K-1 Orbital Vehicle As A LTV?

I was doing some thinking, planning on writing an article talking about why the Orion crew capsule's basic design philiosophy was flawed, when I came upon a much more interesting idea. Basically, I realized that the upper stage of RocketplaneKistler's K-1 vehicle (the Orbital Vehicle or OV for short), could actually make a pretty darned good Lunar Transport Vehicle.

How I got on this Tangent
Here's how I got off on such a weird tangent. When thinking about how the CEV is being designed, I realized that a lot of their problems come from the fact that they're making similar mistakes to what they did with the shuttle. Instead of designing a "space truck", and then designing a "camper" to go with it, they decided to make it a cross between a "space big-rig" and a space "winnebago" and a "space research facility". In other words, they tried to not only cram in a heavy cargo lift capacity, and a pilot/copilot, but they also crammed in a long-duration space hotel (capable of housing 7 people for a few weeks), with research facilities, and several other things. Then they tried to add a bunch of cross-range to it, and when you're all done you get the monstrosity known as the Shuttle.

Quick digression: Just in case you've sucked up the groupthink, Shuttle's problem wasn't mixing crew and cargo. If they had designed it as a crewed cargo delivery vehicle, where the crew was a pilot and copilot, and the crew accomodations were only for short durations, the vehicle would have been many times smaller. Even with the 60klb cargo capacity, if they had cut the crew requirement to two, and used fairly spartan crew facilities, the whole thing would've likely been half as big as it ended up being, which would have made the whole thing a lot easier to work with in spite of all its other flaws. With that kind of a setup, they could have added a "camper module" inside the payload bay (like many of the things SpaceHab has built) for when they needed longer duration habitation capabilities, or research facilities. Trying to cram as much as they did into the basic vehicle was a big part of the problem.

There's absolutely nothing wrong with having a crew member or two on a vehicle. They really don't add that much weight to a reusable vehicle, and add a whole bunch of flexibility. Ok, that's enough on that rant.

Going back to the CEV, they're making some of the same mistakes. Instead of trying to make the CEV modular, so that you add capabilities as you need them, they're once again trying to design a winnebago, and then go back and slim it down for other applications.

So, I was thinking about how I would adapt a commercial earth-to-orbit capsule, like SpaceX's Dragon, so that it could have some of the same general capabilities as the CEV, without being such a bloated, expensive monstrosity. I started thinking about adding a mission module like CSI proposed with their Lunar Express idea that they unveiled back at the Return to the Moon conference last year. Basically, you dock the Dragon capsule to a module that would have longer duration habitation facilities, more room, etc, and then you could use an upper stage to send it to the moon. All it would need would be a slightly beefier heat shield, and you're off to the races (Yes, that is a development project, but one that both we and the Russians have done previously. 30 years ago. I think we can figure it out again.)

So I started thinking about doing something similar with RpK's Orbital Vehicle, when I started crunching numbers...

More Than One Way Home
The simplest and most typical method used for returning a vehicle from lunar orbit is to do a direct return. Basically you do a burn in lunar orbit that slows you down enough that your perigee intersects with the earth's atmosphere, and then you use the earth's atmosphere to slow you all the way down until you're slow enough for your recover system (usually parachutes) to take over. There are some variations on the theme, but the vast majority of missions planned, executed, or even dreamt-up use this technique. The problem with this technique is that it is rather demanding on your TPS. You're coming in at about 11km/s (instead of the ~7.2km/s from LEO), which means you have over twice the kinetic energy to bleed off. You end up getting much higher peak heating loads and G's than from a nominal LEO reentry. Kistler's vehicle uses a radiatively cooled TPS system, much like the Shuttle, which includes a combination of carbon-carbon tiles, and ceramic blankets. These heat shields basically reach a thermal equillibrium where the amount of heat being pumped into the shield is ballanced by the amount reradiated outward from the shield. The problem is that if you greatly increase the heat flux in, the shield has to get hotter to reach thermal equillibrium. With a standard direct return from lunar orbit, it's questionable that a tile-based system like the K-1's would work very well. It might just have that much margin (after all thermal radiation goes with temperature in Kelvin to the 4th power, so it might not need to get that much hotter to reach thermal equillibrium...but proving that out would not be cheap).

