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Thread: How to propel one's spacecraft

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    Administrator lpetrich's Avatar
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    How to propel one's spacecraft

    Wikipedia has a rather comprehensive article on that: Spacecraft propulsion

    I looked in Encyclopedia Britannica (that venerable encyclopedia's online site) and Citizendium (a Wikipedia alternative with expert review of its articles) I could not find anything quite like it.

    I looked for the Encyclopedia Americana and Collier's Encyclopedia, but they have no online sites.

    That Wikipedia article lists methods by
    • Effective exhaust velocity (specific impulse)
    • Thrust
    • Engine-operation duration
    • Maximum velocity change (delta-v)
    • Technology readiness level

    That last one is a measure of how much R&D will be necessary to get a method to work. From the least ready to the most ready:
    1. Basic principles observed
    2. Technology concept formulated
    3. Experimental proof of concept
    4. Technology validated in laboratory conditions
    5. Technology validated in operational environment (in vacuum)
    6. Prototype demonstrated in laboratory conditions (on the ground)
    7. Prototype demonstrated in operational environment (in space)
    8. Flight qualified
    9. Flight proven

    I'd created a previous thread, Rocket engines - from speculations to successful flights but I'd like to go into more detail.

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    I assume Newton's Laws apply. Detaied discusinwuld invole chenistry which I am wek in, thermodynamics, which I know something about same with fluid mechanics, and Newtinia mechanics.


    IOW, I am not a rocket scientist. That being said it comes down to f=ma.

    http://web.mit.edu/16.00/www/aec/rocket.html

    NASA ion engine.
    https://www.nasa.gov/centers/glenn/about/fs21grc.html

    Historical timeline

    https://www.nasa.gov/centers/glenn/a.../timeline.html

    The shuttle engine, it was complex.

    https://en.wikipedia.org/wiki/RS-25
    https://www.nasa.gov/sites/default/f...e_Drawings.pdf
    Last edited by steve_bank; 06-14-2021 at 02:26 AM.

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    https://en.wikipedia.org/wiki/Specific_impulse

    Looks specific impulse relates to how much fuel energy can be converted to dv. More mass less fuel converted to dv.

    s distance
    ds/dt velocity
    dv/dt aceleration
    da/dt jerk rate of chamnge of acceleration

    https://en.wikipedia.org/wiki/Jerk_(physics)
    https://en.wikipedia.org/wiki/Fourth...es_of_position

    Thrust is equal and opposite reaction. For gas particles hitting a surface pressure, Newtons/m^2, is derived using statistical mechanics.
    https://en.wikipedia.org/wiki/Thrust


    I would think max dv equates to max dE/dt which is watts.

    No different than a car. You are are at 1km/hr, what determines how fast you can cahnge to 1.5km/hr.\

    A gas car engine is fundamentally the same as a rocket engine. The difference is the gas from combustion in the pistons is contained during combustion.

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    There are plenty of tradeoffs in design.

    There are several things that one might want to have:
    • High EEV: that means less propellant for some velocity change (delta-v)
    • High thrust: that means high acceleration, like for departing from a planet
    • High run time: that also helps in getting high delta-V
    • Mechanical simplicity: easier for avoiding maintenance
    • Easy storage of propellants
    • Operational convenience, like the ability to stop and restart


    The highest level, 9 (flight proven) has both chemical and electric rocket engines.


    The chemical ones are solid-fuel and liquid-fuel engines, which work by combustion, and monopropellant ones, which work by decomposing their propellant, usually hydrazine.

    Solid-fuel engines are mechanically simple, and are the oldest kind of rocket engine. They can have an EEV up to 3.0 km/s.

    Liquid-fuel engines are mechanically much more complicated, by they can achieve much higher EEV's. Of those that have flown, the champion is H2-O2, at 4.4 km. But H2 has a very low boiling point, 20 K. A common alternative, kerosene-O2 is at 3.3 km/s, but O2 has a boiling point of 90 K. One can use instead of oxygen some oxidizer that is liquid at room temperatures, like fuming nitric acid or nitrogen tetroxide, but such oxidizers tend to be very corrosive. One can get an EEV of 3.1 km/s, however.

    Hydrazine decomposition gives an EEV of 2.2 km/s.

