He reiterated his support for SLS in the long run, but I don't think anybody believed him. If this goes forward, it's certainly the end of SLS Block 1, and likely the end of SLS completely. Opposition will be fierce, but this seems to be the beginning of the end for SLS. Outside of its contractors and its congressional patrons, few will mourn its passing.
But we have to get Orion to TLI using a commercial launcher for this to happen. This post will look at some ways of doing that.
The EM-1 Mission As-Is
To start with, let's look at the interesting parts of the current SLS Block 1 ICPS (the stage that would currently provide the delta-v for TLI) and Orion. Because parts of Orion get jettisoned at various times, this gets a little tricky. Here's a summary of the relevant information:Here's the current EM-1 mission profile, from this:
Note that these are the worst-case delta-v numbers. The min and max elapsed time numbers are from the "selected" mission durations, which vary based on time of year, lighting conditions, eclipses of Orion, and (I assume) missed orbital insertions. I'm going to assume the "min" numbers for analysis below.
Delta-v For Various Launchers
Finally, we can compute a variety of handy delta-v numbers for various launchers. There are three sets of numbers:- The first set are for a "reference" orbit. Launch providers all provide this number for a 200 x 200 km orbit with a 28.7 degree inclination. If you know the size and specific impulses of the various launch stages, and you know their advertised performance to reference, you can figure out how much delta-v is required for each launcher. Note that this number varies by quite a bit, because launchers with higher thrust generally have less gravity drag. This is especially pronounced with the D4H, which is pretty sluggish.
- The second set are a slightly elliptical 185 x 620 km LEO orbit. We'll use this for several of the scenarios we'll examine, because it's pretty much a radiation-free orbit that works out for some of our better candidate architectures. Note that we have two numbers here: the delta-v to get to the parking orbit, and then the delta-v from there to TLI. (The TLI numbers are derived by fitting the C3 (specific energy) and inclination changes of the Apollo launches until I could replicate its numbers, then applying the same altitude orbit and inclination for from the 185 x 620 x 28.7 orbit.
- Finally, there's this whopping huge 185 x 20,220 orbit at the end. This is used by some of our more poorly-performing options. Note that 20,200 is well into the second Van Allen Belt, and consequently would be unhealthy for crewed options--especially options which take multiple orbits to rendezvous. However, this is very close to the altitude of GPS satellites (although those have circular orbits and not this elliptical monstrosity), so we have an existence proof that the hardware ought to hold up OK.
Now that we have some numbers to play with, we have some basic questions to answer:
- Can we make any modifications to Orion?
- Is there any way to make do with one launch?
- What launchers can we use?
- What transfer stages can we use?
- EM-1 is an uncrewed mission. Do we need to design for a subsequent crewed mission?
- In a 2-launch mission, how long can we wait between launches?
- How do we dock the TLI stage and Orion in orbit?
What About Modifying Orion?
I'm going to rule this out, because the purpose of EM-1 is to test Orion as it is. Modifying it would defeat the purpose. Beyond that, getting Orion crew-certified has been hideous, and one can presume that any modifications would require still more hideous.
One thing to note: We can't modify Orion, but we may need to modify the OSA. For our purposes here, we'll assume that the OSA acts as the anchor for the ESM fairings (see here), but the fairings themselves, which are parallel to the air stream, don't change.
Is There a One-Launch Option That Isn't SLS?
The short answer is "no". That's a shame, because a one-launch option is cheaper and more straightforward.
There is one thing that needs to be considered before we dismiss this, though. If we can approximately double the propellant in the S2, there's a kinda skeevy mission profile where the S2 can't quite get to TLI, but there's enough prop margin in the Orion to do the rest. If this were possible, it would probably jump to the front of the line in terms of options. We shouldn't rule this out, but we should also remember that it's unlikely that SpaceX would be willing to do this without a huge incentive. (Getting awarded 5-10 crewed missions to NRHO would do it, I think.)
Launchers
If we're going BEO next year without an SLS, there are really only two other choices: We can use a Falcon Heavy in either its 3-stick-reusable, 2-stick-reusable, or expendable forms (FH3R, FH2R, or FHE), or we can use a Delta-IV Heavy (D4H).
A key factor here will be the availability of D4H cores. As a general rule, the lead time for these is about 36 months, and the pipeline is currently full with two missions for the DoD. However, ULA is already making noises that they can meet the demand if NASA needs them to. Whether that means that they think a National Reconnaissance Office mission can be bumped or re-hosted on some other launcher, or whether they think that they can actually spin up an extra 3 cores in the next 15-16 months, or whether they're just saying whatever they need to say and hoping that Bridenstine's study scuttles the whole idea, remains to be seen.
Assuming we need two launches for the mission, there are obviously 4 combinations of these launchers.
