Tuesday, December 26, 2017

All I Want For Christmas Is a Falcon Heavy Moon Mission

Trump's first space policy directive makes it official that NASA should go to the Moon first, instead of Mars.  Of course, what it really does is continue to pay lip service to the giant pork barrel that is the Space Launch System and the Orion Multi-Purpose Crew Vehicle.

NASA is still slogging away on the planning for the Deep Space Gateway, which is... closer to the Moon than the Earth, but really doesn't do much about getting to the lunar surface, to say nothing of doing something on the lunar surface once we get there.

A big chunk of the problem here is that Orion simply isn't suited for anything other than being a transfer vehicle where you can spend a couple of weeks beyond Earth orbit without dying.  Even if you get use SLS to launch it to trans-lunar injection (TLI), it doesn't have enough delta-v to insert into low lunar orbit (TEI) and then make it back to trans-earth injection (TEI).  It can just barely make it into one of the non-Keplerian lunar orbits like a distant retrograde orbit or a near-rectilinear halo orbit.  Not surprisingly, those are the main candidates for where to put the DSG.

So, to sum up:  the SLS/Orion gravy train continues.  We've currently spent $11.9B on SLS, with another $7.2B expected through 2021.  Orion has consumed $12.5B through 2016, with another $4.5B expected through 2021.  By 2021, we can expect to have flown two crewed missions, one a lunar flyby, and one delivering the first chunk of the DSG (the power and propulsion element) to lunar orbit.

Two crewed missions.  For $36 billion.

It doesn't matter whether the "goal" is the Moon or Mars, because the "goal" is a fantasy.  SLS and Orion suck up every bit of the human spaceflight effort that isn't devoted to the international space station, so there's nothing for landers, or habitation, or lunar resource utilization.

So, is all lost?  For NASA, maybe.  For the private folks, maybe not.

We have three possible private undertakings that could at least replace the SLS, and one (well, maybe one-and-a-half) that could take over from Orion:
  • SpaceX is about to launch Falcon Heavy.
  • SpaceX is also designing the Big Fucking Rocket and Big Fucking Spaceship (BFR/BFS).
  • Blue Origin is working on New Glenn, and then New Armstrong.
  • SpaceX has the Dragon 2 crewed spacecraft for use in Low Earth Orbit (LEO), which might be pressed into service for deep space with a few modifications.
If the BFR/BFS system works, it's a slam-dunk replacement for everything.  Many analyses of this have been done elsewhere, and it's generally awesome, but we really don't have a clue how feasible it is yet.  Given SpaceX's track record, it'll probably work fine, but it'll be 5 years late.  That would put it into service in 2027.

Blue Origin's schedule is somewhat behind SpaceX's, especially for New Armstrong, and their crewed spacecraft efforts so far have been concentrated on the sub-orbital New Shepherd.  My guess is that BO is unlikely to have a lunar-capable system much before 2027, either.

As it stands, neither of these systems is likely to halt the juggernaut that is SLS/Orion, which will so discredit the NASA human exploration program that it probably won't recover.

So what I'd like to do is explore what can be done with Falcon Heavy.

FH is not a heavy enough system to get to the Moon in one go.  SpaceX claims it'll be able to put 63.8 tonnes into low-earth orbit if all 3 cores are expended, but by my model only puts about 18 t into TLI.  That 18 t would have to carry enough propellant to get into low lunar orbit (LLO), let the crew do whatever they're going to do, and then do a TEI burn to get back to Earth.  For comparison, Orion is about 26 t, and even it can't to LLO and back.

So we're going to have to do multiple launches, and rendezvous with stuff in LEO and LLO.  Let's first get a crew to LLO.

Here's a sample mission to put a crew in LLO, using two Falcon Heavy launches.  I'm going to try to do as much as possible of this with existing SpaceX spacecraft and stages, because modifying space hardware is difficult, time-consuming, and expensive, as well as not being what SpaceX wants to do next.  Given that, this is a long shot, but an interesting long shot that could be useful in some circumstances.