    This is less than the orbit velocity for low Earth orbit, 7.8 km/s, so rockets need sizable mass ratios and more than one stage. But chemical engines have some pluses. They can work in air, and they can produce large thrusts. Here are the champions in initial thrust:
    • US Saturn V first stage, 5 liquid F-1: 35 meganewtons (13 flights, all successful)
    • Soviet N1 first stage, 30 liquid NK-15: 45.4 MN (4 flights, all failed)
    • Soviet Energia initial: 4 solid RD-170, 4 liquid RD-0120: 35 MN (2 flights, all successful)
    • US Space Shuttle initial: 2 solid, 3 liquid RS-25: 30 MN (135 flights, 1 launch failure)

    Chemical rocket engines are limited by chemical-bond strengths, and H2-O2 is close to the best case for them. One could do better with fluorine, but it is toxic and corrosive and it does not give much improvement.

    -

    The electric engines are ion engines, using either electrostatic or Hall-effect propulsion (Gridded ion thruster , Hall-effect thruster).

    "As of 2009, Hall-effect thrusters ranged in input power levels from 1.35 to 10 kilowatts and had exhaust velocities of 10–50 kilometers per second, with thrust of 40–600 millinewtons and efficiency in the range of 45–60 percent."

    Of the electrostatic ones, the champions:
    • Dawn spacecraft, 3 NSTAR: EEV 30 km/s, each one thrust 90 millinewtons
    • BepiColombo spacecraft, 4 QinetiQ T6: EEV 42 km/s, each one 145 mN

    The Dawn spacecraft's engines operated for 5.9 years, 54% of the spacecraft's total mission time, and they achieved a total delta-v of 11.49 km/s.

    BepiColombo is on its way to Mercury. It was launched in 2018, and it should go into orbit around that planet in 2025.

    -

    So we have a choice of:
    • EEV high, thrust low, vacuum-only
    • EEV low, thrust high, air-capable

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    Going to 8 (flight qualified), we find resistojet and arcjet engines. They work by electrically heating a propellant material.

    Next down is solar sails. They are listed as 9 and 6, but I'd place them as 7 (prototype demonstrated in space). That is because of IKAROS is the most successful one, and it achieved a delta-v of 100 m/s over half a year: IKAROS and Extended Solar Power Sail Missions for Outer Planetary Exploration - 2011

    Looking at 6 (prototype demonstrated on the ground), I find nuclear-thermal engines like NERVA: heating hydrogen with a nuclear reactor.

    Another one is "mass drivers", linear-motor guns.

    Tether propulsion the article lists as 6, but from the looks of it, it seems more like 7.

    Also at 6 is an air-augmented rocket, making a rocket that is partially air-breathing at low altitudes, to save on oxidizer mass. Likewise, a liquid-air-cycle engine. That one would get its oxidizer by liquefying air and extracting that air's oxygen. It's intended for airplane-to-space systems.

    Going down to 5 (component validated in vacuum), I find some more ion engines, or more precisely plasma engines. These are some more electric engines.

    One of them is the variable-specific-impulsemagnetoplasma rocket (VASIMR), designed to switch between high EEV / low thrust and low EEV / high thrust. The first mode would be for interplanetary space and the second one for getting into and out of orbit.

    At 4 (component validated in vacuum in the lab) is nuclear reactors for supplying electricity to electric engines.

    But the Soviet Union had flown some 35 nuclear reactors in satellites ( List of nuclear power systems in space), 33 of their BES-5 and 2 of their TOPAZ-I. It's not clear how much they were used to power whatever electric rocket engines their satellites may have had. But if they had, that would bump them up to at least 7 and likely 9.

    Another lab-demonstrated technology is solar-thermal rocket engines.

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    So far, I have discussed propulsion mechanisms that have had some physical manifestation. So I now turn to those that have only been the subject of theoretical studies.

    Here are some that are at 3 (validated proof-of-concept). They are presumably far enough along to be worthy of lab tests.

    Project Orion was a proposed nuclear-pulse drive that uses nuclear bombs.

    A space elevator is a super tall tower that extends to synchronous-orbit distance or beyond. Extending beyond that distance would be good for providing force to pull it upward. A space elevator would be impractical for the Earth, because it would require some super material that is only borderline feasible. But space elevators are likely feasible for many smaller celestial bodies.

    Also electric and magnetic sails. An electric sail is an electrically-charged cable that works by deflecting solar-wind ions. A magnetic sail uses a magnetic field to deflect solar-wind ions.

