Only every 2 years to Duna (mars)

[Muddyblack]

Hey guys,

I heared that it would be only possible to get to mars every 2 years (use less fuel).

Well when i play KSP i bring my rocket into an ellipsis around the sun and then choose my target. I get whenever i want to every planet. Now my question:

How is it done in real life do we take an exact course to Mars ? And why start only every 2 years if it works also otherwise or does it takes soooo much more fuel?

Or is KSP in this kind not realistic (am i doing something wrong) ?

 

Thank you 

kind regards Muddyblack

[Superfluous J]

The cheapest (in dV) way to travel to another planet (unless you do flybys for gravity assists) is a Hohnann Transfer.

https://en.m.wikipedia.org/wiki/Hohmann_transfer_orbit

[Rhomphaia]

When you launch into an orbit around the sun and then Hohmann transfer to your target, you are not really saving any time.  You are just waiting for your rocket to get to the appropriate position for its burn around the sun rather than waiting for the planet you are leaving to do the same. 

[Harry Rhodan]

Just look at the available graphical transfer window planers and you can see, how much more a transfer might cost:

https://alexmoon.github.io/ksp/#/Kerbin/100/Duna/100/false/ballistic/false/1/1

[Vanamonde]

Since this is about actual space flight, the discussion has been moved to the Science subforum. 

[mikegarrison]

Short answer: you can go any time you want to, but some times it takes far less fuel than other times.

[Bill Phil]

KSP is interesting in that the rockets are fairly over powered for the KSP solar system. If you put any real rocket in the game it would massively outperform any KSP rocket. We have better engine performance, better tank ratios, and so on.

So in real life rockets are much less powerful compared to the performance requirements. This means that if we want to maximize performance we need to minimize delta-v. 

So trajectory analysts can use a variety tools to optimize performance and one such tool is the porkchop plot - a contour plot where each contour represents a line of equal characteristic energy (also called C3, a way of characterizing escape trajectories based on hyperbolic excess energy). The axes of the plot represent departure and arrival times, or departure and time of flight.

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

^Some more there

It turns out that the minimum C3s - and thus maximum performance - are periodic, in sync with the synodic period, at least usually. For Mars it's a little off since N-body mechanics can perturb the orbit from a perfect ellipse but it's actually quite close. So for Mars a launch period occurs roughly every 2.135 years. 

Now theoretically you can leave Earth at any time, and then make deep space burns to arrive at Mars, potentially earlier than you would if you had waited for a launch period. But such an endeavor requires both a longer time of flight and more vehicle performance. Essentially you get the mission done earlier but you may expose your astronauts to more radiation and you need to build a bigger rocket, potentially much bigger. So that requires more performance requirements. Launching during a launch period means you can use a rocket that's the right size for the job.

Not only that but in practical terms a space probe or rover can take years to develop and the launcher can take over a year to procure, so waiting for a launch period is not as big of a problem. However some launch periods are better than others, if I recall correctly the 2018 window was a pretty low energy window.

Of course you can further minimize launch C3 by using gravity assists. Originally the Galileo mission was supposed to launch from the Space Shuttle with a Centaur stage in the payload bay (the so called "Death Star"), but due to the Challenger this never occurred, so a new trajectory was planned using a series of gravity assists to kick the probe to Jupiter. It took much longer, but the required rocket was less powerful. I believe it still launched on Shuttle, but a solid upper stage was used instead. This increases the time required but reduces performance requirements.

[mikegarrison]

21 minutes ago, Bill Phil said:

KSP is interesting in that the rockets are fairly over powered for the KSP solar system. If you put any real rocket in the game it would massively outperform any KSP rocket. We have better engine performance, better tank ratios, and so on.

So in real life rockets are much less powerful compared to the performance requirements. This means that if we want to maximize performance we need to minimize delta-v. 

So trajectory analysts can use a variety tools to optimize performance and one such tool is the porkchop plot - a contour plot where each contour represents a line of equal characteristic energy (also called C3, a way of characterizing escape trajectories based on hyperbolic excess energy). The axes of the plot represent departure and arrival times, or departure and time of flight.

