Tuesday, August 30, 2011

EML1 Buildup

Today's space launch market is used to place satellites - commercial, scientific and military - into orbit, with the majority going to the geostationary orbit. In all such cases, the launch vehicle does not perform the final maneuver to circularize the orbit. The satellite is dropped off and circularizes its own orbit using on-board propellant. This is a significant delta-v change of about 1.6 km/s, and the remaining fuel is used to maintain the orbit, usually for 25 years or more.

Launch to Geostationary Transfer Orbit, circularize using on-board propellant. This is the standard model for how satellites are deployed into space. It is a mature process which has served us well for decades. However, when planning an exploration architecture, it has always been treated as irrelevant.

Here is a list of some current (and one near future) launch vehicles, their listed throw mass to GTO and the calculated mass that can be placed into the first Earth-Moon Lagrange point using a 312 second specific impulse storable propellant thruster (GTO to EML1 delta-v is 1.27 km/s).

Launch vehicleMass to GTOMass to EML1
Falcon 94680 kg3090 kg
H-IIB 3048000 kg5282 kg
Long March 3B/E5500 kg3632 kg
Proton6360 kg4199 kg
Atlas V 5518700 kg5745 kg
Ariane 5ECA10050 kg6636 kg
Delta IV-H12980 kg8571 kg
Falcon Heavy19000 kg12546 kg

What should be obvious is that there is quite a healthy international stable of launch vehicle providers, and they're all geared up for sending payloads to GTO. What is perhaps not obvious is that by going from GTO to EML1 I am seriously cheating myself. I don't mind because throwing to a lunar transfer orbit is something all of these vehicles can also do and, in all cases, the subsequent transfer to EML1 will be less than a transfer from GTO. As such, we can accept the numbers above as accurate, even if they are overly conservative.

So what does this mean? Suppose we want to land a payload on the surface of the Moon. One option is to simply pick the biggest one of these rockets and fly it directly to lunar orbit and start our descent. The total delta-v for such a mission is likely to be about 3.2 km/s, which means we can land a maximum of 6676 kg.

Suppose, instead, we fly to EML1 and pick up fuel. The table above indicates we can put a maximum of 12546 kg to EML1, and the delta-v from EML1 to the lunar surface is 2.52 km/s, so we need 16043 kg of fuel to make the trip. Because we're using storable propellants, this can be delivered over a long time using whichever provider offers the best price, or over a short time by engaging as many providers as become available.

Although this is just a rough analysis, it shows that we can land twice as big payloads by building up EML1 with propellant, without the need for any new launch vehicles, new technologies or even new ways of doing business, and we could start doing it right now.

Saturday, August 13, 2011

Dreaming About NASA Mismanagement



There's so many things packed up in this clip. For a start, the House isn't trying to cut the James Web Space Telescope (JWST) because "we don't have the money" or to save money or for any budgetary reason what-so-ever. The House is trying to cut JWST because the Government Accountability Office reported that NASA, and the contractor, have been mismanaging this program. They reported this three different times and required reports on what NASA was going to do about it - NASA didn't provide those reports. The House even said that the reason they were looking to cut JWST was to send a message that ignoring oversight will not be tolerated.

Does that mean the JWST isn't important? No.. no-one is saying that. Everyone agrees that JWST is important and that it will give results of significant scientific discoveries should it ever be completed and launched.. but when will that be? Within a two week period - after the House suggested cutting the budget - the program managers said 2020 or 2018 - neither answer was given in writing. Both answers were contingent upon an increase in their budget.. there's a word we use for declining to increase the budget of mismanaged projects: smart.

So is that the end of JWST? In the minds of NASA-can-do-no-wrong advocates, yes. They immediately declare that you're just not throwing enough money at the problem. It goes something like this: Oh, Hubble was massively overbudget and even broken when it launched. If we hadn't thrown more money at the problem we wouldn't even know about [insert discovery of cosmic significance here].

Meanwhile, the cosmologists are going around saying that the JWST is "essentially complete" or that "we've already built it". This isn't just the sulk cost fallacy, they actually think the JWST is ready and Congress is pulling the rug out from under them. This isn't the case at all, and not even NASA is making this claim. I've been suggesting that, if this were true, people who really want to see JWST fly should be calling for a firm fixed price contract - where the contractor covers the cost overruns, and NASA has less opportunity to screw things up - which has been proven time and time again to result in projects that are completed on-time and under budget.

