, PLUS major family commitments none of the others had). Steve Jobs died before 60. No tech entrepreneur was doing that past 60 and no amount of Ketamine will change that. Musk is already fighting depression from all the over-reach things he is doing; he is perhaps the most over-extended and over-relied upon human being in history.

      An atomic rocket should be ready for unmanned flight to Mars by 2030…but it may come from China, because of all the problems mentioned above.

      • “Trump’s arbitrary and highly damaging (and mostly needless) cuts in spending”

        We are 36 trillion in debt. In what planet is cuts in spending when we are 36 trillion in debt needless???????????

        • In the planet Earth which has countries like the U.S. that are monetarily sovereign – meaning they literally cannot run out of money. Inflation? Yeah, that’s a problem only if demand exceeds resources available to meet them, at least in the short term. That’s not a money problem, especially in the modern age when money can be conjured up on a computer, at least by the Fed when Treasury demands it to with Treasury bonds and notes.
          And of the major drivers of so-called debt, social security is actually a stimulus program. I found two well-regarded studies when researching my book “America is Not Broke!” that show for every dollar spent, $0.80 to $1.00 – depending on the study – is created in the economy from the demand. Medicare and Medicaid are not so easy to analyze, but without government healthcare, which, along with the VA, provides more than half of all healthcare spending now, millions of people would sicken and many would die. Surely, that is worth something on the order of billions of dollars. All of these major expenses, are about half the federal budget. $800b in defense spending puts it well over half the $7t budget. Here’s a breakdown, along with where the money is coming from: https://upload.wikimedia.org/wikipedia/commons/d/d6/Federal_budget_2022.webp
          All the little programs Musk & Trump like to make fun of and cut don’t amount to a few percent of federal spending, and they have constituents and rationale too.
          Interest on the debt is a big expense: it could be eliminated domestically by returning to U.S. Notes, first used to pay for the Civil War by president Lincoln, when $450m was issued in 1862-1863, doubling the federal budget, debt-free because they were issued by the Treasury Department, not a central bank (which didn’t exist then). Over time, electronic U.S. Notes could be used to pay off the debt, eliminating interest payments too. Most of this could be done in a single presidential term.

  2. Some smart NBF reader years ago made the argument that fission detonations could be miniaturized to the gram scale if we could briefly achieve [neutron star] pressures high enough to compress metal as if it were an ideal gas. I guess that would be some kind of kinetic or electro-magnetic magic. Still, the concept was interesting enough for me to remember. Orion starts to look a lot less silly with impulses like that to ablate sacrificial graphite or ice nozzles.

    If neutron stars are real, then the gram critical mass is real too, just unobtainable without magic.

    • I don’t think you actually have to get to neutron star density to achieve critical mass at the gram scale.

      If you switch to a better isotope, you can get a head start: U235 has a critical mass of 52kg at normal density. Adding a berylium oxide reflector reduces that to 8.6kg.

      But if you switch to Curium 247, you can lower the critical mass to about 7kg. Maybe 1.2kg with a reflector? (Why Curium 247? Best combination of low critical mass and long half life: You can’t build the bomb if your material is so radioactive the bomb melts before can assemble it!)

      At that point you need to start compressing. In actual fission bombs, a shaped charge is used which applies a high enough pressure to double or triple the density of the fissile material for just an instant before it rebounds. (But it explodes before rebounding…) This reduces the critical mass still further.

      I think you might be able to get a Curium 247 bomb to explode with maybe a half kg of fuel? OK, admittedly 500g isn’t 1g, but it sure beats 52kg.

    • [ density of (known and estimated) neutron stars is about “3.7×10^17 to 5.9×10E17 kg/m3”

      “The entire mass of the Earth at neutron star density would fit into a sphere 305 m in diameter”
      density of planet Earth (average) is about 20.9kg/gal or 5.5kg/liter (while soil for plants is between about 1.1 and 1.6 (with particle density ~2.6-2.7) kg/liter)
      density factor (for the average numbers) is about 1:90*Million*Million (1:8.7*10^13)

      with some average examples,
      the linear factor for the length of the diameters is about 1:40000
      the factor for the volumes is about 1:9*Million*Million (1:9*10^12)

      temperatures on ‘Terra’ are around 60°F or 15°C and
      estimated around 10^11 to 10^12 for a newly formed to “within a few years to around 10^6 kelvin” on an isolated neutron star

      “16 T field” is sufficient for levitating a living frog in a laboratory and a neutron star is estimated for a magnetic field of about 10^4 to 10^11 on the surface (our planet has an average magnetic-field strength of about 3.05*10^−5 T.

