Fuel tank for mission plan bravo | Lunar lander engine and RCS | Forum

It may be the case that we can choose appropriate storage pressures for the two propellants such that the tanks can be equal in size (which would be nice from a fabrication simplicity point of view), but certainly we shouldn't count on it.  The renders showing equally sized tanks should very much be considered "conceptual renders", showing the overall concept and relative position of everything but maybe not having the scale quite right everywhere.

Things are much worse than I've suggested in the past, apparently I made a few stupid mistakes in one of my spreadsheets.

Some calculations for LOX/H2:

Let's assume our lander, with astronaut, has an all up mass of about 500 kg.  To match the Apollo LM's delta-v we want 2220 m/s for ascent and 2470 m/s for descent – a total of 4690 m/s.  LOX/H2 has an Isp (vac) of about 450, so by the rocket equation the total mass with fuel needs to be 1448 kg (meaning 948 kg of propellant).

Now, if the CM has a mass of 1500 kg with astronaut, the MEM 250 kg (probably quite an overestimate) and the LMM a (dry) mass of 750 kg (not sure how I feel about this figure), the total mass we need to do lunar escape on is 2500 kg (CM+MEM+LMM).  The delta-v for escape is around 700 m/s (this is the figure on Gary Snyder's webpage and I've verified that is the difference between circular orbit velocity and escape velocity at 100km above lunar surface – the higher figure of 1000 m/s taken from Apollo was apparently just them trying to get home faster), so by the rocket equation the total mass of this stack plus fuel is 2930 kg.

We need to do TLI and lunar capture on the 2930 kg above plus the 1448 kg of the lander, 4378 kg in total.  TLI delta v is about 3100 m/s and lunar capture is roughly 1000 m/s (I'll try to get a more accurate figure for this) for a total delta-v of 4100 m/s.  The rocket equation gives a total overall mass of 11,094 kg.

The Falcon 9 LEO capacity is 10,450 kg.

We need to lose a bit more than 600 kg (or more if 750 kg for the LMM is an underestimate).

Not only that, it looks like we will need to use LOX/H2 (LOX/LCH4 or LOX/RP-1 will be even heavier) if we want to have a chance of fitting our whole stack on the Falcon 9, and we're going to need a good engine (450s is a high end Isp for LOX/H2, a subpar engine will do worse).  The only way a less than ideal LOX/H2 engine or another fuel engine is going to make this work is if we find another launch option.

Anyway, continuing with the calculations: we're in for a grand total of about 8000 kg of propellant all up.  I have read that the optimal LOX:H2 ratio is 6:1, although brmj has contested this so we may need to look for more sources.  If 6:1 is good, then that comes out to 6857 kg of LOX and 1143 kg of H2.

The Air Liquide gas encyclopedia suggests that these masses of those liquids will occupy 5930 L and 15933 L, respectively (hydrogen has a really lousy density, which is why I was so keen on methane).  Total propellant volume is about 22000 L, meaning our 4 cylindrical tanks will need to hold 5500 L each – more than double their current size.

Now, it may be that we can get the tanks smaller than that (I don't understand supercriticality of liquids at all but it may play an important role here), but this is at least an upper bound.  Note that a single cylindrical tank 2.5 m in diameter could have a volume of 22000 L if only 1.12 m tall.

For comparison, LOX/LCH4 has a vacuum Isp of around 350, so we would need 450/350 times as much mass of propellant, for 10285 kg (exceeding Falcon 9 just in fuel!).  But the optimal LOX:LCH4 ratio is something like 3.5:1, so we would need 8000 kg of LOX and 2285 kg of LCH4, requiring volumes of 6919 L and 5333 L, respectively.  Total propellant volume is about 12300 L, a bit more than half of the volume for the hydrogen option.  Each cylindrical tank would need to hold 3075 L, which is not too much more than they can hold at their current size.  Alas, the methane just weighs too much for a Falcon 9 (with our current vehicle masses).  The Falcon 9 heavy could lift a methane option, but it would cost $90 million instead of $35 milliion, and I don't know if it is man rated.

Also note that all of the above rests on us carrying just enough fuel for the plan.  If something went wrong and we had to make an unscheduled burn we'd be screwed.

Bottom line: we need to shed as much mass as we can if we want to fit on a Falcon 9 or be able to consider easier fuels than hydrogen.  It is time for drastic mass reduction ideas!

What about a 3 legged lander?

Post edited 2:56 pm – January 11, 2010 by Luke Maurits

• Going with a single, cylindrical tank will reduce tank wall materials substantially.

This seems like a perfectly sensible thing to consider.  The tank would be nowhere near the scale of the Saturn V tanks which held the very same propellants, so we know it can be done, structurally speaking.

