MatterBeam

Who are you asking? It is such a vast topic with no clear answers, with the majority of 'solutions' based mostly on how much you believe they are realizable.

I did the calculations. The 85 ton BFS with 1100 tons of propellant has a mass ratio of 13.94. With its 375s Isp Raptor engines, it has 9693m/s of deltaV. RSS players know you can reach orbit with about 9400m/s of deltaV, so you can 'waste' 293m/s of deltaV pushing off on the sea level Raptor engines at 330s to 356s Isp. Estimating payload capacity has proven extremely difficult with the tools I have, but I estimate it at about 1.5 to 2.5 tons. If you want a TWR of 1.3 on liftoff with a fully loaded BFS, you'd need nine 1700kN sea-level Raptor engines though. This will certainly eat into whatever payload capacity you should have left over. The picture changes if SpaceX manages to stick closely to the design figure for dry mass of 75 tons. This gives us 10 tons extra margin to work with. You can increase the payload capacity to 12 tons, or launch 2.5 tons into space and land with 300m/s deltaV for a retro-burn.

Ha! Finally...

Yes, increasing fluid pressure in the eyes causes them to distort over time, and increased blood pressure in the brain might lead to aneurysms.... but these are problems which will take a lot time to develop and don't become very severe. Fluid pressure is eventually equalized as the body re-distributes fluids. Blood pressure in the brain drops as the heart weakens. They are both of very minor concern compared to the risk of cardiac arrest or an artery failing during the 3-6G re-entry to Earth, at least this is the case for today's astronauts.

Why not stay here? is very different question from Venus or Mars? Once we have decided to settle the solar system, either artificially pushed forwards by industrial pursuits or organically motivated by the need to colonize new lands, the only decisions to make is how to allocate our resources towards developing various potential sites for supporting large populations. Cheap and rapid transport of humans across interplanetary distances is an implicit requirement for this question to be valid. As a consequence, none of these potential sites have to be self-sufficient - just like today where we can travel quickly with airplanes or transport goods in bulk with cargo ships, we don't have to extract, process and consume all the things we need in the same spot. Humans living on Mars can be supplied basic elements such as water from Phobos, while their heavy element needs can be extracted from metallic asteroids and converted into machines and electronics on Mars, to be sold to Earth in return for complex items such as rocket turbines and microprocessors... With all of this in place, all we have to do is evaluate the relative value of the potential sites. In contention are Venus, Mars, the Moon and orbital habitats, which can be placed anywhere. Orbital habitats are the simplest. They provide the best living conditions. However, they cost the most to produce and need to import everything they consume. The Moon is the easiest to develop and build on due to its proximity to Earth. It lacks many vital resources, such as large quantities of water or carbon, and the 1/6th gravity can mean very bad things for long term health of adults and the development of children. Mars fares better in terms of living conditions. A bit less than half gravity might not be so bad, and it has a lot of water and CO2 that is just a bit difficult to extract or collect. Bonus points for lots of iron and minerals in the soil to grow plants on. Energy will be expensive however. Venus provides excellent living conditions at much lower cost than orbital habitats, but still needs to import a lot of resources. Energy from sunlight is more available and a thick atmosphere makes importing resources slightly cheaper. Make sense for what? Supporting a population or making money? Requiring the least amount of money to explore or having the best long term prospects. It is also dangerous to mix up terraforming with colonization. People who moved into the Siberian tundra adapted their equipment, homes and lifestyle instead of not bothering with the land because melting all the ice is very difficult. The airships depicted are exploration vehicles. They're not a suitable basis for how a long-term habitat might look like, just like the Apollo Lunar Lander won't look like the underground Martian colonies. Supplies delivered to Venus will likely be delivered by a mothership that performs an aerocapture, then sends down flying vehicles from low orbit. The flying vehicles either slow down and dock with the floating habitat, or just fly past and parachute the supplies before accelerating back into orbit. Also, it's not nice to mock someone's speech impediment. The big danger is not exactly the blood moving, but the heart muscles getting weaker and the artery walls getting thinner. We currently do not have a system which ramps up gravity slowly back to 1g for an astronaut to re-condition themselves before re-entry. This would involve a lot of exercise under increasing gravity to build their heart muscles and cardiovascular system back up to its original strength.

