15

It is easy enough to analyze this in terms of conservation of momentum, we'll assume the velocity through the atmosphere to be 7.8km/s, that is LEO orbital velocity - in actuality it'll be a little different due to rotation of the Earth and eccentricity of the atmosphere-grazing orbit, but not different enough to change the conclusion. Now assume the ship ...


12

Yes. This is considered in human interplanetary missions in the form of pre-placement of chemical rocket stages by solar-electric propulsion (SEP) systems. SEP is very efficient, but slow, whereas chemical rockets are inefficient, but provide fast transit times. You want fast transit times for humans to minimize radiation problems and to reduce the ...


11

The $X/kg is a simplification. Rockets are only available in certain sizes; the launch vehicle with the greatest capacity that is presently operating is the Delta IV-H, delivering a little less than 28 metric tons to LEO. Even hypothetical rockets have limits due to the logistics of assembling and launching them. If you want to launch a large and heavy ...


11

They appear to be nuclear reactor modules, telescoped away from the rest of the station and each other for safety purposes. I googled 'MSFC-70-PD-4000-53' and found NASA's 1970 Space Flight Evolution proposal, which describes (among other dream projects) a broadly similar "space base" design, albeit more general purpose than the fuel depot: The ...


10

It can be made to work, but it would be quite different from the typical SF depiction. As Blake points out, you can't gain anything from mass that is already moving faster than rocket exhaust. However, you do not have to collect the gas at orbital velocity. In the extreme case you could slow to a complete stop and refuel while landed, as long as your fuel ...


9

Humans like fast transit (less supplies, less radiation), so they want to use chemical propellant which is much less mass efficient than electric propulsion. One way to mitigate this and lower the overall mass required for human missions is to use electric propulsion to transport chemical propellant to high energy locations, such as high Earth orbit or at ...


9

There's this proposal floated by Boeing: http://en.wikipedia.org/wiki/Exploration_Gateway_Platform And Russia has had some similar plans, although it was for a station in lunar orbit: http://www.russianspaceweb.com/los.html The Boeing proposal would use some structural elements left over from ISS construction, while also including modules similar to those ...


9

I can't imagine such a system would make any sense to build. At it's heart you have a space elevator--something we don't have the materials to build on Earth yet. A pipe full of fuel hanging on that elevator will be HEAVY--meaning a lot more elevator. Not to mention the weight of all the fuel in it. The pressure requirements won't be a big deal--use ...


7

There are lots of studies. Are any serious? Probably not at this time. That is, nothing is funded. The US is unlikely to do so, due to the fixation on the SLS which is basically a design that says "We don't need no stinkin depots". The Russians seem to have no great ambitions at this time. The Chinese are not yet at a sufficient state to consider this. ...


6

Does all this make any sense? Part of it does. The idea of leaving the tanker as an orbital fuel depot in Martian orbit is OK (if you can reuse a spent upper stage for that), but it would be better to leave propellants themselves on the surface of Mars until you actually need them in orbit, to prevent propellants boil-off and risk of explosion due to ...


6

Even with hydrogen, the tank weight is quite small compared to the content weight. For some examples, you can look here (you may need to do some math to get the actual numbers): http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19750004950.pdf You are right that the heat loss is less of a problem for larger tanks, but structural integrity, especially ...


6

Shuttle orbital manuevering / attitude system tankage used screens to ensure that fluid always remained at the tank exit. Sufficient fluid would cling to the screens to get the engines started. The screens are shown in this schematic of an Orbital Maneuvering System tank. In this case the screens are only at the aft because this system always provided thrust ...


5

Yes. In 1950 Wayne Proell proposed the use of "fuelless box-ramjets" to harness the energy of recombination of atomic oxygen at about 100 km altitude to propel space stations and satellites. (The Significance of Monatomic Gases in Planetary Space Operations, J. of Space Flight, V.2, no.7, Sept. 1950 pp.1-8, V.2, no.8, Oct.1950, pp.1-9.) In 1958 Lionel ...


4

Ullage (i.e. getting the fuel at the end the tank you want it to be at and the pressurant at the other end) for large rocket stages in free-fall is usually handled with small auxiliary thrusters. For small thrusters, it's practical to use bladder pressurization or some other solution; you use the small thrusters to slightly accelerate the large stage, and ...


4

For the Salyut 6 space station, both Progress cargos and the station used bladder tanks. The tanks were pressurised using nitrogen. The nitrogen could be recovered once the operation was done (at least on the station side). To vent the pipes, they just let them open, exposed to vacuum for a week then purged with nitrogen. Source:The Story of Space Station ...