Transfer Stages
If you're going to make a mission out of a D4H and an FH, then obviously the two best candidates for TLI transfer stages are the FH stage 2 (FH S2) and the Delta Cryogenic Second Stage (DCSS), in its 5-meter-diameter form. (Note that the SLS ICPS is a very minor modification of the DCSS-5, and we should at least consider using it as a replacement for DCSS on the D4H launch.)
To that list we probably need to add the existing Centaur 3. I don't think Centaur 5 is ready in time, and ACES is still pretty much science fiction.
The Difference Between Launching a Transfer Stage As-Is, vs. Launching It As Payload
At first blush, it would seem obvious that you need a full, ready-to-go transfer stage when you get on-orbit. Since we don't have time to build up an on-orbit refueling architecture for the EM-1 mission, it would seem to rule out reusing the second stage that got you into orbit.
But that turns out to be wrong. We need to look at how much propellant gets used by the existing second stages if we launch them with no payload. It turns out that both the DCSS and FH S2 can arrive at some kind of parking orbit with enough prop to put an Orion into TLI. There are some wrinkles that are stage-specific, but either will do.
The Centaur 3 is another story, however. If we were to use it, it really would have to go to orbit as payload. We might or might not be able to provide an interstage to make it look more like a third stage than a payload, but it would arrive on-orbit fully fueled with hydrolox.
Crew Rating the Mission Architecture
Bridenstine swears that a commercial EM-1 is a one-off. But he's also sworn that SLS is a key component for any BEO architecture, while he's handed the lunar lander architecture to commercial launchers, stated that the obvious cost-effectiveness of commercial launchers should be employed whenever possible, discussed moving LOP-G modules previously planned to be co-manifested on SLS Block 1B to commercial launchers, deferred the EUS and ML-2 components necessary for Block 1B in the FY2020 budget, re-hosted Europa Clipper to a commercial launcher in the same budget, and now is proposing to pull away EM-1.
It sure seems like Bridenstine has decided to subject SLS to a death of a thousand cuts. If that's the plan, then the 2-launch architecture needs to be crew-certifiable.
Crew certification implies a few things:
- A different launcher will need to be crew-certified by the time EM-2 rolls around in 2023. D4H would be extremely hard to certify, due to structural margins. FH cores and S2's can be certified, but SpaceX has cancelled plans to do so. That would have to change.
- A crew-capable distributed launch really needs to rendezvous in LEO, to avoid subjecting the crew to Van Allen Belt radiation. This turns out to be hard.
- There will be some upper limit on acceleration if it's an eyeballs-out TLI burn.
- Crew-capable launches probably need to get to TLI in one burn. As we'll see, this and requirement #2 are sometimes antagonistic to one another.
Coordinating a Two-Launch Mission
There are three major problems to overcome with the two-launch coordination:
- Stage life.
- Crew safety.
- Pad turnaround time.
Currently, the FH S2 has been proven to be viable for at least 6 hours. The D4H DCSS stage has a typical mission time of 8 hours. Assuming that the Orion launches first, it has a mission life of 3 weeks. We can't use days and days of that, but a few hours isn't going to hurt us.
The Russians have performed successful rendezvous and docking operations with ISS in about 4 hours. NASA has not--yet. However, we will assume that NASA can replicate the 4 hour docking.
Note that this kind of haste is a serious red flag for a crewed mission. Not only do we have to launch the crew first, but an aggressive docking maneuver is going to make the Brown Trouser Brigade very unhappy. Developing a mission profile that's simultaneously safe and fast will be a major challenge.
Finally, while we're talking about major impacts to a few hours' delay, pad turnaround times are going to be measured in at least days. Since we only have two launchers that can accomplish the mission and each of those launchers only has a single pad, we can rule out using two of the same kind of launcher. We'll need both LC-39A for the FH and LC-37B for the D4H.
This rules out the D4H-D4H and FH-FH combinations as options, unless somebody wants to invest in new pads, or radically different payload processing infrastructure.
Docking
There are two ways to dock an Orion and its TLI transfer stage:
- Stage nose to Orion tail, the so-called "eyeballs in" configuration (referring to which way the acceleration would be trying to pull the crew's eyeballs).
- Nose to nose, or "eyeballs out".
One other thing to note: These pictures are fine for docking a TLI stage, but we also need to remember that we have to launch the Orion, as well. Here, it's much easier to launch the Orion on a DCSS or ICPS, because the adapter shown (which is pretty much exactly like the current OSA) can accommodate the ESM fairing panels as-is. The bottoms of the panels seat into the base of the OSA (like this). However, if we chose to launch Orion an an FH, we'd have to do something like this:
This would clearly be a lot more complicated.