Of all the places in the solar system, Earth and Venus are perhaps the only places where gravity is not an issue - in other words, the importance of Venus depends on how vulnerable human life is to the effects of micro or low gravity. Saturn too, but its very far away. Imagine you want to raise a child. You can do so in a habitat spinning in vacuum, trusting the radiation shielding and the suppliers to keep the kid alive until it grows up. Or, you can do so in Venus's atmosphere, trusting balloons and suppliers to maintain viable conditions. The balance tilting to one option instead of another depends on the specifics. For example, if rotating habitats become very cheap to make and are everywhere, plus transporting products between them is easy and quick, then we might never need to build a permanent establishment on a planet. If on the other hand, closed life support for a large population is impossible and supplies take months to travel between destinations, then it might be better to exploit Venus's resources. Or you know, just stay at home. Dragging a bucket across the surface can dig up many of the common minerals found in lava flows, which are essential to agriculture by fertilising a soil to grow on. This includes elements such as calcium, phosphorus, nitrogen, silicon, magnesium, iron ect. The quantities won't be huge, but its a massive step up from a rotating habitat orbiting in a literal void. If we have the technology to travel across interplanetary distances and build floating habitats, it might not be too much of a stretch to implement methods of setting up industries on the Venusian surface. Practically all the mining techniques we are familiar with work on Venus, except at 735K instead of 293K. You must relativise. Compare Venus to other places in the Solar System or to vacuum habitats. It has some advantages and some disadvantages, that is all. Methane is CH4. The H, hydrogen, is excessively rare on Venus.

We do not have the techniques to sustain a floating habitat on Earth, let alone in the inclement environment on Venus. Mars is an even harsher environment, but we have the techniques to handle it.

I seriously doubt that anyone needs radiators the size of aircraft carriers unless you want to radiate at very low temperatures for safety or endurance reasons. The FFRE's design specifically needs to remove megawatts of heat from components running at 140K and 590K! That's extremely low! Because waste heat rejection scales with temperature to the power 4 (!), the low temp radiators are massive. Now, how will electricity be made from a modern spacegoing nuclear reactor? There's two options - thermoelectric, and gas turbine. Thermoelectric allows for very high operating temperatures (rejecting even in 2000K range, but 1500 seems more likely) and no moving parts. It can work even when cells are damaged and requires no maintenance for years on end... but they have terrible efficiency and even worse W/kg ratings. Compact gas turbines do exist, and using something inert like nitrogen or CO2 as a working fluid allows for decently high temperatures at the radiator's entrance - 800 to 1200K. They have high W/kg ratings, so you can compensate for their complexity by just installing several redundant turbines and using probability of failure to your advantage. If failure rate after one mission is 10%, then having three turbines reduces means you'll have a 24% chance of at least one turbine failing, but a 0.1% chance of all turbines failing. If we use electric engines with an Isp of 2000, the mass ratio for a Mars mission of 6km/s is 1.35. An 85 ton craft with 150 tons of payload will need only 82 tons of propellant. If SpaceX uses 1kW/kg reactors and engines, then it can 'invest' 10 tons into propulsion and produce 1kN of thrust... the ship would start accelerating at a measly 0.327 milligees... terrible! This isn't a very accurate calculation however. 1kW/kg is an assumption that even NASA research into powering the VASIMR engines hasn't achieved. A rocket would still need a high thrust chemical engine to make a retro-propulsive landing on Mars, so some of that mass budget will be used for landing. Also, Musk wants to do a rapid transit, so 6km/s isn't enough. Overall, I think I answered my own question. Unless the power density of nuclear electric craft is massively improved (10x or more), then rapid interplanetary travel is best solved by piling on the chemical propellants.

That deltaV advantage doesn't sound very useful if the BFR is only supposed to go to LEO in the short term, and translates into a few extra tons on a Mars trajectory... Exactly this. There might be methods to increase the deltaV advantage, but I am thinking mostly about two things: -TWR. Nuclear rockets have low TWR compared to something like the Raptor. This will mean that for the same acceleration, nuclear rockets will be quite a bit heavier and will cut into the mass ratio advantage they provide. -Refurbishment. Nuclear reactor cores running at high temperature, with sooting from methane and poisoning from radioactive products, will need constant upkeep. When your entire assembly needs to be taken to a special closed environment and worked on by specialized engineers using special precautions, you will have a much harder time keeping up with the rate of launches of a conventional chemical rocket system. Instead of a nuclear thermal rocket, what about a nuclear electric rocket? Double down on the deltaV advantage without covering your entire propulsion unit in radioactive waste particles.

