# RLV Markets Part II: The Black Aluminum Analogy

[Editor’s Note: It’s been too long since I wrote the first article in this series, and I wanted to write some more on this topic.Â  My tendency to try to cram everything into one ginormous ominbus post has been almost completely preventing me from publishing anything original about space lately, so I’m going to try breaking this topic down further than I originally intended.]

The key points I previously touched on were why high flight rates are important for RLVs, and that attaining a high flight-rate would require both technology and market development.Â  While there are many potential markets for launch vehicles that have been discussed over the years, there are three markets–people, propellants, and “provisions”–that I think are particularly suited to early commercial RLV efforts.Â

In this article I want to begin making my case for why I think that flying people is one of the most important RLV markets, by sharing an amusing analogy.

The Black Aluminum Analogy
One of the “fun” classes I took as a grad student was on analysis of composite structures. It was interesting, even though most of the class involved lots of matrix math as we worked our way through from first principles till we could understand how multi-layer composite structures behaved. One of the lectures near the end of the course discussed some more qualitative concepts now that we had a mathematical foundation. One of the ideas Professor Eastman drove home during that lecture was that composites weren’t just “black aluminum”. While it is often possible to make a composite that was roughly isotropic (say using chopped fibers in resin) and then use it as a drop-in replacement for an aluminum part, you’d be wasting a lot of the composite’s potential. To truly unlock the potential of composites, you have to understand and take advantage of their anisotropic nature.

For instance, composites allow you to put strength in the areas and directions you need it, while minimizing the strength in directions it isn’t needed. The upshot being that a properly designed composite part for a given application may, and probably should, look drastically different from an aluminum part for the same job. Another upshot is that there are some applications where what you need really doesn’t line-up well with the advantages of composites, and where an aluminum part might have been a much better choice (X33 LH2 tanks anyone?).

Black Aluminum and RLV Markets
I think this analogy is relevant to the discussion of how to use RLVs. It’s possible to use RLVs as though they were ELVs that just happen to be cheaper, or that just happen to come home from work at the end of the day. Most experienced ELV people I know who look at RLVs treat them that way. They talk about how “there aren’t enough payloads to justify developing an RLV today”, usually followed by a comment that “maybe in 20-30 years there might be”. By payloads, they tend to be thinking of satellites–because that’s one of the only things ELVs are any good for. And for a satellite, especially the way they’re done today, reusability is just a nuisance, unless it happens to make the flight cheaper. It’s just that much less payload available.

Now, this isn’t to say that the only thing RLVs have to offer is the potential of lower launch prices. Eventually, I think you’ll see satellites that take advantage of intact abort capabilities, the ability to do on-orbit checkout before release, etc. I’m just saying that for satellites most of the reusability stuff is of only secondary importance.

People though are different.

People more often then not will be flying round trips. For them, the recovery system isn’t some extraneous feature that is only useful if it makes things cheaper–it’s a fundamental part of the service. Being able to make it back home in one piece even when something goes wrong also tends to be more highly valued by breathing cargo.Â  The interesting thing is that the needs of the personnel transport market actually turn some of the main “drawbacks” of RLVs into strong benefits.Â  That recovery system is no longer “parasitic mass” that can’t be used for payload–Now it’s services already provided by the launch vehicle that don’t have to be deducted from the payload.Â Â  Of course, it is possible for a manned RLV to carry its crew in a separate capsule just like an ELV, in which case you’d lose a lot of these benefits, but it’s never been obvious to me why that that approach makes any sense.

One corollary of this is that a manned RLV doesn’t need to be able to carry anywhere near as much nominal cargo capacity to carry people as an ELV would.Â Â  Depending on the details, instead of needing 10-20klb worth of payload capacity for a 4-8 person capsule on an ELV, you might be able to fly a 2-3 person compliment with an RLV that has only 1000-3000lb worth of cargo capability.Â Â In fact, you could consider the Falcon 9/Dragon stack to actually be a ~6klb to orbit 3STO RLV, just as readily as a 20klb to orbit TSTO with a capsule on top. Of the systems you need for a manned spacecraft, most of them already need to exist for an RLV stage–TPS, landing systems, avionics, RCS, power systems and radiators, abort recovery systems, and possibly even some basic life support hardware (if you’re shipping pressurized/biological cargoes like some of the stuff Dragon will be shipping to ISS).