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

^Some more there

It turns out that the minimum C3s - and thus maximum performance - are periodic, in sync with the synodic period, at least usually. For Mars it's a little off since N-body mechanics can perturb the orbit from a perfect ellipse but it's actually quite close. So for Mars a launch period occurs roughly every 2.135 years. 

Now theoretically you can leave Earth at any time, and then make deep space burns to arrive at Mars, potentially earlier than you would if you had waited for a launch period. But such an endeavor requires both a longer time of flight and more vehicle performance. Essentially you get the mission done earlier but you may expose your astronauts to more radiation and you need to build a bigger rocket, potentially much bigger. So that requires more performance requirements. Launching during a launch period means you can use a rocket that's the right size for the job.

Not only that but in practical terms a space probe or rover can take years to develop and the launcher can take over a year to procure, so waiting for a launch period is not as big of a problem. However some launch periods are better than others, if I recall correctly the 2018 window was a pretty low energy window.

Of course you can further minimize launch C3 by using gravity assists. Originally the Galileo mission was supposed to launch from the Space Shuttle with a Centaur stage in the payload bay (the so called "Death Star"), but due to the Challenger this never occurred, so a new trajectory was planned using a series of gravity assists to kick the probe to Jupiter. It took much longer, but the required rocket was less powerful. I believe it still launched on Shuttle, but a solid upper stage was used instead. This increases the time required but reduces performance requirements.

Short answer again: you can go any time you want to, but some times it takes far less fuel than other times.

Edited May 10, 2020 by mikegarrison

[Nuke]

there's a grey area between the hohmann transfer (most efficient) and the flip and burn of a brachistochrone trajectory (fastest). right now constant thrust drives are really low powered ion drives so fastest in this case is a relative term were quite a ways from having fusion torch drives. but nothing stops you from bringing a little extra deltav. 

my idea is to take a high trajectory with an aerocapture at mars to enable an early arrival. land on mars for a week's stay. then do the same thing on the return. its less efficient and you need to bring more fuel, but you don't have to stay for a long time to make your return window. of course this will probably be headed up by another mission to land a fuel processor or two on mars so you can refuel for the return trip. 

Edited May 11, 2020 by Nuke

[K^2]

26 minutes ago, Nuke said:

there's a grey area between the hohmann transfer (most efficient) and the flip and burn of a brachistochrone trajectory (fastest). right now constant thrust drives are really low powered ion drives so fastest in this case is a relative term were quite a ways from having fusion torch drives. but nothing stops you from bringing a little extra deltav. 

my idea is to take a high trajectory with an aerocapture at mars to enable an early arrival. land on mars for a week's stay. then do the same thing on the return. its less efficient and you need to bring more fuel, but you don't have to stay for a long time to make your return window. of course this will probably be headed up by another mission to land a fuel processor or two on mars so you can refuel for the return trip. 

Because eccentricity of Mars' orbit is quite a bit higher than Earth's, there are transfer options that give you significant time on the surface with almost no delta-V budget increase. But your transfer windows for these might be even fewer than once every two years. In practice, almost any convenient mission to Mars is going to be noticeably worse than optimal in terms of fuel requirement, but it's probably worth it if you're sending humans and not just cargo or experiments.

[Muddyblack]

Thank you for all of your answers

so the major problem is that you need more time -> bad for the moral and need more food what means more weight more fuel needed got it thank you.

But if we would have a rocketlaunchpad-base on Mun would we be able to change our route right ? ^^

And in KSP is it also funny for the performance take some reactionwheels somewhere and everything is fine haha  

[Piscator]

Not quite sure it has been explicitly mentioned yet, but using the two-burn strategy of going into solar orbit first, then choosing your intercept orbit is also always less effective than burning for intercept orbit directly because you don't make the best use of the Oberth effect of your origin's gravity well. Since this can amount to quite a noticeable saving in delta-V, it makes sense to wait until your planet of origin is in the optimal position and only then make a single transfer burn.

