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Rockets launched from earth are often launched in stages. Does this make any sense in space? For instance how about a chemical first stage for an initial burst of speed and nuclear electric propulsion throughout the flight. I suppose the best way to answer this question is how long does it take nuclear electric to generate any reasonable amount of delta v. For instance how much delta v can nuclear electric generate within one month. Obviously this depends on the size of the system but what is reasonable.
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Thing is that most of the stages you are thinking of are burned up just getting to orbit. Sometimes part of the upper stage will be used to inject a satellite to a higher orbit but often, all of the rocket stages will be used up just getting to LEO and then a smaller booster rocket takes the satelite to where you want to get to. Unless you're in orbit, you have to have a high thrust with your engine otherwise you fall back to Earth with unpleasant results.
As far as rockets that provide thrust once in orbit, yes, NEP motors are being considered for this as well as other propulsion options.
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Say for some reason we already have a fully fueled chemical stage in orbit that can dock to another stage.
From [http://www.marssociety.org/news/2004/0123.asp]the mars societies statement on the Bush initiative.
?Without dismissing the important value of NEP for outer solar system robotic missions and other missions involving large velocity changes undertaken across extended time frames, we note that the size of NEP units required to supply propulsion for human exploration missions are on the order of 10,000 kilowatts. In contrast, when used to produce chemical propellants on planetary surfaces, the required reactor size to support human exploration is reduced to about 100 kilowatts. This is because a much smaller reactor stationed on a planetary surface making propellant can emit energy over a long period of time prior to flight, store it as chemical propellant, which then can release the energy as fast as it is needed under flight conditions. The mission mass leverages achieved by such ISRU supported chemical propulsion options are greater than those offered by NEP, while for inner solar system missions, the flight times are less (two orders of magnitude less for Lunar applications).?
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Say for some reason we already have a fully fueled chemical stage in orbit that can dock to another stage.
I think that current plans to use NEP just have the NEP drive as the final booster stage on a rocket - no in-orbit assembly required. NEP does have some nice benefits - an ISP of 8000-10000 allow you to get a lot more tonnage to Mars. Furthermore, you now havea nuclear reactor you can land on Mars and use for generating fuel for the return trip. You can also use the nuke reactor for NEP on the way back as well.
However, the big disadvantage of useing a NEP to get to Mars is the low thrust. It's hard to do orbital insertions and the like when you're getting under a pound of thrust. For example, current plans to use NEP propulsion to get cargo to Mars involve spending a year slowly boosting the orbit of the cargo to progressively higher orbits until a trans-Martian injection burn can be done. Of course, it's impractical to send people to Mars this way and you even have to worry about cargo spending so much time in the Van Allen radiation belts.
NTP propulsion (think NERVA) give lower ISP's of about 900 or so but can have thrusts comparable to chemical. Plus, the nuclear reactor could be also be used for things like ins situ propelant production although the reactor would be less well suited for that task.
I've been thinking of a potential compromise system that might be able to get us to Mars with high cargo loads and still allowing 'normal' orbital trajectories.
I'll go ahead and split the concept to the next message to keep this post from getting overly long.
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*Well...this topic definitely isn't one of my "strong suits." But I do have a question:
Would (solely) nuclear-powered rockets have stages at all?
--Cindy
We all know [i]those[/i] Venusians: Doing their hair in shock waves, smoking electrical coronas, wearing Van Allen belts and resting their tiny elbows on a Geiger counter...
--John Sladek (The New Apocrypha)
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The idea I've got isn't a new one but I haven't seen it being discussed seriously with respect to Mars.
Electro tether propulsion.
Basically, is you have a conductive tether in Earth orbit and apply a current to it, you can generate a thrust against Earth's magnetic field - not unlike how an electric motor works. So far, tether experiments have met with mixed results but it looks as if we're getting close to being able have working sytems. right now, one of the more prominent tether companies is Tethers Unlimited - check out their website, they have some nice technical documentation. NASA's been interested in using Electro Tether Propulsion (ETP) to keep the ISS orbit stable. If they can get the system to work, ETP will use excess power from the ISS solar panels to keep the orbit boosted, saving over $2 billion in launch costs to keep lofting hydrazine to the station over its lifetime. There's the additional benefit of getting softer thrust that is more compatible with long term microgravity experiments.
Tether Unlimited is somewhat enamored with the idea of rotating tethers flinging cargo up into higher orbits. Personally, I'm dubious about the engineering practicalities and safety of this approach. What I'd be more interested in is an ETP powered space tug.
Since ETP thrust is pretty much 100% efficient and requires no propellant, the size of your power source is the most important factor. A very large photovoltaic array win combination with a large battery array could give decent amounts of thrust for boosting vehicles into high orbits that minimize the amount of fuel needed to get to Mars, vastly increasing the total amount of cargo that we can deliver or allowing the use of smaller Earth to LEO boosters, lowering the cost.
Basically, loft cargo to LEO with a heavy booster and dock it with the ETP barge. The barge can then so a series of perigree boosts that greatly reduce the amount of fuel needed for a Trans Martian Injection(TMI). After releaseing the Mars mission, the barge uses ETP to bring its orbit back down to boost the next mission. The boosting period would occur over a month or two.
I haven't looked at the power requirements for the barge yet but here's some numbers I crunched for how much of an advantage you get for getting mass to Mars.
