Using Hohmann transfer orbit to get to Mars

[fatbutt]

Just been reading the BBC article about the Indian Mars probe and it highlighted the Hohmann transfer orbit to get to Mars which got me thinking: why can't you use that for manned flights to Mars by keeping the system unmanned for the majority of the 'orbits' until the very last one where you send up a short(er) range orbiter to get the crew to the Mars system in its last Earth flyby?

The Mars system carries lots of supplies, equipment, etc. but less fuel. The 'manning' system has more fuel to get up to speed and transfer the people but less food/ equipment.

Daft or has someone already thought of this?

BBC Article: http://www.bbc.co.uk/news/science-environment-2516...

Edited by fatbutt on Sunday 1st December 19:46

[Eric Mc]

Buzz Aldrin - who is a bit of an orbital mechanics guru - has worked out a cost effective system for getting to Mars which he calls the "Mars Shuttle".

[CrutyRammers]

You basically need a given amount of fuel to get a given amount of speed from a given mass, whether you do that in stages or in one go. To match orbits with the first bit, then boost that to mars, wouldn't save anything over docking the two bits at the begining, and sending the whole lot.

There are good reasons for sending up stuff in multiple launches and assembling it in orbit, but that's because getting up through the atmosphere will put a maximum practical limit on the size of rockets. But once in orbit, it's largely a question of total mass and total amount of thrust.

[CrutyRammers]

BTW, the Hohmann Transfer doesn't refer to the sequentially raising of the orbit used by the Indian probe - the article is misleading there. It's an eliptical transfer between two orbits, with one burn to leave the first orbit, and a second burn when you get to the second orbit to circularise again. But it relies on effectively an instantaneous, or at least short ( a few minutes) burn. If your engine hasn't got the power for that, you do what the Indians are doing - break the burn down into a number of shorter ones and do it in stages.

[fatbutt]

I was thinking less of fuel, more of the period of time the crew would be aboard.

[Eric Mc]

Unmanned probes have the luxury of taking the long way round. Human spacecraft have to shorten the journey as much as is practicable.

[fatbutt]

I got the impression from the BBC article that there were a series of orbits that gradually slingshotted the craft further out. If this was the case, what I'm saying is that you put the crew on for the last orbit so you only need provisions for that stage.

[MartG]

ash73 said:

Kerbal is the best way to learn about orbits, imho

[fatbutt]

Aye, I keep meaning to get around to installing Kerbal and having a go.

What I'm getting at is two vehicles, both say 50Te.

Vehicle 1: 90% mission equipment (inc. a food allocation for the crew), 10% fuel
Vehicle 2: 90% fuel, 10% crew support

Blast vehicle 1 off using the Hohmann orbit and have it loop around as often as needs be to get sufficient speed using minimal fuel. On its last pass by Earth, blast off vehicle 2 that is all about interception rather than mission longevity. Vehicle 2 carries the crew and minimal life support. Crew transfers from Vehicle 2 to Vehicle 1 at big speeds, maybe even join vehicle 2 to vehicle 1 so as to have more crew space. This way you have your vehicle(s) up to speed with minimal crew provisions using small enough launch vehicles instead of a big ass vehicle and a gazillion day voyage for the crew.

Just a thought.

[Eric Mc]

You might find this interesting -

http://en.wikipedia.org/wiki/Mars_cycler

[CrutyRammers]

fatbutt said:

Aye, I keep meaning to get around to installing Kerbal and having a go.

What I'm getting at is two vehicles, both say 50Te.

Vehicle 1: 90% mission equipment (inc. a food allocation for the crew), 10% fuel
Vehicle 2: 90% fuel, 10% crew support

Blast vehicle 1 off using the Hohmann orbit and have it loop around as often as needs be to get sufficient speed using minimal fuel. On its last pass by Earth, blast off vehicle 2 that is all about interception rather than mission longevity. Vehicle 2 carries the crew and minimal life support. Crew transfers from Vehicle 2 to Vehicle 1 at big speeds, maybe even join vehicle 2 to vehicle 1 so as to have more crew space. This way you have your vehicle(s) up to speed with minimal crew provisions using small enough launch vehicles instead of a big ass vehicle and a gazillion day voyage for the crew.