      And rotation could be up to maybe ~1122Hz (1/s) compared to 1/24h or 0.000012Hz for the Globe.

      “In addition to pulsars, non-pulsating neutron stars have also been identified, although they may have minor periodic variation in luminosity.”

      “The fastest-spinning neutron star known is PSR J1748−2446ad, rotating at a rate of 716 times per second or 43,000 revolutions per minute, giving a linear (tangential) speed at the surface on the order of 0.24c (i.e., nearly a quarter the speed of light).”

      “At present, there are about 3,200 known neutron stars in the Milky Way and the Magellanic Clouds, the majority of which have been detected as radio pulsars.”
      closest about 400LY

      “Neutron stars are only detectable with modern technology during the earliest stages of their lives (almost always less than 1 million years) and are vastly outnumbered by older neutron stars that would only be detectable through their blackbody radiation and gravitational effects on other stars.” (thx) ]

  3. In these matter of space, I’ve come to take an attitude: what you have is what you have, no more, no less. PowerPoint rockets are zero, nada.

    So chemical rockets of the Musk, Bezos and Chinese kind is all there is now and for a foreseeable future. Lunar trips, mars trips are on the possible menu.

    Fancy probes and rovers sent by a hefty refillable launcher, why not? Solar sails? maybe.

    But a helluva Isp fusion or fission rocket? nope, show me the credible project and experiments building them and I’d think otherwise.

    And I love space, rockets and I still believe we’re going to the high frontier. But show me the hardware or get moving.

    • How about this from the 1950s & 60s: https://youtu.be/7dUYfDg3G2A?si=NW_BkSC2H0L_gXFE
      It’s easy to find more history on atomic rockets, which were abandoned in the 1970s when NASA starting focusing on the LEO Space Shuttle which didn’t need such speeds. NASA never really returned to these proven engines, which were a lot closer to being used in a spaceship than people realize today. A lot of stuff was cancelled back then, stifling manned space travel for 2 generations, at least. We still can’t send people to the Moon, which we could do 60 years ago. This is a lesson to all the government cost-cutters about how hard it will be to regain that level of science once it’s gone. And no, Starship doesn’t replace Apollo, at least not yet.

      • [ “We still can’t send people to the Moon, which we could do 60 years ago.”

        for the overall support in society there was a higher return for the energy invested in extracting fossil fuels, including no constraints from utilizing fossil fuels and very reduced rocket starts compared with today’s activity and a nuclear industry expanding and nuclear weapons industry (budget) for testing effects (with above surface tests) researching new possibilities for enhancing the ‘balance of deterrence’ and one ‘sur un pied d’égalité’/’on par’, USSR, counterpart/contender towards space, lower constraints from rules and a more general acceptance of risks and a smaller global population, ‘flower power’ stability and fewer wars and people determined with showing being ‘fed up’ with wars and people more focused on contemporary ‘time line’ events(?)
        Technically it’s probably possible, but not with a today’s rules(?)

        ‘golden times for the Voyagers'(?)

        and on the other side:
        computing devices itself, on comparable computing power level with the 60’s, capable of controlling/regulating a moon transfer are the size of ‘mm^2’s for $1 or even less(?) (thx) ]

  4. Oh, for the love of Beezelbub.

    IF WE HAD MAGIC … we could, we could … what? Do ANYthing! You most certainly wouldn’t need to choose 0.3 G (or 3 m/s^2) as a transit goal. Go All Out, Ladies! You’ve got physics magic, so might as well design around 1 G, since it is so darn convenient for the protoplasm blobs contained within. Jeez…

    Second, since we got Physics Magic well handled, and available at a Space Depot near you, we might as well envision all nature of hauling around the asteroids themselves, by the teraton. Seriously! Push (or pull, whatever) some (fvck it: ANY) of the asteroid-belt objects, or for that matter, the Kuiper, or Oort belt objects into not just ‘near earth’ orbit, but actual Earth orbit. Those L3 and L4 waypoints are plenty handy. Parking lots for mining operations.

    But wait, … we have magic. Might as well not ‘mine’ the asteroids, but instead zap ’em into conveniently small chunks so they can be soft landed on Earth, for dissection here. Much cheaper, when the magic wands are finally figured out.

    And while we’re at it, might as well use the magic to fly across the interstellar void, in a couple of years (certainly less than 10 apparent years to nearly anywhere in a centaparsec), take a whole lot of pretty pictures, send out samplers, figure the sheetz out, and come vaunting back post-haste. To a possibly long-gone civilization that dies on the sword of too many carbs, too few kids, and no stomach for work.

    Just saying, Goats.
    Just saying.

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