There are four modules associated with this mission:

  1. A fairly standard Dragon 2, with a crew of 3.  Payload for this D2 may be a little lighter than for a normal D2.
  2. A second "support" D2.  This is a standard D2 frame, but the guts have been reconfigured to allow it to act as extra living space, provide extra power, deep space communications, modest environmental control and life support systems (ECLSS) capacity, and various pieces of equipment that are required for longer-duration flights.  (A toilet is an important thing here.)  At the same time, some things aren't required:  a heat shield, the SuperDraco engines, some of the ECLSS equipment (to the extent that the D2 Crew has some as well), etc.
  3. A "TLI stage".  The current Falcon Heavy doesn't have enough oomph to launch something, then push it and something that it rendezvoused with into trans-lunar injection.  One possibility here would be to refuel the FH second stage, but refueling technology with liquid oxygen is tricky and hard to develop.  (This is something that is being developed for BFR/BFS, but is unlikely to be developed for FH.)  So, instead, I've assumed that we can launch a chopped-down version of the existing FH S2 as a payload of the FH itself.  Usually, making shorter propellant tanks isn't a huge modification to an existing vehicle.  However, there are some other challenges associated with this.
  4. A "lunar maneuvering stage" (LMS).  Once you're in the lunar transfer orbit, cryogenic propellants like LOX become very difficult to use, because they boil off after more than a few hours.  Since it takes 3 days to get to lunar orbit, execute the LOI burn to enter LLO, do the mission for a few days, and then do the TEI burn to get back to an earth transfer orbit to go home, these two maneuvers need to be done with "storable" propellants, usually a hydrazine fuel and a nitrogen tetroxide oxidizer.  SpaceX has two low- to moderate-thrust engines, the Draco and SuperDraco, that use these propellants, but they're not quite efficient enough to do the trick.  For that reason I've assumed that the LMS is based on a space shuttle Orbital Maneuvering Engine, which is itself a variety of the same engine used for this purpose on Apollo and the Space Shuttle, the AJ10.
The trick is to figure out the mass and delta-v budgets for all of these.  Here's what I got:

Module NameDry MassPayload MassPropellant MassWet MassEngineSpecific ImpulseDelta-v NeededDelta-v AvailableLaunch
Dragon 2 Crew6,4008831,0008,283Draco300-430FH #1
Dragon 2 Support4,2002,6485007,348Draco300-207FH #2
LMS1,000-14,58515,585AJ103161,9501,950FH #1
TLI Stage3,000-52,97055,970Merlin 1D3483,1903,190FH #2
FH #123,86711.2
FH #263,31814.4

The payload masses (i.e. the amount of mass needed for humans and consumables, along with mass above and beyond the D2 base mass aren't quite SWAGs, but they're close.  Note also that I'm guessing that more than 2 tonnes can be gutted from the support D2, when we remove heat shields, SuperDraco thrusters we don't need, parachutes, etc.

Note that the order of these is a bit odd.  The idea here is that the things at the bottom of list push all the things at the top.  So any change to the LMS or D2 masses will make the TLI stage need more propellant to meet its delta-v budget, which will likely make it too heavy to launch with anything else.

The delta-v budget (i.e., the change in velocity each module must produce to fulfill its mission), is ultimately what controls the size of the TLI and LMS systems.  More delta-v takes more propellant, which requires more propellant to accelerate the extra mass, which makes everything still more massive, which requires more propellant, and so on.  Rocket delta-v is determined by the ratio of the "wet mass" (the mass with propellant) to the "dry mass" (the mass when all the propellant for a particular stage has been expended), and the exit velocity of the rocket exhaust, which is the "specific impulse" (oddly measured in seconds, for mostly historical reasons) times Earth's gravitational acceleration.  For the math geeks:

Δv = 9.8Ispln(Mwet/Mdry)

Note:  "ln" is the natural logarithm function, and the "Mwet/Mdry" term is called the "mass ratio". The natural log falls off quite sharply as you reduce the mass ratio close to 1 (which would mean that you had no propellant), so keeping the payload and parts of the spacecraft that aren't propellant as light as possible is incredibly important, as it keeping the propellant mass of stages further up the stack (which just looks like payload to the currently thrusting stage).