While it’s outside of the scope of this blog post, before I go on, it is worth mentioning that there are two technologies/techniques that accentuate the advantages of RLVs even further–tugs and fast rendezvous techniques. But that’s a discussion for a different day. It’s also worth mentioning that the “black aluminum” treatment of RLVs extends not just to how people think about using them, but also in how people think about developing them. But that is also a discussion for another day.

In my next post in this series, I’m going to discuss a counter-intuitive result that this line of thinking led me to.

The following two tabs change content below.

#### Jonathan Goff

President/CEO at Altius Space Machines
Jonathan Goff is a space technologist, inventor, and serial space entrepreneur who created the Selenian Boondocks blog. Jon was a co-founder of Masten Space Systems, and the founder and CEO of Altius Space Machines, a space robotics startup that he sold to Voyager Space in 2019. Jonathan is currently the Product Strategy Lead for the space station startup Gravitics. His family includes his wife, Tiffany, and five boys: Jarom (deceased), Jonathan, James, Peter, and Andrew. Jon has a BS in Manufacturing Engineering (1999) and an MS in Mechanical Engineering (2007) from Brigham Young University, and served an LDS proselytizing mission in Olongapo, Philippines from 2000-2002.

#### Latest posts by Jonathan Goff (see all)

Jonathan Goff is a space technologist, inventor, and serial space entrepreneur who created the Selenian Boondocks blog. Jon was a co-founder of Masten Space Systems, and the founder and CEO of Altius Space Machines, a space robotics startup that he sold to Voyager Space in 2019. Jonathan is currently the Product Strategy Lead for the space station startup Gravitics. His family includes his wife, Tiffany, and five boys: Jarom (deceased), Jonathan, James, Peter, and Andrew. Jon has a BS in Manufacturing Engineering (1999) and an MS in Mechanical Engineering (2007) from Brigham Young University, and served an LDS proselytizing mission in Olongapo, Philippines from 2000-2002.
This entry was posted in Commercial Space, Launch Vehicles, RLV Markets, Space Transportation, SpaceX, Technology. Bookmark the permalink.

### 37 Responses to RLV Markets Part II: The Black Aluminum Analogy

1. Bill White says:

If lots of people are flying to LEO and return home, genuine RLVs make tremendous sense.

Okay then, what business models are based on flying lots of people to LEO and then returning them home?

Hotels are an obvious example. A LEO sports stadium is another example if the teams are rotated once every week or ten days even if the competitions are almost exclusively television events. Of course, Hyatt could have its space hotel attached to the sports arena and showing the sports events would be terrific advertising for the hotel.

Excellent point, and I eagerly await the counter-intuitive result.

Also, thanks for chopping things up. My own blogging has gone easier since I’ve decided to let ideas develop piece by piece.

3. Jonathan Goff says:

Bill, I’ll be getting more into that in future posts in this series. But one of the interesting things is that closing the business case for a 1st Gen commercial RLV, might be closer than you think.

4. Jardinero1 says:

What I see frequently, and which may confuse some readers, is that the word cost and price per flight are used interchangeably. May I suggest that the word cost be used exclusively with reference to the provider of flight services and price be used exclusively with reference to the consumer of flight services.

I think that is important, because, while it is true that a higher flight rate may reduce the cost of each flight for the provider; it doesnâ€™t necessarily follow that the price to the consumer will be less. If demand for flight exceeds supply then providers can charge more. If supply exceeds demand then providers will have to charge less regardless of their internal costs. At least, thatâ€™s the standard micro-economic model of supply, demand and prices.

5. Jardinero1,
Fair point. I can sometimes get sloppy. Especially since one person’s price is another person’s cost. Ie if you’re say in Bigelow’s shoes, the launch operator’s flight “price” for you is part of your “cost”, and the full-wrap “price” you offer to a customer wanting to fly to your station is going to be different still (and hopefully higher if you want to make a profit).

~Jon

6. Excellent and intriguing post. I’m also glad you have been able to get back to these kinds of posts, what with all the distractions and stuff.

I do have a sort of OT comment on this sentence:

“””Of course, it is possible for a manned RLV to carry its crew in a separate capsule just like an ELV, in which case youâ€™d lose a lot of these benefits, but itâ€™s never been obvious to me why that that approach makes any sense.”””

There can be compelling reasons for designing an RLV so that the crew compartment/payload bay is a separate component. One is for easier escape in case of a catastrophic abort during launch. Another is to simplify the RLV design by not having a crew/payload compartment interfering with the propulsion and tankage structure, making for a cleaner, simpler vehicle design. Still another is it allows for the RLV to be more versatile instead of having, separate, dedicated vehicles for different launch functions.

