5 ms·

I had no idea before reading this article that it was hard to escape from the attraction of the sun! It makes me wonder if we now have the capabilities to build

by S4M 14y ago

I had no idea before reading this article that it was hard to escape from the attraction of the sun! It makes me wonder if we now have the capabilities to build a rocket that can do it without the help of the attraction of the outer planets. Does somebody here who's knowledgeable on the subject have some reference?

mturmon 14y ago
It's very common to use gravity assist, especially in outer planets missions. Fuel is very expensive and has diminishing returns (the more fuel you put on, the more fuel you need to lift the extra fuel you put on). After Voyager, Galileo, Cassini, and New Horizons (among others) have used gravity assist to get to Jupiter, Saturn, and Pluto respectively (NH is still en route). Even Stereo, a two-satellite pair in earth-trailing and earth-following orbits looking at the Sun, used gravity assist (with the Moon). Gravity assist is one way to provide "delta-v" (velocity change, the coin of the realm in propulsion). You can provide negative delta-v using aerobraking -- orbiting your destination, Mars say, and slowing down using atmospheric friction. This is used on the Mars orbiters. As usual, see: http://en.wikipedia.org/wiki/Delta-v_budget http://en.wikipedia.org/wiki/Delta-v_budget
grecy 14y ago
I watched the Dawn spacecraft[1] launch, it uses an ion propulsion drive which Wikipedia says can perform a velocity change of over 10 km/s over the mission. My understanding is that if they left the ion drive on long enough, the craft would continue to accelerate. [1]http://en.wikipedia.org/wiki/Dawn_%28spacecraft%29#Propulsion_system http://en.wikipedia.org/wiki/Dawn_%28spacecraft%29#Propulsio...
- uvdiv 14y ago
  It can be much better than that. The ion thrusters in Dawn have a specific impulse of 3,100 seconds (30 km/s exhaust velocity). Later thrusters designed for the Jupiter Icy Moons Orbiter achieved 9,600 seconds in a lab (94 km/s e.v.) JIMO itself is designed for a delta-v of 40 km/s. https://en.wikipedia.org/wiki/High_Power_Electric_Propulsion https://en.wikipedia.org/wiki/High_Power_Electric_Propulsion https://www.lpi.usra.edu/meetings/jimo2003/hartman.pdf https://www.lpi.usra.edu/meetings/jimo2003/hartman.pdf delta-v is proportional to the specific impulse (or exhaust velocity), and logarithmic in the propellant ratio. For instance, with a propellant ratio of e = 2.7, you can get a delta-v equal to the exhaust velocity. For a delta-v twice that, you need a propellant ratio of e^2 = 7.4. Obviously it's not practical to go much faster the exhaust velocity; the propellant demand grows exponentially. https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation
- deleted 14y ago
  [deleted]
arscan 14y ago
If you are interested in gaining a more intuitive understanding of the forces in play (and have some time to kill), I recommend grabbing a copy of Kerbal Space Program. Its only in alpha right now, but its a heck of a lot of fun already and does a really good job of teaching you how to use orbital mechanics to land on other planets (or to shoot a probe out of the solar system, if you want to try to do that). Their model is definitely simplified (no lagrange points, for example), but the major aspects are modeled well enough to give you an idea of how interplanetary space travel works. http://kerbalspaceprogram.com/ http://kerbalspaceprogram.com/
uvdiv 14y ago
Yes, easily. Solar escape delta-v is about 21 km/s from earth orbit (not including earth's gravity). The Jupiter Icy Moons Orbiter is designed with a delta-v twice that, 40 km/s, using nuclear-powered ion thrusters. To be used to maneuver within Jupiter's gravity well, moving from one moon to another. http://www.lpi.usra.edu/meetings/jimo2003/hartman.pdf http://www.lpi.usra.edu/meetings/jimo2003/hartman.pdf It's a completely doable project sidelined by budget priorities (NASA thinking space tourism more important than science).
johngalt 14y ago
It's entirely possible to build a rocket that doesn't need help from gravity assists, but rocket fuel is very expensive to send to the outer planets. Better to roll downhill towards Jupiter rather than uphill away from the sun. There are other fun things that we can do with gravity assists as well: A rocket burn is significantly more effective when close to a large gravitational source. http://en.wikipedia.org/wiki/Oberth_effect http://en.wikipedia.org/wiki/Oberth_effect If you are willing to wait long enough there is a way to get almost anywhere in the solar system for very little fuel. http://en.wikipedia.org/wiki/Interplanetary_Transport_Network http://en.wikipedia.org/wiki/Interplanetary_Transport_Networ...
S4M 14y ago
Thanks for all the links guys!