• We could put the LM in front of the CM to avoid a big, nasty support frame like the current design calls for. We could use lighter weight materials, like titaniam, extensively.

I think this is the most promising idea in your list.  The original motivation for having the LM at the back was to keep the whole stack aeordynamic.  However, if we are launching inside a Falcon 9 fairing then aerodynamics can be damned.  This approach removes the mass associated with the support structure, removes entirely the need for a Lunar Mission Module which is distinct from the Propulsion Module (a huge simplification!) and makes the transfer from the CM to the lander a lot shorter and thus safer.  This is an absolute winner (although it rules out the plan where the LM's engine does everything because we'd have to route fuel from a tank behind the CM to a lander infront of it).

EDIT: Although, it does leave the lander unguarded against micrometeoroids during the trip to the moon…

• We could make the pressure vessel inside of the cone out of a fabric, stretched over a frame for mounting things to.

The big problems I can think of with this is that it means all the radiation shielding needs to come from the cone (which is a larger structure overall) rather than some coming from the pressure vessel, and also that insulating the pressure vessel from the cone becomes even more vital.

• We could head back towards an unpressurized cabin.

This introduced so many headaches the last time we tried it I think it should be a last resort.

• We could have the only rcs systems be on the lander and cm.

This would probably work well enough, especially if we have 4 engines in the PM to add extra steering.

• We could look into scaling down the CM a little bit.

In what way?  I think the intent has always been for it to be as small as possible subject to it holding the things it needs to.  What could we take out and where could we put it?

• We could cut down on the lander's electronics and fly it with more pilot intervention.

I'm actually hesitant to do this.  Electronics and computers are so light and this is one of the biggest advantages we have over Apollo.  Also, software is the one thing that we can freely produce on a large scale and at high quality just by working over the internet.  It would be a shame not to leverage these two things the fullest by having a really intelligent lander that can practically fly itself.  This also makes training astronauts a lot easier.

• We could replace the nitrogen in the atmosphere with a lighter inert gas, such as helium.

This sounds perfectly sensible.  Are there any biological consequnces to this?  I presume this has been done before in diving/submarine situations?

• We could look into whether or not solar panels would be a better choice for the main part of our stack than the fuel cells.

This is worth doing, it's been a while since I looked at the fuel cell figures, I forget how much of their methanol/water mix they consume.

• We could replace the window with a camera.

I don't know how much mass this would save – but it would simplify construction somewhat, I think. EDIT: Although it would leave the astronaut "blind" in the event of a camera/power/computer failure.

Luke Maurits said:If we can get ou…CM down to 1000 kg (Mercury was 1118 kg so this may be possible)…

I am actually becoming increasingly convinced we could do this.  Encyclopedia Astronautica has a break down of the 1118 kg Mercury mass into individual subsystems.  I've put this information into a spreadsheet and began replacing their values with what I think we can achieve.  The savings are considerable.  E.g. Mercury's navigation system had a mass of 40 kg – but Crossbow's fanciest IMU has a mass of around 1.6 kg, so our total navigation masses end up being on the order of 5 kg.  Mercury's power system had a mass of 80 kg (it just used batteries), but the Ultracell fuel cells we have been considering have a mass of 1.6 kg each, with 24 hours of fuel having 1.2 kg.  Even if we use many of these for redundancy/extra power, we are not going to exceeed 20 kg.

After working through the entire list and making what seem like reasonable adjustments where I can but leaving the structure mass as it is (for one thing I have no idea how much lighter a Mercury could be built today and for another I suspect we will want a slightly larger capsule and I don't know by how much we'll have to grow it) the mass is down to just over 800 kg.  If the combined effects of the structure getting lighter due to modern materials and the structure getting heavier due to being a little larger add up to less than +200 kg overall then we can hit 1000 kg.

At any rate, it seems clear that the 1500 kg figure we had been using until now is too high.  We should use 1250 kg as an absolute expected maximum and remain optimistic about achieving 1000 kg.

Luke Maurits said:

Note that a single cylindrical tank 2.5 m in diameter could have a volume of 22000 L if only 1.12 m tall.

Such a tank made out of 1cm thick aluminium walls (which would surely be thick enough, to my non-engineer's intuition?) would, by my reckoning, have a mass of 502.6 kg (not including the separation bulkhead for the fuel and oxidiser).  Of course we'd need insulation as well, which would push things up a little.  This means to hit our dry mass target of 750 kg we may only have around 200 kg of mass to spare for the structure and the engines.  Of course, the 22000 L capacity was calculated based on a 1500 kg CM which I now believe we can beat.

Putting an actual aluminium skin around the LMM seems a little wasteful if it is itself mostly just a big aluminium tank.  Maybe we could get away with providing solar shielding and a little extra micrometeoroid shielding by wrapping the frame in some kind of tough, reflective foil?