Here's what the mission profile looks like:
  1. FH launch #1 takes the D2 Crew, mounted on top of the LMS, to LEO, where the crew waits for the second launch.  Note that the total payload is a little less than 24 tonnes, which is easy for an FH.  I'd expect that the FH's 2 boosters and 1 core can all be recovered and reused from this launch.
  2. FH launch #2 takes the D2 Support, mounted on top of the TLI stage, to LEO.  This mass is a bit more than 63 tonnes, which is right on the hairy edge of what an expendable FH can take to orbit.  This is a very near thing, and even small changes in the payload or structural mass of the two D2's will probably blow the budget and require a third launch.  For various practical reasons which will be explained, this would be a big problem.  One would also think that this should be launched first, since there's no point in risking the crew if the TLI/D2 Support launch fails.  Again, there's a reason for this.
  3. The TLI Stage/D2 Support and the LMS/D2 Crew rendezvous with each other.  The two D2's are equipped with docking adapters, and dock nose-to-nose.  When this is done, the LMS is (facing backward) at the "front" of the 4-module system, followed by the D2 Crew (also facing backward), the D2 Support (facing forward), and finally, the TLI stage at the back.
  4. Since the D2 Crew module is currently facing backward, the crew has to transfer to the D2 Support module before the TLI burn.  Otherwise, they'd have a significant acceleration trying to pull them out of their seats.
  5. The TLI stage fires to insert the stack into a lunar transfer orbit.  This requires about 3190 meters per second of delta-v.  Once the TLI burn is complete, the TLI module is jettisoned.
  6. On the way to the Moon, the crew uses both D2 modules as their living space, as they do while they're in LLO.
  7. When it's time to enter LLO, the three remaining modules slow down by using the LMS to do the Lunar Orbit Insertion burn.  LOI takes about 940 m/s of delta-v.
  8. Now that the crew is in LLO, they perform their mission.  Presumably, this will involve rendezvousing with some pre-positioned chunk of hardware to do stuff, since everything we're talked about so far is  only there to get them to and from LLO, not do anything there.  How you launch that hardware is an exercise left for another day.
  9. When it's time to come home, the LMS does a second burn (the "TEI burn") to inject the spacecraft into an Earth transfer orbit.  This requires about 1010 m/s of delta-v.  After this is complete, the LMS is discarded.
  10. The only thing left now are the two D2's, mated nose to nose.  As they approach Earth, the crew climbs into the D2 Crew, seals the hatches, and discards the D2 Support module.  It'll either burn up in Earth's atmosphere because it doesn't have a heat shield, or a small maneuver from its thrusters can launch it out into deep space so it doesn't clutter up Earth orbit.
  11. The D2 Crew jettisons the Dragon 2 trunk (which has most of the solar panels on it), then turns to reenter Earth's atmosphere.  Note that, just like Apollo, it doesn't try to enter Earth orbit.  That takes too much delta-v.  So it just barrels straight into the atmosphere, using it to slow down.  (The D2 heat shield was designed for this in mind.)
  12. The D2 Crew lands, probably using parachutes.
This whole sequence has been designed to use as much existing SpaceX hardware as possible.  But, since there are always mission-specific tasks required of any space systems design, some things are not perfect.  Here's the list of the things that are the biggest issues.
  1. First and foremost is a little oddity called "dwell time" or "maximum mission duration", and it applies to the TLI stage.  Simply put, it's the amount of time the stage can live in space and still fire its engine.  The FH S2 uses kerosene and liquid oxygen as propellant.  While the kerosene isn't much of an issue, the LOX literally boils as it warms up.  This, along with how long you can maintain power to the stage (it has no solar panels, only batteries), and how long you can keep enough gaseous helium to pressurize the stage, means that all the burns the stage needs to do need to be done quickly.  That's why we launch the FH with the TLI second.

    Even so, it takes a while to rendezvous, dock with the other stack, do checkout, and line up for the TLI burn.  Right now, the longest the FH S2 (which is identical to the second stage on the Falcon 9, BTW) can manage is about 45 minutes.  My guess is that that needs to be at least 4 earth orbits, or about 6 hours.  That's a hefty design change.

    To do that, changes need to be made to better insulate the LOX tanks (which in turn keeps the helium tanks, which are inside the LOX tanks, cold) and either to add more batteries to the stage (which is heavy), or to get it to deploy temporary solar panels for power. Remember also that the TLI stage is shorter than the FH S2, which also requires testing. All-in-all, it's a fair amount of work, even though we know that the Merlin 1D engine is well tested and capable of doing on-orbit restarts.

    An obvious question here is why we don't just use storable propellants, as we do in the LMS. The answer has to do with specific impulse. Storable propellants don't have the necessary exhaust velocity to keep the TLI stage at a size that's capable of being launched with the D2 Support module on an FH. However, if we decide we want to put everything together in 3 launches instead of two, a storable TLI stage might be a better option. However, rendezvousing three separate spacecraft is something that we have no experience doing.
  2. The other problem with the LOX on the TLI stage is that it involves filling the stage just before launch. Since the FH would consider this stage "payload", there are no on-pad facilities to do this. They'd need to be added. That's time-consuming and expensive.
  3. SpaceX doesn't have any piece of hardware that's comparable to the LMS.  It has engines, the Draco and SuperDraco, that use storable propellants, but the SuperDraco is designed more for thrust than specific impulse, and the Draco is more of an attitude control thruster and doesn't have enough thrust.  In any case, it would require developing an entirely new stage.

