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Bender222

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Would it be feasible for a possible mars mission earth departure spacecraft to stay in earth orbit for a period of time prior to departure and gradually build up speed so that the journey would be faster and cheaper? and if there was any problem during the time they are accelerating they are just in orbit and not halfway between earth and their destination. The trip home would be alot longer but doing ths might reduce some of the risk involved.
 

TWRackers

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Sorry, orbital mechanics don't work that way. If you started accelerating while in Earth orbit, you'd start spiraling away from the Earth until you reach escape velocity, beyond which you won't fall back into an elliptical orbit if you cut the engines off. At that point you'd be in a Solar orbit.

You COULD do as you propose, sort of, IF you had an extremely large supply of fuel onboard, but you'd use a whole lot more than if you used the usual technique of well-timed short burns to change orbital trajectories, and coasted 99+% of the time. It's all rather complex and mathematical, but fascinating stuff (to me, anyway).
 

Bender222

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using a low thrust / high impulse motor for acceleration and have a high thrust motor to fire short bursts at regular intervals to maintain orbit.
 

shrox

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using a low thrust / high impulse motor for acceleration and have a high thrust motor to fire short bursts at regular intervals to maintain orbit.
You're still going to use about as much fuel. It would be like skidding through a turn in a car, although the physics are different. It's all about conservation of energy.
 

luke strawwalker

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Would it be feasible for a possible mars mission earth departure spacecraft to stay in earth orbit for a period of time prior to departure and gradually build up speed so that the journey would be faster and cheaper? and if there was any problem during the time they are accelerating they are just in orbit and not halfway between earth and their destination. The trip home would be alot longer but doing ths might reduce some of the risk involved.
Similar trajectories have even been suggested for lunar missions... It's POSSIBLE but has a LOT of downsides. As you accelerate, your orbit gradually rises higher and higher, which before too long puts you smack dab in the middle of the Van Allen Radiation Belts (all those charged particles from the solar wind whipped around the Earth by the Earth's magnetic fields, which generate a lot of radiation.) Such radiation plays heck with electronics over time and is downright dangerous to human life. About the only upside is that you can use low-thrust high-efficiency boosters like Solar Thermal rockets, VASIMIR, or solar electric rockets (plasma/ion engines) which have PHENOMENAL ISP (fuel efficiency) generally speaking, but have VERY low thrust levels, and so can only accelerate payloads over a period of weeks or months at a very low rate.

There have been some folks advocating a solar-electric space tug equipped with solar electric propulsion or VASIMIR engines that would slowly 'spiral out' an unmanned lunar lander to the vacinity of the moon... eventually your orbit is SO vast that it 'intersects' the moon's orbit, but then you have to reverse your thruster's direction and 'spiral in' to lunar orbit. Such a system, depending on the size of it's solar panels and number/power of it's ion thrusters, would take anywhere from a month to several months to get from LEO to LLO (low earth orbit to low lunar orbit).

For a Mars trip, it would basically require the switch to a nuclear thermal rocket or nuclear electric propulsion. Nuclear thermal rockets were designed and tested back in the 60's (NERVA, etc.) but were never developed and soon cancelled due to budget and environmental concerns, and possible treaty problems with orbiting nuclear powered equipment, etc. A nuclear electric propulsion system would use a nuclear reactor to generate electricity that would be used in an ion engine to provide propulsion, but the efficiency is not too good because most of the energy released by nuclear reactors is thermal, or heat energy, and conversion to electricity is both heavy, bulky, and not very efficient from a loss standpoint. Coupled with the very low thrust levels of ion propulsion, despite the high propellant gas efficiency of the rocket engine, it would be a very slow and expensive propulsion method, compared to nuclear thermal. Solar electric isn't really feasible for Mars missions because of the nature of the propulsion systems-- for low-thrust systems like this 'spiralling' out to their destinations, you start slow, and gradually over a long period of time accelerate to pretty high velocities, BUT then you have to reverse your thrusters in the middle of the flight and start slowing down so you're going slow enough when you get to the destination to brake into orbit. The intensity of solar power is about 1/2 what it is at Earth out at Mars, so you'd need TWICE as big a solar array to provide the same power as you'd need at Earth, all of which has to be accelerated by the low thrust engine and braked by the engine down to speeds low enough for a Mars Orbit Insertion (MOI).