There will be a significant delay between the capability appearing and the satellite companies retooling to make full use of the capability. Considering how tightly regulated the industry is, it won't switch over quickly...

I personally don't think there's a market for the 150 ton payloads of the BFR, much less the hundreds of tons the ITS would have put into LEO. A lot of research has gone into making satellites smaller, to the point where there are no commercially available space-based systems that will fill even 10% of the BFR's cargo capacity. Sure, there are government plans to put tons of stuff into orbit for a Mars mission, but as their habit of cancelling everything proves, it is unwise to rely on government spending. So, we must focus on the commercial sector. The BFR would transport significant numbers of people and amounts of payload across intercontinental to interplanetary distances. That's good. But until you have companies clamoring for dozens of tons in space per launch, the BFR will not be commercially viable. SpaceX might add up the payloads of several companies to fill one BFR. It must do so at a quick rate, rapidly enough to fulfill its promise of low $/kg rates. The problem is, there are not enough companies putting that much mass up for delivery to space! It is like building a super tanker to transport diamonds. You might eventually fill it up, but it'll take a long time and the demand for it will be very low! The main advantage of the miniBFR suggestion is that it can handle the vast majority of space contracts with about 15 tons of payload capacity, and it will remain profitable when you cannot fill the full 150 tons of the BFR. In today's world of microsatellites as low-mass, high-efficiency satellites, it is especially relevant. Imagine a SpaceX-2. A billion-dollar start up that tackles Elon Musk at large-scale low-cost space launches, instead of the plethora of small-scale attempts. I wonder what solutions it will come up with.

Probably unmanned. The ITS was designed for a mass ratio of 6700/275: 24.36. The BFR was designed for a mass ratio of 1185/85: 13.9. If the miniBFR only achieves a mass ratio of 10, it would have a dry mass of 11 tons. This gives it a payload capability of 15.6 tons, which can be satellites, maneuvering propellant or retropropulsion propellant. If it needs 400m/s to de-orbit and land, then it needs to save at least 1.75 tons of propellant using 275s Isp sea-level engines. Therefore, the maximum payload with return is 13.8 tons. That is a useful payload to LEO! On the other hand, if the entire payload capacity was replaced with propellant, plus a 1.75 ton reserve for landing, it would achieve a deltaV of ln(111/11.75)*375*9.81: 8261m/s. That's enough to go practically anywhere in the solar system... enough for a 6.57km/s flyby of Jupiter plus 1691m/s for maneuvers! It would be unmanned of course, and the human habitation amenities replaced with radiation shielding, long-range communications and extra thermal protection for the faster re-entry.

I wonder how it would look like, or what its performance would be. One thing for sure, it would have to be quite performant and might not be able to make a retro-propulsive maneuver once it achieves orbit. The latest Falcon 9 stage has a mass ratio of 19.5 and an Isp of 311s. The second stage is assumed to use the 375s Raptor engine. We want a deltaV of 9500m/s total, so putting everything together, I get a second stage dry mass of 26.6 tons and wet mass of 111 tons. This means that the Falcon 9 booster can launch the usual payload (111 tons upper stage) and return, while the second stage pushes into orbit. Adding two tons more of propellant to the upper stage (dry mass 26.6 to 24.6 tons) gives it an extra deltaV of 300m/s, which should be juuuuust enough to deorbit and make a retro-propulsive landing. That's 4296m/s of deltaV in the Falcon stage, and 5526m/s in the miniBFR stage. I don't know about you, but a 25ish ton spaceship in orbit which can be fuelled to 5.5km/s deltaV is very very cool. That's enough to go to Mars!

Definitely. Everyone else is building sub-scale versions of the new designs. I believe in SpaceX's ingenuity, but not in a near-magical ability to escape standard testing procedures.

4am? The flight control staff must be on edge after two (?) cancellations.