I know you recall the early post-shuttle RLV designs from AIAA publications and NASA slide shows which showed a payload fairing mounted topside on an orbiter kind of like the cockpit on a WWII fighter plane. I’m talking about something like that, but removable and replaceable. A payload pod would simply be bolted on, while a crew pod would also be plugged into the RLVs avionics and diagnostic systems, etc. For a VTVL, the pod/cockpit would be on top. Thus, you could have a “Dragon” on top of a Blue Origins orbiter.

7. While your post did veer towards passenger RLVs, you mentioned propellant and provisions early on. I think it’s the combination of these three sub-markets that could create a high flight rate for RLVs.

I really hadn’t thought of RLVs being a good sollution for propellant and provisions, because I was stuck on the idea of cheap ELVs, or some other means of delivery (like electromagnetic “capapults,” or whatever). But why not RLVs?

Will you be going into more detail at another time about propellant and provisions?

8. Daniel Chisholm says:

It’s probably simpler, without being misleading, to continue to use ‘cost’ and ‘price’ interchangeably (standard micro-economics usually mumbles about prices being pressured toward the cost of production, long run risk-weighted return on capital, etc)

Jon, I quite like the “beware black aluminum” caution (and I too anxiously await the promised counter intuitive result)

9. Jonathan Goff says:

Rod,
Yeah I’ll be talking about propellants and provisions in future posts in this series. I was originally trying to cram it all into one article, but the thing likely would’ve been 20 pages long by the time I was done.

~Jon

10. Jonathan Goff says:

The capsule vs. integral personel cabin discussion is probably best left for a full blog post. But the big drawback I see to this approach is that you’re effectively adding an additional stage to the whole system if it’s a separable reenterable capsule. This loses you many of the main benefits of an RLV, and requires a much bigger RLV to service the people-flying market. Now, having that crew cabin be something that is easily reconfigurable for flying other payload sorts is probably not a bad idea. But this is an RLV–not only is it supposed to make it home at the end of the day on nominal flights, but in order to make the economics work, you need to make it robust enough that it can gracefully handle abort modes as well.

To be fair though, this is a legitimate argument to have, and at least my boss agrees with you.

~Jon

11. Daniel Chisholm says:

Let me presume that getting to space is an extremely marginal proposition. This will favour integrated design approaches (put the crew inside the structural shell that you need anyhow) over modular approaches (attach an independently pressurized living compartment, to a carrier spine that is independently structurally adequate) particularly strongly in an RLV. So unless flexibility and modularity come for free, or at a negative cost, they perhaps ought to be considered bugs not features. This would suggest that successful RLVs should be designed and optimized to perform specialized tasks, rather than to be general workhorses.

People and propellants are different. For one thing, the person buys a return ticket whereas the fuel shipper buys a one-way. For another, the intrinsic value of a fuel cargo is very low, whereas for a people cargo it is very high. So the cost of a catastrophic failure of a propellant delivery mission is little more than the flight vehicle – perhaps this should influence the cost-vs-reliabilty-vs-survivability considerations in designing an economically optimal fuel tanker, and possibly result in very different design approaches being taken to an RLV tanker vs. an RLV peoplebus?

And speaking of fuel haulage – is this an intrinsically one-way proposition, or has anybody been able to imagine a useful backhaul cargo? If it truly is an empty-backhaul job, what does that tell us to do to our designs?

12. gravityloss says:

Interesting thoughts!
Hadn’t thought how far an integrated people carrier (Kistler K-1 is a nice example where the crew is in the second stage) is from a fuel deliverer.
In a propellant delivery vehicle, you probably don’t have a payload section at all, you just stretch the second stage tanks and don’t burn em to empty. High payload fractions of empty mass should be reachable this way.

13. Bob Steinke says:

I was thinking that the counter-intuitive result was related to the fact that people in orbit will need propellant and provisions so growing one market grows the other two, but with this talk about people carriers not necessarily being good propellant or provision carriers I’m not sure.

14. “”””””But the big drawback I see to this approach is that youâ€™re effectively adding an additional stage to the whole system if itâ€™s a separable reenterable capsule.””””””