So, while feasible, the problems are daunting, and there are VERY SUBSTANTIAL development programs required and a lot of hurdles both real and imaginary to overcome. Ion propulsion has been proven on Deep Space One and is being used on some other probes, but it's in it's infancy as a space propulsion system. Solar power is feasible for a power source for the engines, but it's low density and rapid falloff as you go away from the sun means it requires heavy, large, easily damaged solar panels, and even larger ones as you go toward the outer planets (that's why all the outer solar system probes beyond Mars are powered by NTG's-- nuclear thermal generators, which are essentially take the heat of plutonium decay and use that to drive a thermocouple to generate electricity, but the power density is VERY low (unsuitable to power anything much beyond instrumentation, certainly not enough for a viable Ion engine). Nuclear would also have to overcome all the resistance from Greenpeace types that are convinced launching a nuclear powered rocket, even one with an unactivated reactor in a 'safed' condition that wouldn't be activated until it's in orbit, would somehow lead to a global Armageddon if it went awry... plus possible diplomatic resistance related to the Space Treaty banning certain types of nuclear stuff in space. Then there's the budgetary problems, as to how to pay for it all, which is the largest and most fundamental problem.

Project Prometheus was going to revive nuclear thermal propulsion for a Mars trip by Constellation; it was cancelled almost before the ink was dry on the VSE...

Later! OL JR :)
 

luke strawwalker

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Then there's also the proposals to use the libration points as staging points for missions, because it minimizes the delta-v requirements, which means your rocket can get more cargo where it's going for the same amount of fuel (or your lander/cargo can fly on a smaller rocket with less fuel). The libration points exist in every 2-body system orbiting each other between those bodies. The Earth-Moon libration points (EML's) lie around the Earth-Moon system, EML-1 is between the Earth and Moon where the gravity 'balances' between the two. EML-2 is an equivalent distance BEHIND the moon (beyond it's orbit AWAY from Earth) but is actually easier to get to because of orbital mechanics-- to get to EML-1 you have to essentially "stop" in space at the libration point, which takes a lot of fuel, or do some complicated lunar flyby's combined with engine braking. EML-2 takes a few days longer to get to, but can be done mostly by lunar flyby's to reduce your speed and save propulsive braking. EML-3 and 4 lie on the moon's orbit 60 degrees ahead and behind it, and EML-5 is on it's orbit directly on the other side of the Earth from the Moon. A spacecraft can 'orbit' the EML points just like they'd orbit Earth or the moon, using a slight amount of fuel for station keeping, as the EML's are a bit unstable because of slight variations in gravity, orbits, etc. The upside of EML's is that they reduce the propulsion requirements and expand the capabilities, since it is equally easy to land on ANY POINT on the moon from the EML, propulsion wise, unlike launches from Earth that require propulsive plane-changes to the orbit to land at say the lunar north or south polar regions. The downside is it's a 7-8 day trip to EML 2 from earth and another day or so to descend to the moon from EML-2, and about the same from EML-1, and it requires some 'station keeping' and more sophisticated mission with more propulsive manuevers (burns) and a more complicated trajectory to perform an EML lunar mission, which means you're not in any kind of "free return" trajectory like you are with the standard "Hohmann Transfer" type of lunar mission trajectories we flew during Apollo. The benefit is, better performance, anytime communications with Earth through leaving your Orion at the EML point while the lunar lander descends to the moon, which can provide 'far side' communications for a farside landing on the moon, less fuel is required, and you can land anywhere without expensive plane changes to your orbit, and return to the EML point equally as easily, and you can go/return at any time equally easy, without having to loiter in LLO or LEO for your orbits to come into phase-- the correct alignment to launch to the moon or go back to Earth. Anytime return was a big requirement for Constellation which could potentially require a crew to loiter on the lunar surface or in lunar orbit for several days waiting for the orbits to phase before coming home, or else pay the propulsive penalties of a high delta-v (acceleration or deceleration) plane change manuever.

There are also Sun-Earth Libration points (SEL 1,2,3,4,5) and around Earth's orbit around the sun, in similar positions relative to each other (Google libration points for a better explanation and visuals) The James Webb Space Telescope is going to be sent to one of the SEL points and enter a 'halo' orbit around it, so that it will not suffer the same observation limitations imposed on Hubble because of it's low Earth orbit (Earth is between the telescope and whatever it's looking at 45 minutes out of every 90, or thereabouts:)) In a halo orbit around an 'imaginary' point, there's NOTHING in the way, so you can have the telescope sit and look at a single object for WEEKS if you so desire!