by S4M 14y ago

mturmon 14y ago

It's very common to use gravity assist, especially in outer planets missions. Fuel is very expensive and has diminishing returns (the more fuel you put on, the more fuel you need to lift the extra fuel you put on). After Voyager, Galileo, Cassini, and New Horizons (among others) have used gravity assist to get to Jupiter, Saturn, and Pluto respectively (NH is still en route). Even Stereo, a two-satellite pair in earth-trailing and earth-following orbits looking at the Sun, used gravity assist (with the Moon). Gravity assist is one way to provide "delta-v" (velocity change, the coin of the realm in propulsion). You can provide negative delta-v using aerobraking -- orbiting your destination, Mars say, and slowing down using atmospheric friction. This is used on the Mars orbiters. As usual, see: http://en.wikipedia.org/wiki/Delta-v_budget http://en.wikipedia.org/wiki/Delta-v_budget

grecy 14y ago

I watched the Dawn spacecraft[1] launch, it uses an ion propulsion drive which Wikipedia says can perform a velocity change of over 10 km/s over the mission. My understanding is that if they left the ion drive on long enough, the craft would continue to accelerate. [1]http://en.wikipedia.org/wiki/Dawn_%28spacecraft%29#Propulsion_system http://en.wikipedia.org/wiki/Dawn_%28spacecraft%29#Propulsio...

uvdiv 14y ago
It can be much better than that. The ion thrusters in Dawn have a specific impulse of 3,100 seconds (30 km/s exhaust velocity). Later thrusters designed for the Jupiter Icy Moons Orbiter achieved 9,600 seconds in a lab (94 km/s e.v.) JIMO itself is designed for a delta-v of 40 km/s. https://en.wikipedia.org/wiki/High_Power_Electric_Propulsion https://en.wikipedia.org/wiki/High_Power_Electric_Propulsion https://www.lpi.usra.edu/meetings/jimo2003/hartman.pdf https://www.lpi.usra.edu/meetings/jimo2003/hartman.pdf delta-v is proportional to the specific impulse (or exhaust velocity), and logarithmic in the propellant ratio. For instance, with a propellant ratio of e = 2.7, you can get a delta-v equal to the exhaust velocity. For a delta-v twice that, you need a propellant ratio of e^2 = 7.4. Obviously it's not practical to go much faster the exhaust velocity; the propellant demand grows exponentially. https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation
deleted 14y ago
[deleted]

arscan 14y ago

If you are interested in gaining a more intuitive understanding of the forces in play (and have some time to kill), I recommend grabbing a copy of Kerbal Space Program. Its only in alpha right now, but its a heck of a lot of fun already and does a really good job of teaching you how to use orbital mechanics to land on other planets (or to shoot a probe out of the solar system, if you want to try to do that). Their model is definitely simplified (no lagrange points, for example), but the major aspects are modeled well enough to give you an idea of how interplanetary space travel works. http://kerbalspaceprogram.com/ http://kerbalspaceprogram.com/

uvdiv 14y ago

Yes, easily. Solar escape delta-v is about 21 km/s from earth orbit (not including earth's gravity). The Jupiter Icy Moons Orbiter is designed with a delta-v twice that, 40 km/s, using nuclear-powered ion thrusters. To be used to maneuver within Jupiter's gravity well, moving from one moon to another. http://www.lpi.usra.edu/meetings/jimo2003/hartman.pdf http://www.lpi.usra.edu/meetings/jimo2003/hartman.pdf It's a completely doable project sidelined by budget priorities (NASA thinking space tourism more important than science).

johngalt 14y ago

It's entirely possible to build a rocket that doesn't need help from gravity assists, but rocket fuel is very expensive to send to the outer planets. Better to roll downhill towards Jupiter rather than uphill away from the sun. There are other fun things that we can do with gravity assists as well: A rocket burn is significantly more effective when close to a large gravitational source. http://en.wikipedia.org/wiki/Oberth_effect http://en.wikipedia.org/wiki/Oberth_effect If you are willing to wait long enough there is a way to get almost anywhere in the solar system for very little fuel. http://en.wikipedia.org/wiki/Interplanetary_Transport_Network http://en.wikipedia.org/wiki/Interplanetary_Transport_Networ...

S4M 14y ago

Thanks for all the links guys!