Simply because the launch vehicle can only lift a certain amount at any time - and that size or mass will be the maximum module size - and the only way to get larger masses and structures is to bolt them together in the low Earth's orbit (LEO).
LEAMOR stabnds for Light Extended Apollo Mars Orbit Rendezvous.
|SLS can lift about 150 .. 300 tons (lbs) to LEO|
The maximum amount the latest and greatest (2015) launch vehicle (SLS) can lift is about 150 000 lbs and maybe later up to 300 000 lbs. So about 150 tons (lbs) will be our design module size. Later you can send two or more modules in a single launch .. or maybe even make the module a bit larger. But what ever it will be it must be sent in parts if it can not fit into the single launch limits. (And bolted together later if required).)
Here are the Mars Vehicle burn calculations for descent and ascent with masses. Safety factor 1.3 for propellants. (The Rocket Equation used).
- MV full 150 t (about max SLS, later maybe more)
- MV dry 27 t
- AS full 20 t
- AS dry 3.5 t (3500 lbs is not a large mass for such a stage .. send some material using robots also)
- DS dry 7 t (7000 lbs is not a large mass for such a stage .. send some material using robots also)
- Isp both 311 s
- Results delta-V: MV = 5.2 km/s and AS = 5.3 km/s (~ 1.3 x 4.1 km/s)
But why not use smaller structures to avoid any launch limits .. that is impossible because of the rocket equation (see this article) and the target vehicle's mass in the Low Mars Orbit. The unit that is required there is the Mars Vehicle (MV) and since it needs to make the delta-V 4.1 up and down to get the job done it requires propellants .. and after solving the problem with the rocket equation we will need a Mars lander in the Mars orbit which mass is about 150 000 lbs. So it kind of "happens" that the SLS just has exactly that lifting capacity that is required and later maybe more. So our MV's mass is 150 tons or maybe later more.
|Mars Vehicle (MV) about 150 000 lbs is similar to the Lunar Module LM with ascent and descent stages.|
We also need propellants for the Earth LEO departure stage (Trans Mars Injection TMI) and the Mars Orbit Insertion (MOI) stages. So we choose a general propellant-carco-rocket-etc. module size 150 t or so as follows.
|L-Modules - each about 150 000 lbs or so except the capsule - measures: diam. 5 m x length 11.75 m|
Now using this L-module we will build our final Mars spacecraft that will carry our cargo and astronauts to the Mars and back. Basically here is how we calculate everything (about).
- Simple propellant module = 140 t propellants + 10 t dry = full 150 t
- Rocket module with propellant = 130 t propellants + 20 dry = full 150 t
- Module with capsule = as above but allow 20 t for the capsule = full 150 t
If the SLS can later carry more .. either send several modules at one launch or maybe even put some more mass to some modules .. or some less to some .. so that the full launch capacity can be used. That will be the fine tuning of the mission.
|LEAMOR MV compared to Apollo LM|
(In the next part of this article series we will design the whole spacecraft from LEO to LMO).
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