Then of course there's the relatively 'new' phenomena of the 'interplanetary superhighway' that has been proposed... which is a sorta ever-changing path through space caused by the various positions of the planets relative to each other and the sun, which creates 'highways' that a craft could fly along and get to VERY distant points for MUCH lower delta-v than they could flying more standard Hohmann transfer trajectories to their destination. But that's a whole other topic in and of itself... but a fascinating read!

Fascinating stuff all right... I grasp the principles Ok if not the mathematics... :) OL JR :)
 

bobkrech

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using a low thrust / high impulse motor for acceleration and have a high thrust motor to fire short bursts at regular intervals to maintain orbit.
The only reason you have to perform an orbital reboost is to make up for atmospheric drag losses in LEO (Low Earth Orbit). Once you're above ~450 km it's not an issue for years.

For orbital corrections and drag makeup, the preferred motors are low thrust-high Isp motors. These are frequently used in GEO orbits for station keeping to correct for solar wind drag. The current motors have high Isp, but have poor electrical efficiency, but since this is the domain of communication satellites with lots of power, it's not an issue.

The only way to obtain rapid acceleration is to have high thrust motors. Storable biprop motors have Isp in the 300 s to 350 s range. Low thrust-high Isp motors have been proposed to greatly reduce the transit time on interplanetary missions. They provide very low acceleration for a long period of time. Ion engines such as the one on Deep Space 1 have operated for more than a year but have relatively little thrust and aren't very efficient from an electrical standpoint.

The Variable Specific Impulse Magnetoplasma Rocket, VASIMR is a new high-power plasma-based space propulsion technology, initially studied by NASA and now being developed privately by Ad Astra. http://www.adastrarocket.com/home1.html

It is claimed that a VASIMR engine could maneuver payloads in space far more efficiently and with much less propellant than today’s chemical rockets, however I have not seen any numbers on the actual electrical efficiency of this type of engine. A laboratory version of the engine has operated ate the 200 kw level, but the claim of a potential electrical efficiency of 60% has not been demonstrated. In any case, it needs a nuclear reactor to power it. Nuclear reactors are not light so the spacecraft will also be high, and while it is claimed to have an Isp up to 30,000 s where the thrust of a 10 MW unit is only 40 N which is what you get from a G40 motor. Furthermore, the propulsive efficiency of a motor operating at that Isp is poor, as you get the best propulsive efficiency when the Isp is ~100 times the exhaust velocity in km/s so at 10 km/s vehicle velocity you are most efficient when the exhaust velocity is 10 km/s and the Isp is 981 s.

Bob
 
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shrox

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I though the OP meant something like this, this is the only way I can think of to have a "fast orbit" at a give distance.

FastOrbit001.jpg
 

TWRackers

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I though the OP meant something like this, this is the only way I can think of to have a "fast orbit" at a give distance.
Yep, that would allow you to maintain a circular orbit at a higher than normal (i.e. coasting) speed, BUT you're burning fuel the whole time and throwing away some of the acquired kinetic energy to maintain that orbit. That's kinda silly, in my opinion.
 

shrox

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Yep, that would allow you to maintain a circular orbit at a higher than normal (i.e. coasting) speed, BUT you're burning fuel the whole time and throwing away some of the acquired kinetic energy to maintain that orbit. That's kinda silly, in my opinion.
Yes, exactly.
 

mjennings

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On the topic of Radio Isotope Thermal Generators (RTG) and other nuclear payloads it takes 5 high level signatures to get off the launch pad, not to mention town hall meetings, protest and the like they are (I reasonably sure this is the right list):
The President
NASA Administrator
Department of Defense
Department of Energy
and the Nuclear Regulatory Commission

For those reading the thread who liked to learn more about Orbital Mechanics I suggest the following books
Fundamentals of Astrodynamics by R. Bates Et Al
http://www.amazon.com/dp/0486600610/?tag=skimlinks_replacement-20

Spacecraft Mission Design by Charles Brown
http://www.amazon.com/gp/product/1563472627/?tag=skimlinks_replacement-20

Introduction to Space Dynamics By W Thompson
http://www.amazon.com/dp/0486651134/?tag=skimlinks_replacement-20

Spacecraft Mission Design is small but expensive as it is an AIAA book, the others are fairly cheep, used them in school and really enjoyed them, but as a warning some higher math is required

Does anyone know if Liberation point has surpassed the old Le Grange point terminology?
 
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