After thinking through that, I was just about to give up on the idea, when I realized that the direct return isn't the only, or even the preferred way to come back from lunar orbit, especially if you have a reusable transfer vehicle. A much better, and more workable method would be to use a combination of aerobraking and propulsive braking to return the OV from lunar orbit to LEO, and then continue from there to earth's surface. For aerobraking, since you're trying to bleed off less velocity, you end up targetting a higher (and hence thinner) part of the atmosphere than you do with a direct return. It turns out that the peak heating loads and total heat loads are quite similar for aerobraking into LEO from a lunar return trajectory as compared to a return from LEO to earth's surface. Basically, by using aerobraking, you get to split your reentry into two phases, neither of which is particularly worse on the vehicle than a nominal reentry, and with as much time as you want between the two. This means that you can let your vehicle cool down between those phases, you can inspect your heat shield for damage or wear, or you can dock to an orbital facility or another vehicle to transfer crew or cargo. Also, this means that you might not have to do anywhere near as much requalification of the TPS design for the OV--you might even be able to use a "stock" OV for the mission.

Aerobraking Challenges
Now, aerobraking is a bit tricky. While we've done a lot of aerocapture, and a lot of multi-pass aerobraking (particularly for space probes going to Mars for example), we haven't got a lot of experience with "single-pass" aerobraking. Let me explain a little bit first. "Aerocapture" is when you have some incoming vehicle or probe that isn't actually in orbit around the target planet, which then uses the target planet's atmosphere to slow it down enough that it enters an elliptical orbit around that planet. This doesn't take a huge amount of delta-V, and so it can be done at fairly high altitudes, low heating rates, and low stresses. "Multi-pass Aerobraking" is using several passes through the upper atmosphere of the target planet (once you're in an elliptical orbit around the planet) to slowly drop your apogee until you're in a nearly circular low-orbit around the target planet. You do need a tiny bit of a circularization burn to bring your perigee back up at the end, but if you're patient enough, and can take enough passes, that propellant requirement goes way down. With single pass aerobraking, you try to bleed off just enough energy to lower your apogee to your target orbital altitude, all in a single pass, without lowering it so far that you end up accidentally reentering the target planet.

It's that last part that's the kicker. If you hit a part of the atmosphere that's a little too dense, your apogee can drop into the atmosphere, and then it's all downhill from there. If your vehicle isn't capable of taking a reentry, you're toast. If it is capable of taking reentry, you're likely to end up with a very hot, emergency landing somewhere completely unexpected. Neither of those is particularly good. In order to avoid that, you need to have fairly detailed information of the density of the upper atmosphere, and have good control of your vehicle during the maneuver. Not impossible, but dicey.

The reason why you really want to do single-pass reentry, in-spite of it being more difficult, is that it cuts dramatically down on the duration of the return flight. A return from lunar orbit usually takes like 3 days. With multi-pass aerobraking, you could end up taking another 2-3 weeks or more as you slowly keep dropping your perigee lower and lower. For humans or sensitive equipment, having to pass through the van Allen belts repeatedly is a major drawback. Also longer duration flights require more supplies, more food, etc.

So, how can you lower the risk of single-pass aerobraking? By beating the problem with a "delta-V" stick. Basically, if you keep a fairly beefy propellant reserve (say ~750 m/s worth), then if you hit a little too hard, you can do an engine firing to bring your apogee back up above the atmosphere, and if you hit it too soft, you can either come back for another pass (if you're close enough that your second pass will come up soon enough), or you can do a retro burn to lower your apogee the rest of the way. Now, whether that 750m/s is enough will depend quite a bit on how well we figure out the aerobraking in the first place. If we have good enough data about the atmosphere, 250m/s might be sufficient. I imagine that with a good star-tracker/GPS fix right before atmospheric interface, and with a good IMU, the vehicle computer can probably recalculate the apogee in real time, and let the pilot know (or adjust itself if it's unpiloted) if adjustments are needed. For this discussion, we'll use the 750m/s for reserve, and 250m/s for raising the perigee at the end of the breaking maneuver, but these numbers need more research before they can be considered gospel truth.