    That's not the end of the world. The AJ10 is just about the best-understood engine in existence, and it's been used for many different applications, from the Apollo service module to the Space Shuttle Orbital Maneuvering System. Still, a stage is a stage, and it comes with a minimum amount of design and test effort to make it flight-ready. Considering that we'd be launching it with the D2 Crew module (which contains, you know, crew), getting it flight-rated is especially onerous.
  4. As I mentioned above, while the D2 Crew is (or will be) pretty much off-the-shelf, the D2 Support is going to be gutted to the studs and filled up with new stuff.  The D2 Crew doesn't have enough environmental support, power, or deep space communications to be used beyond Earth orbit as-is.  All of that stuff needs to got into the D2 Support.  Again, it's non-trivial work.  That said, working with a flight-proven spacecraft design is a huge step forward.  But mixing and matching equipment and payloads for both D2's so that we stay within our mass budget is tricky.
  5. While docking in LEO is well-understood, docking two pairs of modules would be a little hair-raising.  Long thing things have a high moment of inertia, which requires a hefty thruster system to control.  This further complicates the construction of the TLI stage and the LMS, and increases the possibility of a failed docking.
  6. Note that, at one point, the TLI module is firing while the D2 Crew module is backwards.  As it turns out, because the Merlin 1D wasn't designed for this type of orbital maneuvering, it actually has more thrust than we'd like.  By my calculations, gee forces between 1.2 and 1.8 g would be trying to pull the crew out of their seats, even if the Merlin was throttled down to minimum thrust.  Not only does this require having crew couches and controls in the D2 Support (which adds mass), but it also means that the D2 Crew module needs to be qualified to be accelerated ass-backward into the unknown.  Since this has probably never been tested, it probably doesn't work without some effort.
  7. There are currently no plans to use the FH as a crew-capable vehicle.  All D2 launches are currently planned only for the Falcon 9.  NASA is incredibly risk-averse, and launching a D2 Crew on an FH would require months of certification work.  Add on to that the fact that it would launch on top of the LMS and rendezvous with a stage filled with LOX and kerosene, and you'll have quite a time convincing NASA that this is adequately safe.
  8. While D2 spacecraft are considerably shorter than the fairings used for launching satellites on the F9 and FH, the combinations of the D2 and the TLI stage or LMS are somewhat longer than the usual fairing.  That requires a careful reexamination of the structural and aerodynamic loads that the new spacecraft would put on the launch vehicle.
  9. The current D2 solar panels are attached flat to the trunk.  Whether this will generate enough power to enable crew operations in lunar orbit is an unknown.  If more solar panels are needed, they'll have to be deployed after the TLI burn, which would put too much structural stress the solar panel mounts.  That's a bit of a no-no from a mission safety standpoint, because you don't want mission-critical stuff done after the checkout in LEO, in case you need to abort.
  10. And of course we haven't talked at all about why you'd send crews to LLO in the first place.  Just as the SLS/Orion/DSG is a system with no mission, there would also have to be a lunar lander system, launched on at least one, and probably more, FHes, or some other launch vehicle.  That's still more design work, more fabrication of new hardware, more testing, and more crewed certification.
Would SpaceX want to do this work?  Almost certainly not without major encouragement from NASA.  But if they get antsy about not having a presence on the Moon before the late 2020's (if, for example, the Chinese made a push to have a presence), counting on the SLS/Orion system is economic folly, and counting on the BFR/BFS or New Armstrong might be political folly.  In that case, cobbling together a system mostly out of SpaceX's leftovers might be the least of all evils.

The other possibility is that SLS/Orion suddenly falls out of favor.  It's increasingly more difficult to justify these programs as anything other than pork, with some of its erstwhile patrons getting more and more nervous about appearing to be stupid in public.  If the dam finally bursts, a lot of money would be freed up for NASA to develop real live spacecraft.  If NASA were to get out of the launcher business and wish to remain in the space hardware business (which they should--on both counts), FH would be an awfully attractive way to get started.  I doubt very much that NASA will wish to cede the entire spacecraft business to SpaceX and the BFS.  Given that, using the FH creatively might be pretty attractive.


redneck said...