Just a thought.

...but it wouldn't save any time over sending both up into orbit, getting everything into one ship, then sending it straight to mars. Your vehicle 2 has to have the thrust to do that, anyway, so you gain nothing by docking and transfering en route. Plus the rendevous would be a great deal more difficult and dangerous, with no escape route. (If you miss and have to go around whilst in orbit around the earth, no biggy, just go round a few more orbits. If you miss when transfering out to Mars, you'd need to do a few more orbits of the sun to rendevous again...)

Then don't forget you need considerable thrust when you get to Mars to enter its orbit - and your vehicle 1 probably wont have the thrust to do it without taking a number of ever decreasing orbits, the reverse of what it did round the earth.

And yes, Kerbal is a great way to try these things out

[Einion Yrth]

CrutyRammers said:

Then don't forget you need considerable thrust when you get to Mars to enter its orbit -

To be fair aerobraking is an, admittedly scary, option at Mars.

[fatbutt]

I was assuming two 50Te units would be easier to launch than one 100Te unit given that 50Te appears to be Space X's upper payload limit.

[fatbutt]

Einion Yrth said:

To be fair aerobraking is an, admittedly scary, option at Mars.

I always think of '2010' the movie when I hear aerobraking

[fatbutt]

Eric Mc said:

You might find this interesting -

http://en.wikipedia.org/wiki/Mars_cycler

There you go, that's pretty much what I'm saying. I knew someone else would have thought of it...

[Einion Yrth]

fatbutt said:

There you go, that's pretty much what I'm saying. I knew someone else would have thought of it...

Then you massively confused matters by mentioning Mangalyaan, which is entirely different from the cycler concept.

[CrutyRammers]

Thing is, the thing that docks and undocks from the cycler will still need enough delta-v to get to mars, and match orbits when it gets there. I'm not sure what the cycler gives you, apart from more space for the astronauts during the transit. Which may be worth it from a human point of view, but certainly isn't saving anyone any fuel.

[Einion Yrth]

CrutyRammers said:

I'm not sure what the cycler gives you, apart from more space for the astronauts during the transit.

Radiation shielding

CrutyRammers said:

Which may be worth it from a human point of view, but certainly isn't saving anyone any fuel.

Except that you only have to accelerate it once, the shuttle vehicles can be much lighter.


ETA:-
Obviously it's pointless for "flags and footprints", it's part of "space to stay".

[CrutyRammers]

You don't get a "free ride" by docking with the cycler - we're talking about things on ballistic trajectories here. If the shuttle can dock with the cycler, then the shuttle has already attained the speed to get to mars on its own.

Ultimately, once in space, it takes X amount of energy to move Y amount of mass into a given orbit. Doesn't matter if you split it up into different parts; they're all meeting the same orbit, and hence the total energy is going to be the same*

• actually probably more for the cycler as you've got more "dead" mass such as engines and fuel tanks, and its orbit overshoots Mars, so the shuttle will actually have to accelerate more to dock with it than it would just to get to Mars alone, but anyway...

[Einion Yrth]

CrutyRammers said:

You don't get a "free ride" by docking with the cycler - we're talking about things on ballistic trajectories here. If the shuttle can dock with the cycler, then the shuttle has already attained the speed to get to mars on its own.

Ultimately, once in space, it takes X amount of energy to move Y amount of mass into a given orbit. Doesn't matter if you split it up into different parts; they're all meeting the same orbit, and hence the total energy is going to be the same*

• actually probably more for the cycler as you've got more "dead" mass such as engines and fuel tanks, and its orbit overshoots Mars, so the shuttle will actually have to accelerate more to dock with it than it would just to get to Mars alone, but anyway...

You use the cycler more than once, that's the point. Apart from a little fuel to correct course aberrations the cycler continues Earth-mars-earth-mars.... more or less ad infinitum, thus the mass of the cycler only has to be accelerated once. The living space and shielding can be used multiple times. Yes the shuttles have to be capable of the delta-v necessary to get themselves to Mars, but the really heavy stuff only has to be accelerated once.