But, no matter which launcher we choose, this nose-to-tail stuff is still really hard. Even if you provide a standard docking ring to mediate the capture and retraction of the Orion onto the stage, the stage will have to be latched all the way around its periphery. That's a 5 m circle for a DCSS/ICPS, and a 3.7 m circle for an FH S2. Getting the alignment and latching exactly right, and reliable enough to use, is a hard job.
In contrast, Orion is designed with an NDS docking port in its nose, so a nose-to-nose docking is almost trivially easy: Just slap a docking port on the standard payload attach fitting for your stage and you're done:
The question is whether the Orion, can survive the thrust of the transfer. And, since we're trying to do something that could work for a subsequent crewed mission, the term "eyeballs out" suddenly takes on some context.
What we're interested in here is the dry acceleration, i.e., the acceleration just as the last of the propellant is being burned, which will be the maximum acceleration. Turns out that things aren't terrible, but they're not great for the FS S2:
The FH S2 will definitely need to throttle down the Merlin 1DV to keep acceleration to just a bit over 1 gee. The RL10 in the DCSS produces accelerations of less than a third of a gee.
Time to Look At Some Missions!
After combing through the various constraints we've enumerated above, we're down to 5 possibilities:
- Modify FH to be able to support an Orion direct to TLI in one launch.
- Launch Orion on D4H, then launch a no-payload FH and use its FH S2 as the transfer stage.
- Launch Orion on FH, then launch a no-payload D4H and use its DCSS as the transfer stage.
- Launch Orion on an FH3R, then launch a second FHE to act as TLI stage.
- Launch Orion on FH, then launch a Centaur 3 on D4H.
Option #1: Modify FH to do Orion to TLI in One Launch
Modifying the F9 core isn't an option, but it might be possible to stretch the FH S2, or both stretch and widen it, to provide enough propellant to get Orion to TLI directly. Unfortunately, you simply can't make the numbers close if you're keeping the same three cores on the bottom. You can get to the point where the Orion can make up the missing delta-v for TLI with an S2 that's double the prop, but it requires a ridiculously high HEEO (185 x 94,000) to do so.
I'm not going to spend much time worrying about this one, but it is by far the cheapest way to get crews to NRHO (at least on a marginal cost basis) until Starship comes along.
Option #2: Orion on D4H, No-Payload FHE's S2 As Transfer Stage
Since the Orion Stage Adapter is designed for an ICPS, and an ICPS is almost identical to the D4H DCSS-5, the first thing to try in the 2-launch category is launching Orion on the D4H, then using an FHE with no payload to get its S2 to the rendezvous orbit with enough prop to do TLI. We can do this--just barely--by launching to a 185 x 620 km parking orbit, which has the nice property of being radiation-safe through the whole orbital path.
Here's a mission profile:
Note that this profile is limited by the height of the orbit to which the D4H can take the Orion, but the FHE is in only slightly better shape when it comes to being able to take it on to TLI. In general, these margins would be unacceptably low. However, the Orion has plenty of extra prop on board, and can easily make up for any shortfalls, either from the D4H or the FHE.
This profile has the simplest set of work items associated with it (which almost certainly makes it the best for an EM-1 replacement):
- Modify the existing OSA to work with the D4H DCSS-5. This should be almost no work at all, since the OSA is designed to attach to the ICPS which is almost the same as the DCSS-5.
- Assuming the eyeballs-out nose-to-nose configuration, build an SH S2 payload attach fitting with an NDS-compliant docking ring on it. SpaceX has experience with the NDS from the Dragon 2 work for the Commercial Crew Program, so this doesn't seem very hard. If we have to dock nose-to-tail, things are vastly more complicated, and likely unfeasible in the time allotted.
- Develop a reliable rendezvous, docking, checkout, and TLI profile that will accommodate the FH S2 stage life.
If the rendezvous, docking, checkout, and burn can't be accomplished in 6 hours, another option is to make improvements to FH S2 stage life. Installing heaters for the RP-1 tank and RP-1 fuel lines, adding insulation to the tanks, or adding solar cells to the stage fairing, and/or adding extra batteries are all possible--but they're not cheap in terms of R&D and, as you can see from the mission profile above, the extra weight will require adding in a little delta-v from the Orion.
The big drawback of this option is that the Orion is on the D4H. This pretty much rules out crew-rating the solution, since D4H isn't crew-certifiable for structural issues. An alternative to this would be to launch the Orion empty, then transfer the crew on-orbit from a CCP system (either the F9/D2 or the AtlasN22/CST-100). However, the crew would need to be transferred before the FHE launch, or we'll get bitten by the FH S2 mission life problem.
