PDA

View Full Version : Spacecraft Trajectory Questions



cjackson
2014-Jan-22, 06:48 AM
I have multiple questions about the same topic.

1. I know spacecraft rely on gravity slingshots and changes in velocity rather than just going from point A to B due to it being more economical. However, it seems to take months to years to get anywhere. Rosetta was launched in 2004 and will finally reach 67P in November this year. Similarly, MESSENGER was launched in 2004 and didn't arrive at Mercury until 2011. Why is it that it takes longer to reach objects closer to the sun than further away?

2. If money weren't as tight and spacecraft didn't have to rely on gravity assists but could carry enough fuel to 'gun' it, so to speak, how much would transit time be cut down? Would nuclear thermal spacecraft be able to go faster than present spacecraft?

3. What is the best near term solution for making spacecraft faster?

NEOWatcher
2014-Jan-22, 01:24 PM
I know spacecraft rely on gravity slingshots and changes in velocity rather than just going from point A to B due to it being more economical. However, it seems to take months to years to get anywhere. Rosetta was launched in 2004 and will finally reach 67P in November this year. Similarly, MESSENGER was launched in 2004 and didn't arrive at Mercury until 2011. Why is it that it takes longer to reach objects closer to the sun than further away?
It's hard for me to explain, so I won't try. I suggest doing some research on orbital mechanics. It's going to answer a lot of your questions on this board.



2. If money weren't as tight and spacecraft didn't have to rely on gravity assists but could carry enough fuel to 'gun' it, so to speak, how much would transit time be cut down? Would nuclear thermal spacecraft be able to go faster than present spacecraft?
As fast as you want below the speed of light. But the fuel requirements do not scale up linearly, so before you know it, your ship could exceed the resources of the Earth.
A nuclear craft's fuel has a higher power to weight ratio. So, with the same power, you are pushing much less weight, making you much more efficient. The nuclear spacecraft has faster thrust, so your acceleration can be faster.


3. What is the best near term solution for making spacecraft faster?
Near term, not much. Nuclear can do it, but there's going to have to be a lot of technological research to do it correctly, and that's only if it can be done politically.
Decades ago they did have working models of engines, and their thrust was on the order of 3 to 4 times faster.

noncryptic
2014-Jan-22, 01:55 PM
> could carry enough fuel to 'gun' it

Just carrying more fuel won't help because it increases the mass of the ship and thus reduces the ability to 'gun it'.
What would be needed is a several orders of magnitude increase in engine/fuel efficiency (with the same or better engine power).

> What is the best near term solution for making spacecraft faster?

There is none in the foreseeable future. Only hope is to keep on doing science and research.

Hornblower
2014-Jan-22, 03:19 PM
The trip to Mercury took a long time because it required numerous orbital close encounters with the inner planets, each of which gave a relatively small tweak to the trajectory. We did not have the benefit of the one-shot wallop Jupiter can give to a spacecraft bound for the outer planets.

Hornblower
2014-Jan-22, 03:26 PM
It's hard for me to explain, so I won't try. I suggest doing some research on orbital mechanics. It's going to answer a lot of your questions on this board.



As fast as you want below the speed of light. But the fuel requirements do not scale up linearly, so before you know it, your ship could exceed the resources of the Earth.
A nuclear craft's fuel has a higher power to weight ratio. So, with the same power, you are pushing much less weight, making you much more efficient. The nuclear spacecraft has faster thrust, so your acceleration can be faster.


Near term, not much. Nuclear can do it, but there's going to have to be a lot of technological research to do it correctly, and that's only if it can be done politically.
Decades ago they did have working models of engines, and their thrust was on the order of 3 to 4 times faster.
My bold. What sort of propulsion system is this? The only one I ever heard of is a nuclear powered ion thruster that delivers about 1/100 g or less over a very long time. Its high specific impulse enables a given top speed with far less propellant than is needed with chemical rockets, but it might not save any time on a trip to Mars. For the outer planets it is a different story.

If you know of a type of thruster that can give a large thrust to mass ratio, please bring me up to speed.

Glom
2014-Jan-22, 03:32 PM
It's worth pointing out that mechanical energy is inversely proportional to semi-major axis.

So in fact, the change of energy is in fact greater going to Mercury than it is going to Jupiter.

NEOWatcher
2014-Jan-22, 04:11 PM
My bold.
Forgive me if I got a little discombobulated there.
I was thinking of the high ISP of the NERVA, and thinking the lighter fuel load would offset the weight and amount of thrust of the engine.
I looked it up, and see that it's not quite what I imagined. I got effiency and acceleration mixed up.

Glom
2014-Jan-22, 05:37 PM
It's worth pointing out that mechanical energy is inversely proportional to semi-major axis.

So in fact, the change of energy is in fact greater going to Mercury than it is going to Jupiter.

Okay, nevermind.

The most important thing is delta-v and it turns out to go to Jupiter does require more, but only about 1.5 times as much.

Elukka
2014-Jan-23, 04:19 PM
If you know of a type of thruster that can give a large thrust to mass ratio, please bring me up to speed.
I think more modern nuclear thermal rockets can qualify. Pebble bed reactor rockets (Timberwind/SNTP) can do thrust-to-weight ratios of 20-30 at an isp of 1000 seconds. Aerojet at some point also did tests regarding LOX injection into the propellant stream of an NTR, which, if memory serves, they figured would raise thrust by 50-100% at the cost of some isp.

IsaacKuo
2014-Jan-23, 08:08 PM
Okay, nevermind.

The most important thing is delta-v and it turns out to go to Jupiter does require more, but only about 1.5 times as much.
Actually it takes much LESS delta-v to get to Jupiter than it does to orbit Mercury (via direct Hohmann transfer, rather than a complex series of multiple gravity assists).

For a flyby or impact mission, it takes a bit less delta-v to reach Mercury than Jupiter--but not much.

From LEO to Earth-Mercury transfer requires 5.5km/s delta-v.
From LEO to Earth-Jupiter transfer requires 6.3km/s delta-v. That's only 15% more than getting to the Earth-Mercury transfer--not 1.5 times as much.

Now, if you care about actually staying at Mercury (an orbit mission), then that's where things get really difficult for a Mercury mission. The Oberth effect is stronger for planets with stronger gravity wells. Mercury's Oberth effect is very weak, while Jupiter's Oberth effect is very strong. So strong, in fact, that it requires very little delta-v to enter Jupiter orbit. But it takes a lot of delta-v to enter Mercury orbit. For an orbiter mission, the delta-v requirements are:

From LEO to Earth-Mercury transfer to high Mercury orbit requires 11.7km/s (an additional 6.2km/s for orbital insertion)
From LEO to Earth-Jupiter transfer to high Jupiter orbit requires 6.6km/s (an additional 0.3km/s for orbital insertion)

Furthermore, Jupiter has an atmosphere, so you can use aerobraking for free to lower your orbit from this high elliptical orbit down to a closer orbit (more suitable for most missions). With Mercury, you are left in an inconveniently high elliptical orbit which may or may not be suitable for your mission.

Without using complex and time consuming multiple gravity slingshot flyby maneuvers, the delta-v costs to get to Mercury orbit are prohibitive. It's just plain easier to get to Jupiter than Mercury, by an extremely large margin.