On-Orbit Refueling
The one other assumption in this plan is that there is a way to do on-orbit refueling. As per my previous discussions, this doesn't necessarily imply that you need a propellant depot to do this. Propellant transfer could be done by docking/berthing the fueler to the K-1 OV, then spinning the two like a baton to settle the propellants. Or it could be done using a non-depot station, with two or more docking/berthing ports. Just dock the OV to one port, have quick disconnects inside, dock the fueler to the other port, and then manually run plumbing runs and pumps between the two. Or you could use a depot. Regardless of how it's done, this idea does require on-orbit refueling before it can be done. Once you see the numbers I've run however, you'll see why I think this is a good way of approaching things (for NASA, or even for a privately funded project). The number of launches needed to refuel an OV will be rather large (about a dozen Falcon 9 flights, or about 30 K-1 flights, or even more of a smaller RLV), but in my opinion, that's a good thing. Higher flight rates will drive prices down, and flying more often tends to help you up the learning curve faster. With a bulk buy in that size, I'd be surprised if you couldn't get the price down as low as $1k/lb or less. That's still over $250M for a translunar flight, but that's less than the estimate cost of a single Ares I launch (estimated at $280M)!

Running the Numbers
Ok, here's what I found when running the numbers. Due to the weird "Return To Launch Site" maneuver that the K-1's first stage does, the upper stage only gets about 1.5km/s of it's orbital insertion velocity from the first stage, and has to provide the rest itself. What that means is that the K-1 upper stage (the OV) is a very high performance stage. From the Kistler website, and an AIAA article that I found, the relevant stats are:

• 290,000lb fully loaded without payload


• 348s Isp with 395,000lbf from their main engine


• 27,000lb dry mass (according to the AIAA article)

That comes out to about 8.5km/s of Delta-V from the stage. However, if you're using aerobraking, the most delta-V you need for a round trip is about 6.2km/s, which means that the K-1 OV can actually push a lot more than 10klb to lunar orbit and back. Here's a few sets of numbers I got (email me for a copy of the Excel spreadsheet I used):

• If you want to carry the cargo all the way to lunar orbit and back (leaving nothing behind), you can carry 24klb of payload there and back. If this were say a lunar tour group, you could probably carry 5-10 passengers and 1-2 crew (depending on how much space you needed/wanted per person).


• If the vehicle is flying unmanned, and drops all of it's cargo off in lunar orbit, it can deliver 51klb of payload in lunar orbit. This is enough capacity to deliver a Nautilus Module to lunar orbit (even easier if you want it in L-1 instead.


• If you posit a 10klb "crew module", you can still deliver 30klb to lunar orbit while bringing the 10klb module back to LEO. This is enough for 2-4 people and a small reusable 2-4 seat lander.


• If you posit a 6klb "crew module", you can deliver 38.5klb to lunar orbit, while bringing the crew module home. This is probably sufficient for a 2-3 person crew, and a 2-3 person reusable lander.

Not too shabby all in all. The most surprising thing I found was when I compared the K-1 OV to NASA's EDS stage. Now, admittedly there are a lot of numbers floating around for the various parts of the ESAS architecture, and it's hard to tell what the currently accurate numbers are. While I understand NASA not wanting to post numbers while the design is still in flux, it makes it a bit harder to do valid critiques. Some numbers I've seen put the fully fueled stack in LEO at about 374klb, with about 147klb of that being the CEV+LSAM stack. If that is the case, and assuming a 455s Isp out of the J-2X on the EDS, that gives you about 3050-3100m/s of delta-V, which is just about right for a Trans Lunar Injection (Apollo numbers and most other numbers I've seen come in around 3050 m/s). Unfortunately, these numbers are confusing because the NASA website claims that the Ares V is capable of putting 290klb into LEO, which would mean that the Shaft either is putting up 84klb, or these numbers are obsolete. NASA's site also claims about 143klb for the CEV/LSAM stack. Based on NASA's numbers, the EDS may impart as little as 2800m/s of the TLI burn, with the LSAM taking up the rest of the slack.

So, the current EDS is capable of giving the stack somewhere between 2800-3100m/s of Delta-V. It turns out that a fully fueled K-1 OV can give the stack anywhere from 2950-3050m/s of Delta-V (depending on how much propellant you assume for your aerobraking margin). Which means that in the absolute best case (for ESAS), the two are almost identical, but in the worst case, the K-1 OV actually provides more total impulse to the CEV+LSAM stack by over 300m/s.