Option #3: Orion on FHE, No-Payload D4H's DCSS as Transfer Stage
Well, if D4H isn't crew-certifiable, why not launch the Orion on the FHE and use the DCSS as the transfer stage? The problem is that the mission is considerably more difficult using the DCSS, because the D4H simply doesn't have as much oomph to get it there with enough prop to do TLI. To do so, we have to launch the Orion into an extremely high 185 x 20,220 highly elliptical earth orbit, which has serious radiation issues, as well as serious issues for DCSS mission life.
(A quick aside: Some of you may be going "Huh? right now. Why do we solve a delta-v shortage problem by going to a higher energy parking orbit? The answer is because it's much cheaper to loft a no-payload transfer stage to the parking orbit in order to save delta-v at TLI, because we have to move the Orion with us at TLI. It's kinda weird, but it'll make sense if you think about it.)
Here's the mission profile:
The high apogee on this is a problem, even for an uncrewed mission, because it's in the middle of the second Van Allen Belt. This is technically the altitude of a medium earth orbit, and things like GPS satellites are quite happy here, but I doubt this is the radiation environment the Orion designers had in mind. The high apogee also implies a long orbital period (in this case about 6 hours). The minimum rendezvous is going to be a launch to LEO, a coast for 45 minutes, boost to the apogee (3 hours), one additional orbit to get everything squared away (6 hours) and then a coast to perigee to do the TLI (3 hours). Total mission time from launch: 12.8 hours--well beyond the DCSS mission life.
SpaceX would have to build a completely new Orion Stage Adapter, with a boat-tail to fan the 3.7 m F9 S2 out to the 5 m width required for the OSA to handle the bottoms of the ESM fairings. This is a lot more work than using the D4H as the Orion launcher and the F9 S2 as the TLI stage.
What About 2 FHEs?
However, the more complex OSA might be worth SpaceX's while if they thought they could get some crewed missions out of it. I'd think that SpaceX would be rather circumspect in its criticism of SLS in general, but if they're offered the cover of killing it completely dead (which is what a crewed commercial launch would do), they'd probably go for it.
A moment's examination of the two detailed mission profiles above should convince you that it would be very simple to do double FHE mission from a performance standpoint. I'm fairly confident that putting an Orion into LEO on an FH3R (3-stick reusable) would be doable, which would save some cost. It makes the issue of crew-rating the launcher at least possible, which is attractive. There's absolutely no radiation or orbit period length issues.
But SpaceX would have to figure out a way to launch two FHEs within 4-5 days of each other. (I figure that's about how much Orion mission life could be wasted.)
I don't think that the LC-39A payload processing would allow that. So SpaceX would have to invest in building a second pad for FH.
This... might not be as silly as it sounds. In general, there's no way that the launch cadence for FH justifies a second pad. However, there are plenty of reasons to think that SpaceX might want to be able to launch SuperHeavy/Starship from Canaveral, where the range of inclinations is vastly wider than it is at Boca Chica. Building a pad that can handle both heavy launchers might be attractive.
SpaceX has also gotten dinged for not supporting vertical integration, i.e., stacking the payload on the erect stack, rather than bolting it onto the PAF in a horizontal position as SpaceX does currently. Horizontal integration is great for quick launch turnaround, but imagine an entire Orion stack cantilevered off of the FH S2 in the horizontal position. Some payloads simply can't handle this, and SpaceX can't launch 'em today.
A second FH/Starship pad could also support vertical integration.
Talk of Centaur Should Remain Just That
Several people have talked up launching a filled Centaur as a payload, either on FHE or D4H. I can get this to work on a D4H, but it pretty much has all of the disadvantages of the Orion/FHE + Empty/D4H, with none of the simplicity. ULA does have some experience loading hydrolox into stuff in the fairing (they gained this experience by blowing up a couple of missions), but it's all on the Atlas V side of the house.
This is just a terrible idea.
Now, when Centaur 5 or ACES comes along, this should be revisited. But ACES is too heavy to launch as a payload, and I've lost the will to live when it comes to doing the numbers on Centaur 5. Just say no.
Your Handy Scorecard
Here's a summary of all the options, with pros and cons:
I didn't score the Centaur. So sue me.
Overall, Orion/D4H + Empty/FHE is by far the best choice for an EM-1. If NASA wants to go with a crewed version later on, I'm pretty sure they'd have to go all FH (which would drive Boeing nuts), and I'm also pretty sure that they'd need another FH pad at the Cape.
Update 3/23/19 14:51: OK, here's a stupid problem: The Orion ESM solar arrays apparently have only been designed to handle a 0.5 g acceleration, which is the maximum that the ICPS would generate. If we use an FH S2, we'll be at a bit more than double that.
Update 3/23/19 14:51: OK, here's a stupid problem: The Orion ESM solar arrays apparently have only been designed to handle a 0.5 g acceleration, which is the maximum that the ICPS would generate. If we use an FH S2, we'll be at a bit more than double that.
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