Also, when orbital refuelling is in play and the deltaVs are under 10km/s, the benefits of NTRs are quite limited. They only produce high exhaust velocities using liquid hydrogen. Using denser fuels such as ammonia or methane only increases Isp by about a 100 to 200 seconds over Methalox. You'd be investing massively in a very strictly regulated technology, developing it to modern standards, testing it to higher rigor than manned spaceflight (no-one wants a rocket launch accident spraying nuclear dust over all of Europe... again) and mounting it on the ITS for a drastic thrust reduction just to save on propellant, arguably the cheapest part of a spaceship refuelled in orbit. DeltaV to Mars is about 5km/s, down to 4km/s using aerobraking. Mass ratio using 375s Isp methalox (Raptor) is 2.96. An 85 ton rocket will need 167 tons of propellant. A methane NTR might achieve 644s Isp. Mass ratio for 4km/s is 1.88, so that 85 ton rocket will need 75 tons of propellant. All in all, you'd save about 92 tons of propellant. I think this might be the right place: What do you guys think about an ever smaller ITS/BFR/Recoverable lander able to lifted most of the way into orbit by the existing Falcon 9 booster? It would allow SpaceX to continue its production of Falcon 9 boosters, but expand its capabilities and lower the $/kg rating through orbital refuelling and easier reusability.

A more useful measure is power to weight ratio. W/kg. Solar thermal rockets can achieve higher W/kg ratings that solid core nuclear rockets. Since space systems in the forseeable future will be restricted mostly by how much mass you can put into orbit, you'll be comparing mass budgets for the solar thermal and NTR propulsion systems. If we use W/kg, we can directly compare the two propulsion systems. This Solar Moth concept, designed using 1979's materials and technology, achieves 167kW/kg. Modern materials, such as using ultra-thin Mylar instead of aluminium sheets, or carbon fibre struts and THC heat exchanger instead of tungsten, might improve this figure by quite a bit. Using this figure, a Solar Moth of 10 tons would produce anything between 378kN@900s to 1702kN@200s. At the highest specific impulse, it can push 3856 tons at 0.01g. That's ten milligees. A bit more maths on the acceleration time of a 3856 tons craft with a 0.01g initial acceleration: 3856 tons contains 10 tons propulsion. If we assume a mass ratio of 4, deltaV is 12.2km/s. Let's assume 6.1km/s is spent on the departure burn. Propellant mass is 2892 tons and dry mass is 964 tons. Propulsion fraction is 1%, which is an absolutely tiny engine on a very big rocket. Anyways, mass ratio for a deltaV of 6.1km/s is 1.995, meaning 959 tons of propellant. In other words the initial burn consumes 1932tons of propellant, and the average mass during departure is 2890tons. Average departure during acceleration is 0.13m/s^2 (0.0133g). The 6.1kms burn is completed over the course of 13 hours. A lot, but it's just over half of one GEO orbital period. If we have a propulsion fraction of 10%, which is much more reasonable, then the craft will produce 3780kN and accelerate at 1.3m/s, completing the burn in just under an hour and a half! Not bad! Now, all we have to do is to find figures for nuclear thermal rockets of similar kW/kg ratings. Running all the maths again will give you very similar results, as the maximum Isp of the NTR is pretty much the same as for the solar thermal rocket. Since we are using kW/kg, is is scalable to craft of any size. If sending 964 tons to Mars seems unreasonable, you can just reduce it by a factor 100 (9.64 tons) and get the same results. Be careful however of citing reports on NTRs from the 60s, as that was before any physical testing was performed. The Reusable Nuclear Shuttle designs suggested power to weight ratios of about 212kW/kg before radiation shielding is considered, so we're actually not far from the performance of solar thermal rockets. The departure burn can be done on the night side of Earth, because Earth's shadow at that altitude only covers a tiny portion of the orbit. if you enter a GEO orbit other than over the Equator, you might not intersect Earth's shadow at all.

Fascinating this is going to be the 'new normal' for the next generation.

The scale of investment! Putting tethers into orbit large enough to be useful will demand an enormous up-front cost, both in terms of the resources for the tethers themselves, but also the development costs of actually making them work. Rocket on the other hand are a well known technology, and mining Phobos is a combination of technologies we are familiar with. Phobos is useful immediately, with even the smallest solar-powered water splitter being useful on its surface. The massive benefits of a tether over rocket-powered flight is obvious and they cannot be ignored in the long term. However, it might be better to go with what we know and set up a propellant cycle centered on Phobos, instead of waiting for someone to put the money into tethers.