Now, I'm not suggesting that NASA should fund this instead of Ares V and EDS (though one really starts wondering what the advantage of going that route would be), just trying to point out how capable of a vehicle the K-1 would be.

Drawbacks
There are a few drawbacks to using the OV as a lunar transfer vehicle. First off, it requires a lot of propellants for the job--almost twice as much as the EDS stage would by weight (but much less by volume due to the much higher density of Kerosene than Hydrogen). This would require a lot of propellant delivery flights (12 Falcon 9 flights at least, or 30 K-1 flights). While that's a lot of demand, and will drive the flight rates and reliability up for the vehicle supplying the propellant, while simultaneously dropping prices, that's also a lot of logistics one has to handle. Even at one flight per week, you're talking at least 3 months for the Falcon 9 fueled vehicle, or 6 months for the K-1 fueled vehicle. While that's comparable to the expected flight rate for the ESAS stack, that's still kind of low. If RpK were to build a few additional airframes (say a fleet of 5), you could possibly cut that down to more reasonable times, but that would require more people to process, and a much higher up front capital investment. SpaceX probably can't ramp the Falcon 9 flight rate up much higher than 1 flight per week due to their reusability scheme for the system. And, it may take a while for other competitors with higher flight rates to hit the market. So in the near term, refueling the OV on orbit (once it actually exists) will be a non-trivial task.

More importantly, two technologies still need some work before this can be done--on-orbit transfer and storage of propellants (particularly LOX, the kerosene should be pretty easy in comparison), and aerobraking. We know a lot about aerobraking, and the precision, and advanced knowledge of the atmosphere needed to do this right aren't that much harder than what is needed for a lunar return (with those you face the same issues--hit too fast and you burn up or go squish, hit to slow and you bounce), but this is an area where some low-cost demonstrators would be in order. RpK could probably do those internally (or with the help of a private consortium) for not too much if they can actually get K-1 built and flying. So, while there are technological obstacles, they aren't insurmountable.

The most obvious problem however is that K-1 doesn't exist yet, and has never flown yet. It's partially built, and it looks like RpK has enough money being promised that they could just pull it off, but it's a big project, and the team doesn't have much of a trackrecord yet for actually flying things. Until they've flown something, all of this is just fun speculations.

Additional Details
Now for details. If the OV that's being used for lunar operations is only intended for exo-atmospheric use, it might be worthwhile to remove the airbags and parachute. Kistler was following an approach of making things modular so they could be easily replaceable, but I'm not sure if they've made it so modular that you could remove or reinstall those on-orbit. If you can that'd be great (and if they can tweak the design to allow for that without costing too much extra weight, that'd also be good). The parachute plus airbags probably weigh in the 3000lb range (based on historical comparisons). With those removed, you'd have more than enough mass to have a docking interface, some solar panels, radiators, star trackets, and the plumbing interfaces for refueling. Add a Canadarm Mini, and you're off to the races! The modular design of the K-1 may actual make doing that relatively straight forward, since the design is meant to have it's payload bay swapped out depending on the mission. That would require some work, but the modularity they've built in will make it easier.

Crew/Passenger facilities for long duration flights also require both space and development. The larger of the two standard payload bays that Kistler has is about 3.4m in diameter by about 5.6m wide. That's a fairly decent size, almost 50 cubic meters of space, which is almost 4 times as much space as the combined CSM and LEM from Apollo. That is about 2.5 times smaller than an ISS module. So, while it's more spacious than what was used for Apollo for three people, it may be a bit cramped for say a 12 person tourist flight. There are a couple of ways of dealing with this, such as having an inflatable extension, or making the lunar payload bay version bigger (though that would require requalifying the aerobraking dynamics), or having a temporary extension.

Larger payloads like lunar landers would likely need to be docked externally (especially if the internal volume is being used for crews). This shouldn't be too hard, so long as they're left in lunar orbit before return. Anything that is outside of the OV moldline when it aerobrakes would either need it's own TPS (and would require requalifying the whole vehicle for flying with that external payload), or would get really toasty fast.

Anyhow, there's more details that would need work, but the overall concept has some merit. What do you all think?