Failing to mention any of this, Tyson then goes off into one of his standard rants. Oh, we've stopped dreaming. We don't look up. We've turned inwards. Can you imagine why? Hint: it has something to do with NASA mismanagement.

Back in the 1960s people dreamed of going to the Moon. Guess what? NASA went to the Moon. Was NASA not grossly mismanaged back then too? Of course they were, but they were given the mandate to "waste anything but time" and that is one thing government does well: waste.

What did people dream about in the 1970s? Space settlement. These dreams became plans, that wasn't the problem. All the engineering analysis at the time indicated that NASA could do it, so what happened? The plans called for cheap access to space and that requires the opposite of government: efficiency.

Instead, NASA became a government agency focused on "international cooperation", with first the Shuttle-Mir program and later the International Space Station, and while I'm sure there was plenty of people out there dreaming about more cooperation between nations, it had little to do with looking up.

Saturday, August 06, 2011

We've Already Got Propellant Depots

The solution to so many space logistics problems is: use a bigger rocket. Propellant depots allow you to add another solution: use more rockets.

Now, for some reason, many people who are advocates of propellant depots object to just using existing space storable propellants because that would mean you'd need to launch more mass than if you used cryogenic propellants. Well, so what? More launches - that's a good thing!

We don't need technology development to make propellant depots work. They already work.. we already have one in orbit!

Suppose you want to send 100 tons to Mars transfer orbit. You need either 236 tons of storable propellant or 144 tons of cryogenic propellant (and that's being overly generous to cryogenics). Instead of 5 Falcon Heavy launches you now only need 3. So what? How much is that worth?

So, ya know, NASA has selected companies to study storing cryogenic propellants in space.. and that's great. Technology development, in general, is fantastic and it makes things better in the future. Unfortunately I'm already hearing people say "woohoo! Now we'll have propellant depots and we won't have to waste $38 billion on a heavy lift vehicle to no-where!". Well, no. We already have propellant depots and we already don't need heavy lift to go beyond LEO.

There is, however, a few things that we are in desperate need of.... the political will to go, anywhere, on the government side, and an outspoken willingness to go it alone, if necessary, on the commercial side.

Friday, August 05, 2011

All we need is a really long tether!

I've written previously about non-rotating artificial gravity in Earth orbit. After recently watching this stinker I broke out the code I use to figure out gravity gradient effects. Surprisingly, this seems pretty good:




AltitudeMassGravity
LEO Station300 km273 tons0.999 g
GEO Station35786 km30 tons0.38 g


Of course, this is a much longer tether than in the film.. but hey, Danny Baldwin is in it - he doesn't make good movies. Anyway, the high station could be an ISS-style module with airlock and docking ports for satellite servicing vehicles. The low station would be a true permanently inhabited facility where people can live for years at a time without fear of bone mass deterioration or the other negative effects of zero gravity. To maximize space we may be tempted to use inflatable Bigelow modules, but we have to consider how they will behave in full gravity.

The only sticking point left is radiation. On the LEO station crews have much less exposure to cosmic radiation thanks to the Earth's magnetic field, however they receive just as much from flying through the South Atlantic Anomaly. As a result, radiation on the GEO station would be 2.19 times as high during solar maximum and 6.568 times as high during solar minimum. If that seems confusing, just remember that the Sun's magnetic field provides most of our protection against cosmic radiation, and it does that more at maximum than at minimum.

One solution may be minimagnetospheres but, again, technology developed for zero-g rarely works unmodified in full gravity. The best solution may simply be appropriate mass. The requirement that the low station be 9.1 times more massive than the high station means that both will have to grow simultaneously but getting mass from LEO to GEO is pretty easy when you have a tether joining the two altitudes.

You may be asking: how plausible is this? Or even: isn't this just the Space Elevator? I estimate it is at least two orders of magnitude easier to do than a space elevator and would only require (vast amounts of) existing tether materials. The cost is most likely dominated by launch costs that should be around $800M in a few years time.