Well, the solar thermal rockets we are discussing probably accelerate at milligee rates, so structural support is of little concern. Being so thin, the Mylar can be held up by a combination of solar light pressure and centripetal force. Myar absorbs between 5 and 0.1% of the sunlight it receives, depending on the manufacturing quality. Even at 5%, it is absorbing 65W/m^2. If the anti-solar face is moderately black, with an emissivity of 0.8, it will reach equilibrium temperature when radiating at 194K. That is very cool! Your perform burns at the periapsis until you break orbit, to make the most of the oberth effect. Make sure it is on the sunlit side! If you can't burn on the sunny side, raise your orbit first. This will increase your orbital period and how long you can stay in the sun. Going from LEO to GEO takes about 4km/s. Going from GEO to Mars costs another 2km/s. You can burn the 2km/s with continuous sunlight available. Nuclear thermal technology is great, I agree. However, a lot of 'solutions' that are great on paper run head-on to political, social or economical problems. The OP requested a propulsion technology which you or might might have access to in the future. I am certain governments will not care if you're touting around large sheets of metal to focus sunlight, but might be bothered by the tens of kilograms of refined uranium you'll get with any solid core nuclear engine.

The key to the asteroid belt and most of the inner solar system's small rocky bodies is Mars's moons. Phobos and Deimos are natural propellant depots, with thousands of tons of ice sitting in low gravity, just waiting to be used. Perhaps a surface installation on Mars can build bulky components like pressure vessels, solar panels and propellant tanks, assemble them with hard-to-make components like rocket engines and avionics shipped in from Earth, and launch them towards Mars's moons to start mining propellants. The propellants from Phobos and Deimos can then supply spaceships heading towards asteroids at rather low cost. Bringing back metals and ores from the asteroids to Phobos is cheaper than sending it all the way down to Earth and aerobrake to the ground. @kerbiloid: There was also the military aspect. The Space Shuttle could theoretically capture enemy satellites and change orbital inclination unexpectedly using aerobraking maneuvers. It was a bigger, more useful, possibly cheaper replacement to the DynaSoar. Basically, you had a spaceship that could serve the Air Force, NASA and the Army equally well.

My man! Despite the numbers posted by Elon Musk, Mars will not inherently generate money for a very, very long time for the simple reason that it has no services or products to offer back to Earth beyond scientific research. Research is a valuable and profitable goal, but it is strongly capped and cannot contribute to the commercial expansion of a human settlement on its own. I strongly believe that Elon's projects will be funded for a long time by people buying into the project, including himself. If you pay $25000 for a ticket, you're paying for the opportunity to live on Mars - that's the product. What you do once you've landed and been assigned a tent has been unanswered. You have to rely on more people arriving and paying their tickets for the supplies to keep coming. A basic buy-in pyramid scheme unless something gets added to the plan. Now, pyramid schemes are not an evil thing. They can bootstrap incredible projects that are not viable on day one. They sell hopes, dreams and the future and stay alive on cash injections. However, all schemes fail unless those hopes are realized. The delay between the project beginning and its collapse depends on how many people believe in and pay into Elon's vision. There seems to be a lot of that, so establishing a base and running it for some time might not be an issue. My view is that the solution to turning this scheme into a valid business is to leverage your 'start-up' capital of humans, equipment and infrastructure on Mars into a profitable industry. That is the true core of the discussion: What can you do with a few thousand skilled people living on Mars, that people on Earth will pay for?

I believe this fraction will depend on the thickness of the atmosphere, its composition and the albedo of the surface. This will determine how much energy is absorbed by the atmosphere following this general equation. Energy absorbed: Atmosphere thickness * Atmosphere Absorption * ( 1 + Albedo) * Solar Power The thickness will be dependant on gravity. For example, on Earth, atmospheric density drops by about 63.2% every 7.64km. This is its scale height - density decreased by a factor 1/e every 7.64km. Convection, solar heating and different elements complicate this slightly. On the moon, gravity is 6 times lower, so the scale height will be 45.8km. For the same surface pressure, the atmosphere will be 6 times taller and/or 6 times denser. Absorption is a factor of the atmospheric composition. Empirical studies have tell us 77% of the solar intensity reaches the ground. A lunar atmosphere will have to be 6 times denser to achieve 1 atm on the ground. Albedo is between 0 and 1. Average lunar albedo is 0.12, so 12% of the incident radiation is reflected back through our hypothetical atmosphere.

Its interesting to watch another space launch doing live webcasts, other than SpaceX. Its much more formal with ULA, with many uniformed men

Hello. Is it possible to also have the 'tooling' function, where reusing the same parts again becomes cheaper over time, as a separate mod? The intention is that while RO is difficult to move into 1.3 and then into 1.3.1, perhaps this much smaller part of it, which is applicable to non-RO saves, can be updated. Thanks.