I used numbers out of Case for Mars and did a very simplistic set of calculations. Someone better versed in orbital mechanics than I should feel free to correct any mistakes I've made:
Delta V's:
LEO to Near Escape eliptical orbit: 3.1 km/s
LEO to TMI (for slow 270 day cargo transfer) 3.7 km/s
LEO to TMI (for faster 180 day crew transfer) 4.3 km/s
NE to TMI (cargo) 0.6 km/s
NE to TMI (crew) 1.2 km/s
ISPs:
H2/O2: 470
NTP: 900
NEP: 8000
(mass of spacecraft + mass of propellant)/mass of spacecraft = e^(delta V / engine exhaust speed)
Engine exhaust speed (km/s) = 0.0098 * ISP
Crunching these numbers, I get figures quite a bit higher than what Zubrin quites in Case for Mars - I assume that means that he is not including the mass of the transfer engine and associated fuel tanks as part of the total Mass sent to Mars as it is not useful mass. To get my figures to match with his, I've divided the tonnage by 1.36 for checical and 1.23 for nuclear propulsion. The space barge figures basically assume that the velocity boost gained is 'free' since the barge doesn't use propellant and detatches from the spacecraft before it launches off for Mars.
Here are the figures for how much tonnage we can get to Mars orbit (the usable amount we can get to the surface of Mars is lower by about 1/2 and I don't have the necessary data to properly calculate it) assuing a 140 ton to LEO heavy booster. (NE stand for Near Escape - the orbit the space barge will raise the spacecraft to for 'free')
.....................O2/H2.......NTP.........NEP
LEO to TMI
cargo.............46.1..........74.8........109.4
LEO to TMI
crew..............40.5..........74.8........108.6
NE to TMI
cargo.............90.4..........106.3.......112.9
NE to TMI
crew..............79.3..........99.3.........112.1
As you can see, the space barge allows one to greatly increase the mass to Mars - even with chemical rockets on the spacecraft. In fact, it would seem that chemical propulsion is a fairly good competitor to NTP with the barge. For cargo missions, the barge is especially effective.
Alternately, instead of larger masses, we can get away with smaller heavy lift vehicles, greatly reducing the cost of getting to Mars. With purely chemical propulsion, you can use a booster 1/2 the size compared to a booster without the use of the barge. Therefore, Zubrin's Mars Direct plan could use 70 ton to LEO boosters and otherwise be identical.
NTP engines have the disadvantage of having a heavy nuclear reactor that is designed for heat, not power generation. NEP engines suffer from a lack of thrust. In both cases, the level of development of these engines is far lower than what you have with chemical engines. Use of the ETP barge allows one to get enough power out of plain old chemical rockets to make large Mars expeditions possible.
I suppose that one could argue that ETP is even more of an untried system than either NEP or NTP and that would be correct. However, the principles behind tether propulsion are very simple and if successful in boosting the ISS, we will have an ample amount of operational data to work from.
I'll try to come up with some good estimates of what sort of magnitude of thrust and barge mass we're looking at here.
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*Well...this topic definitely isn't one of my "strong suits." But I do have a question:
Would (solely) nuclear-powered rockets have stages at all?
--Cindy
Assuming a nuclear rocket was assembled in space (as I can't imagine nuclear rockets blasting off of Earth a la Orion anytime soon), these type of rockets would essentially be single-stage rockets, as there would be no need for "disposable" stages like the Saturn V, etc.
For this reason, nuclear rockets would be far more economical to operate than traditional chemical rockets, and it would behoove NASA and other space agencies to develop nuclear propulson technology as quickly as possible.
B
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I'm certainly not qualified to calculate whether you could have a single stage nuclear rocket. (I'm assuming you're referring to a NTP Nuclear Thermal Propulsion rocket which has enough thrust to launch from the surface of the Earth.) I think that the proposals for that kind of launcher do call for a SSTO otherwise you'd have to try and recover falling nuclear reactors - not an idea I'd relish)
As for NEP nuclear driven ion engine proposals - they would be single stage but you're already in orbit since those engines produce at most a few pounds of thrust and can't get you off the ground.
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OK, I ran some numbers for the ETP barge idea I had and the numbers are considerable less attractive than I originally thought. The idea is plausible but probably only practical for cargo shipments - it's too slow for sending people.
Taking data from the Tethers Unlimited proposal for a 100 km rotating kinetic energy boost tether and optimizing it for propulsive force generation using 16 times the power output of the original design, I get the following numbers:
100 km tether, composed of 10 - 10km segments.
16 - 80 km long copper force generating electrodes.
Solar cells capable of generating an average of 980 kW of electrical power.
Unlike the original design, this does not rotate, greatly simplifying the strength requirements and allowing for greater thrusting efficiencies.
All data taken from the original paper is linearly extrapolated, assuming that non-linearities in mass scaling will end up approximately evening out. Note that the original numbers are taken from experimentally derived numbers and existing commercial subsystems.
The projected mass of the whole thing is about 40 metric tons. Assuming a spacecraft that masses 140 metric tons, the amount of time to boost the spacecraft to the Near Escape orbit is about 4.8 months. Too long for manned crews but not too bad for cargo.
Carrying the example to extremes, boosting the power capacity of the tether to 32 and 64 times the original power levels give tether masses of 73 and 141 metric tons and transit times to Near Escape orbit of 2.85 and 1.9 months respectively.
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