    Also launch loops and orbital rings. A launch loop is a linear-motor track that rises from the ground above the atmosphere and then returns to the ground. It would be used to send spacecraft into orbit aboard sleds on the track. When a sled is at highest point and full speed, it would release its spacecraft. Orbital rings are somewhat similar.

    Beam power is shooting a laser beam at a spacecraft to heat its propellant.

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    With that kind of thrust, how long does it take to get to say Jupiter?

    It seems to me that for a viable commercial application for space travel, we need to be getting to that distance in a matter of say a few months or so. That might make asteroid mining practical. We could send a team out there, get the stuff, and get back in less than six months. Less than an ISS deployment.

    Now if only we could adequate radiation shielding. Water?

    https://www.newscientist.com/article...iation-shield/

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    Quote Originally Posted by lpetrich View Post
    Chemical rocket engines are limited by chemical-bond strengths, and H2-O2 is close to the best case for them. One could do better with fluorine, but it is toxic and corrosive and it does not give much improvement.
    Does it offer any improvement?

    From Wikipedia I see the H-F bond energy as 569 kJ/mol and the H-O bond energy as 497 kJ/mol. You get two bonds per water molecule vs one bond per hydrogen fluoride for the same molecular mass. Wouldn't that make the hydrolox engine better?

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    Quote Originally Posted by Loren Pechtel View Post
    From Wikipedia I see the H-F bond energy as 569 kJ/mol and the H-O bond energy as 497 kJ/mol. You get two bonds per water molecule vs one bond per hydrogen fluoride for the same molecular mass. Wouldn't that make the hydrolox engine better?
    You'll also need the H-H, O-O, and F-F bond energies. Or you can look up heats of formation in the NIST WebBook

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    We next go down even further to 2 (technology concept formulated). They presumably need a lot of work before they would be ready for testing.

    Nuclear pulse propulsion (Project Daedalus), uses inertial-confinement fusion. It works by focusing pulses of laser light onto small pellets, making the outer layers explode and crushing the inner layers, thus making nuclear fusion. The pellet's expanding gases would then be deflected by magnetic fields, thus transmitting their momentum to the rocket engine.

    Though ICF has been researched for several years, it is still far from achieving energy breakeven, and the big lasers it uses don't seem very suitable for spacecraft duty.

    Several other types of nuclear-fusion reactor have also been researched, like magnetic confinement (tokamak, etc.). They could either be used for direct drive, like in Project Daedalus, or indirect drive, to power electric rocket engines.

    I remember once read the Project Daedalus feasibility study. All of it looked at least halfway plausible except for the stuff on the control computers. That struck me as pure handwaving. It talked about hardware that directly understands high-level programming languages, for instance. The real problem here is the level of AI that an interstellar spacecraft's control computers will need. It may be hard to avoid strong AI.


    Also at this level is antimatter. This is not bizarro matter but just like ordinary matter with some properties reversed in sign. This reversal cancels out in what determines its macroscopic properties, so one can use data from the corresponding ordinary matter to get an idea of its properties.

    Antimatter is hard to make in quantity. One needs a *lot* of energy to do so. Part of that energy is what's involved in making it: pair-production reactions, where one needs to supply all the energy in the mass of an ordinary particle and its antiparticle. Pair production is a great demonstration of Einstein's famous equation, E = m*c^2. A further problem is the low efficiency of making it, something like 10^(-3) of the particle-beam energy for positrons (antimatter electrons) and 10^(-8) for antiprotons. A further problem is the energy efficiency of the particle accelerators needed to make those beams, and it's hard for me to find good numbers on that. One doesn't need very big accelerators, so one can use some medical linac (linear accelerator) as a reference.

    Antimatter is also very hard to store, because it must be kept from reacting with ordinary matter. Their particles annihilate with each other, making energetic photons and pions and the like. Positions electrically repel each other, as electrons do, and likewise for antiprotons, like protons. Antineutrons don't repel each other, but they decay at the same rate as ordinary neutrons. Hydrogen is the simplest combination that is both stable and electrically neutral, so one can consider making antihydrogen. But it will have the same properties that ordinary hydrogen has, meaning that one has to chill it to a few K to keep it from evaporating.

    One can try to make heavier elements by irradiating antihydrogen with antineutrons, but there are two stability barriers on the way, at 5 and 8 nucleons. One won't be able to surmount them by adding antineutrons. One must smash together antinuclei, electrostatic repulsion and all. But anticarbon should be refractory enough to easily store, at least by antimatter standards.

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