[jimmymcgoochie]

In KSP, the solar system is about 1/10th the size of the real Solar system and orbital velocities are considerably slower than in reality. Delta-V is cheap as a result because you can bring more fuel along pretty easily, meaning you can quite literally judge your transfer burns by eye and it has a good chance of still working despite not being the most efficient way possible of getting there.

In real space projects, delta-V is expensive (fuel is heavy and heavy makes you slower so you need more fuel for the same delta-V which means more weight, etc. etc.) but there are literal supercomputers to crunch the numbers and save every last gram of fuel by taking some strange trajectories like going to Mars by flying past Venus once and Earth twice.
 

To get a crewed mission to Mars wouldn’t really be that much easier starting from the Moon, unless you could make fuel on the Lunar surface and fuel up the Mars ship in orbit rather than trying to launch it full of fuel on a truly huge rocket from Earth. That would require a substantial mining and fuel production colony on the Moon though, which isn’t likely any time soon.

[Scotius]

Let's not forget that unlike Kerbals, human astronauts require living space, constant supply of food, water and oxygen. All of it cuts into dV budget. To get the same results you need to take more fuel.

[Nuke]

14 hours ago, jimmymcgoochie said:

In KSP, the solar system is about 1/10th the size of the real Solar system and orbital velocities are considerably slower than in reality. Delta-V is cheap as a result because you can bring more fuel along pretty easily, meaning you can quite literally judge your transfer burns by eye and it has a good chance of still working despite not being the most efficient way possible of getting there.

In real space projects, delta-V is expensive (fuel is heavy and heavy makes you slower so you need more fuel for the same delta-V which means more weight, etc. etc.) but there are literal supercomputers to crunch the numbers and save every last gram of fuel by taking some strange trajectories like going to Mars by flying past Venus once and Earth twice.
 

To get a crewed mission to Mars wouldn’t really be that much easier starting from the Moon, unless you could make fuel on the Lunar surface and fuel up the Mars ship in orbit rather than trying to launch it full of fuel on a truly huge rocket from Earth. That would require a substantial mining and fuel production colony on the Moon though, which isn’t likely any time soon.

im rather astonished by some of the orbit-fu nasa and others can pull off for deep space missions. 

Edited May 11, 2020 by Nuke

[K^2]

3 hours ago, Nuke said:

im rather astonished by some of the orbit-fu nasa and others can pull off for deep space missions. 

Don't underestimate the power of the brute force methods. There are some wild trajectories out there, but you rarely find something good near these, and if you did have such a mission profile, one little error would mean a total loss. So all of the practical missions are found in fairly large "valleys" of reasonable solutions. Once you constraint where you're coming from, where you're going, and how many transfers you want to make, there are not that many initial conditions you have to try before you can just gradient-descent the rest of the way. Some general ideas, like Earth-Earth fly-by used by Rosetta sound silly until you do the math, and somebody must have had a stroke of genius to suggest them initially, but once you know the kinds of "points of interest" you might encounter, iterating through all the possible combinations doesn't really take all that much computing power.

The hard part is actually coming up with a "map" of the Solar system that lets you integrate motion precisely enough that your answers aren't total nonsense. Computing optimization problem for one ship, even with all the iterations and false starts that any search will have to do, is nothing compared to building up the map of every single object of any significance, as well as reconstructing average contribution from all the other orbital junk.

And the truly amazing thing about all of this is that NASA is sharing their data via convenient interface. JPL Horizons provides positions of the Sun, planets, over 200 moons and planetary satellites, thousands of comets, and nearly a million asteroids. With this data, you can actually plan missions on your home computer, provided you have solid enough grasp of numerical methods or have software that handles that, of course.

Once you have a mission plan, you still have to execute on it, of course, and that's another matter entirely. Space telemetry is hard. I'm sure a lot of otherwise good mission plans end up on the cutting floor because they require too much precision in execution. A mission to "maybe make a crater on a moon of Jupiter, or maybe miss it entirely," is not going to get a lot of backing even if you promise to cut delta-V requirements in half.