4

The LiftPort Group is planning to station the "top" of their Lunar Space Elevator at EML1. From that point, they would drop an anchor station to the lunar surface while extending a counterbalance to 250,000 km above the lunar surface. Below is a graphic illustrating the proposed Lunar Space Elevator. According to Michael Laine, the LiftPort CEO, the Lunar ...


4

The Non-Atmospheric Universal Transport Intended for Lengthy United States Exploration (Nautilus-X), a concept developed by the Technology Applications Assessment Team of NASA, is intended to initially take up residence at EML-1, after being assembled at the ISS. The spacecraft was designed for long duration (one to twenty-four months) exo-atmospheric space ...


4

As it costs $X/kg to launch something into space, wouldn't it cost the same to launch fuel by itself as it would to launch it as part of the vehicle that's going to end up using it? Why assume that the launch cost is the same? Here are some hypothetical scenarios why storing fuel in space might be a feasible idea. (Assuming a need for fuel in space.) ...


4

Lots of plans. 1) The Canadians are apparently working on a spacecraft that is part space tug, part robo-mechanic & part filling station: After years of planning, Canadian company MacDonald Dettwiler and Associates (MDA) announced it is building the first space gas station, with plans for a 2015 launch. The Space Infrastructure Servicing vehicle will ...


4

I can think of three uses for fuel depots: For a specific mission where payload and fuel are launched separately and dock in LEO. Either to achieve a larger spacecraft, or to make a safer smaller launch for a crew or other irreplacable payload. Prelaunched and placed on the surface of, or in the orbit around, the mission target, like ...


4

Capillary action will not get your fuel to space as this article in Wikipedia points out the height of lift is related to the diameter of the tube, where a narrower tube gets you more lift measured in inches. Trees use capillary action combined with other actions to raise fluids. In no case would capillary action by itself get you anywhere near orbit. ...


3

Any fuel sent to a depot has to be launched, and so needs to be in a sufficiently anti-slosh container. Sending up deflated tankage to be filled from depots presumes inflatable tankage capable of handling the fuels; the most common space fuels use cryogenic fluids, and the others tend to be corrosives. (Hydrazine, monomethylhydrazine and nitrogen tetroxide ...


3

You need to keep in mind that every kilogram you add for a recovery system on the upper stage robs an equivalent kilogram from the payload. With a refueled stage that can shed most of the 9000 m/s ΔV in the deorbit burn, we don't need a heat shield (or at least as robust a heat shield). We could probably get away with something like the inflatable heat ...


3

It's close. Plugging in the numbers for the Falcon 9 upper stage I get 11,300m/s of delta-v. Since 9000m/s will get you to LEO that's enough to get you back down with something to spare. However the upper stage doesn't have landing gear. Since you have 5000kg of payload capacity (brings the delta-v down to 9111m/s) you could land it. The weight of the ...


3

ESA is currently (2018) studying a quite similar project. In short, it does not use oxygen but any gas in upper atmosphere to feed an ion thruster instead of flying with a tank of gas to feed this thruster.


2

To onboard 1kg of gas requires many kg of exhaust at lower-than-orbital speed. The key to doing that is for the ship to burn atmosphere while going by, rather than bringing it to rest relative to the ship first which requires speeding it up in the wrong direction. Instead, use a scramjet approach: Burn as much as needed, pulling off a small amount to ...


2

As others have noted, exhaust velocity needs to be higher than orbital velocity. Some ballpark velocities for low, circular orbits around various bodies: Venus 7.2 km/s Earth 7.8 km/s Mars 3.5 km/s Jupiter 42 km/s Saturn 25 km/s Uranus 15 km/s Neptune 17 km/s Lox/hydrogen's 4.4 km/s exhaust velocity is about as high as you're going to get with chemical ...


2

Interestingly, a fuel depot is one of the objectives of the Lunar Space Elevator project that a company called LiftPort is pursuing. The idea is that an elevator could be constructed from the lunar surface to EML1 using COTS technology available today. Using a solar-powered lifter to transit payloads from EML1 to the lunar surface and back solves the problem ...


2

Launching fuel from Earth without a specific purpose, waiting for a need to turn up, can't be a good idea. I've heard of fuel depots in the context of producing fuel on the Moon. So that is in a vision of a much more developed space flight than what we have today. The benefit would come from the lesser energy needed to launch from the Moon's lower gravity. ...


2

Warning I've written this answer and there have been no opportunities for citations - that means you're just trusting this random guy on the Internet if you believe me! I suspect the figure of merit for judging which is better (upgrading or refueling) is more a question of practicality than anything else. We are extremely unlikely to develop a new ...


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