I think you are missing a trick here. By dismissing the possibility of LOX transfer in orbit, you are adding a TLI stage and launching one of the FHs about 30 tons light. Developing the ability to transfer LOX would allow you to eliminate developing the TLI stage and save the mass of launching it. The second FH as pure tanker could deliver something in excess of 60 tons to the rendezvous. This gives at least 90 tons of fuel for the mission compared to your reference mission, which is sufficient for a TLI burn with the upper stage. If You want two D2s for the mission, they could be stacked on FH1 and launched with the crew. The TLI burn could then be eyeballs in instead of out. Saves on development of the inverted acceleration crew accommodations and vehicle mods plus re-certifications.

In short you can refuel on orbit, or do the NASA thing of developing several new components that may eventually become operational.

TheRadicalModerate said...


My main reason for rejecting refueling is that I think the mods to the S2 are more extensive to get it to work than doing the chopped TLI stage. But the eyeballs-in thing is a big deal--I like that a lot.

If you're not going to hard-dock your tanker stage to the target stage, you'd have to:

1) Execute a precision rendezvous with close to zero deviation along all six degrees of freedom for both vehicles. (This is why hard dock is so important--it nulls that stuff out by rigidly coupling the structures.)

2) Attach two refueling lines via boom between the two vehicles.

3) Execute simultaneous ullage burns on both spacecraft, with a control system capable of nulling out differences in acceleration across all six degrees of freedom for both vehicles).

4) Safely jettison the refueling lines.

5) Modify the F9 S2 to handle even longer coast times--precision rendezvous and refueling would take multiple orbits.

That's a lot of new tech. I suspect that it's way more than the chopped TLI stage (although the longer coast time is common to both methods and is a hard requirement).

A better solution would be figure out how to hard-dock, but you'd have to figure out how to do a sufficiently rigid docking between two S2s, either tail-to-tail (aka nozzle-to-nozzle with two MVacs), or nose-to-tail. Maybe you can deploy 3 rigid booms from the nose of the S2 tanker? It's obviously a pretty big mod to the payload attachment to get something like this to work.

OK, I'm intrigued. Just for grins and giggles, let's work this out assuming that the prop transfer is magically solved.

First, you cannot launch the LMS, D2 Crew, and the D2 Support on the same FH. Even if you figure out how to fix the nose-to-nose problem with the D2's (think Apollo S-IVB and the LEM for some idea), the stack is simply too long to launch on an FH. So the best you can do is launch the LMS + D2 Crew on FH #1 and keep its S2 mated, then launch the D2 Support on the second FH.

Second: D2 Crew + D2 Support + LMS wet mass + F9 S2 dry mass (4 t) = 34,220 kg. Mass ratio for 3190 m/s @ Isp= 348: 2.55. So we need 53 t of propellant to do the TLI burn.

Third: FH #1 launched expendably with the D2 Crew and LMS will arrive in LEO with 29.3 t of prop remaining in its S2. If you launch just the D2 Support of on a 2-booster-reusable FH #2, you wind up with 39.7 t of prop left over. So if you can get the FH #1 S2 to refuel the FH #2 S2, you're in business. That's interesting.

But now you have to deal with the refueling magic. See above.

OK, that's not bad. It also has your very nice property that the D2 Crew is now facing forward for both burns:

1) Launch FH #1 expendably, with LMS + D2 Crew. S2 stays attached and becomes the TLI stage.
2) Launch FH #2 2-booster-reusably, with D2 Support. S2 will be the tanker.
3) Soft rendezvous.
4) FH #2 jettisons "tanker" S2, leaving D2 Support free-flying.
5) Tanker S2 hard-docks nose-to-tail with FH #1 "TLI" S2.
6) D2 Support docks nose-to-nose with D2 Crew.
7) Ullage burn and fuel transfer. NASA crew safety people wear their brown trousers that day.
8) Undock FH #2 tanker.
9) Perform TLI burn with "TLI" S2. Crew stays in D2 Crew.
10) Jettison TLI S2.
11) Do LOI and TEI with LMS.

Big ticket problems:

a) Figure out the hard dock.
b) Re-plumb the fuel system to accommodate transfer.
c) Even longer coast time improvements. I'm guessing you now need 8 orbits (12 hours).
d) Crew safety is a nightmare during this.

I think I like it, though. Good idea.

redneck said...

I think we mainly disagree on the difficulty of developing orbital refueling and storage of LOX. I also think that the historical reason that orbital LOX transfer is not yet common is that it would render many of the NASA monstrosities obsolete. Ares I,V, and SLS would have been stillborn given the capability. That's more than enough Dirksens for real development.