You are correct, in my view. The thing is, though, I wasn’t calling for a separate module capable of reentry. I only meant it to be a way to add flexibility to an RLV system, while also offering an abort option where the crew module (and perhaps the same for a payload module) would separate during a catastrophic abort, say, during the boost phase. No heat-shielding or re-entry technologies, so, only minimal additonal weight penalties. I’d offset weight penalties by putting as much of the propellant in a drop tank anyway. Hey, that reminds me, here’s a good PowerPoint presentation on drop/external tanks for RLVs: http://www.jupiter-measurement.com/research/jpc_06b_talk.ppt

15. To Daniel:

Good points, like Jonathan’s. I’m not wedded to my suggestions above. The economics and operational issues may indeed be different enough for non-human vs. human cargo. Perhaps, though, this modular approach is still viable for other non-human payload deliveries vs. passengers.

Discussions for future posts, I guess.

16. Speaking of propellant delivery, what might be a dedicated, cost-effective propellant delivery-dedicated system that is realistic to implement in the fairly near-term? I.e., no space elevators or electromagnetic catapults.

17. Anonymous says:

Jon,

Excellent post.

Before your next post, you maybe want to consider the analysis of another blog post that suggests that the Falcon 9 first stage could tale a 2 -3 ton payload to low earth orbit (LEO) as a single-stage-to-orbit (SSTO) vehicle. SpaceX is already trying to build re-usability into that 1st stage using their Dragon heat shield technology, so maybe the Falcon 9 first stage could be that RLV SSTO carrying 1,000 – 3,000 lbs or 2 – 3 people to LEO that you are talking about.

You also may want to research that the Russians supposedly plan to use a propulsive landing and not parachutes for their ACTS manned spacecraft that will replace their Soyuz capsule.

You also may want to look at the Rotary Rocket Wikipedia page to see that the Roton was a composite shell built by Scaled Composites in 1998 for \$2.8 Million (probably without fuel tanks or flight-worthy structure however).

I think if you combine some of this information with the flight control systems that you are developing at Masten Systems, then your next post will probably show that the business and financing case might be able to close for a manned RLV SSTO easier and faster than some think.

18. duke says:

RLV launch rates are limited by ozone depletion, which of course affects the business case. Everyone here will roll-eyes-and-groan about “those environmentalists” but the fact is rockets cause ozone loss and big rockets launched often will cause a lot of ozone loss, maybe too much for the Montreal Protocol to swallow. See here:

http://news.nationalgeographic.com/news/2009/04/090414-rockets-ozone.html

19. Jonathan Goff says:

Daniel,
I’m not sure we have enough data to know for sure yet whether RLVs will have to be specialized or not in the way you suggest. I can see a couple of possible ways to allow you to at least switch back and forth between people, propellants, and provisions. However, I don’t have anywhere near enough detail to be sure how much of a performance penalty such interchangability will exact.

As to the fact that propellant often leads to “dead heading” the stage back (sending it home empty), I’m not sure yet what that implies for designs or operations…

In other words good questions!

~Jon

20. Jonathan Goff says:

Bob,
I was thinking that the counter-intuitive result was related to the fact that people in orbit will need propellant and provisions so growing one market grows the other two,

I actually agree with this statement (and plan to go into that in a later post), but that’s actually pretty intuitive to me….ok, I see I have people on pins-and-needles, let me see if I can get that next post out soon. 🙂

~Jon

21. Jonathan Goff says:

Roderick,
Iâ€™d offset weight penalties by putting as much of the propellant in a drop tank anyway. Hey, that reminds me, hereâ€™s a good PowerPoint presentation on drop/external tanks for RLVs: http://www.jupiter-measurement.com/research/jpc_06b_talk.ppt

I skimmed the link earlier, and while I agree that drop-tanks definitely would help an SSTO concept…I think they’re still missing several important drawbacks to using droptanks:

1-Drop tanks mean you are dropping something that you’re not controlling (and probably not recovering/reusing). That makes your launch site options a lot more limited. You’re pretty much stuck operating like any other ELV–flying out over the ocean, because dropping big random bits pretty much kills any chance of getting your E-sub-c low enough for ground overflights.
2-Drop tanks can actually increase the development cost–if you’re developing your RLV the right way. If you’re doing incremental testing, flying-early and flying-often, having to continuously fab drop tanks gets to be obnoxious quickly.
3-Drop tanks drive up vehicle complexity. You now have to deal with more “staging” events, more complex aerodynamics, loadings, etc….

I guess if you were trying to do a SSTO they’d make sense, but I’m not really sold.

~Jon

22. Jonathan Goff says:

Roderick,
I still think RLVs are the best way of developing propellants to orbit once they’re available.

~Jon

23. Jonathan Goff says:

duke,
As mentioned in the article, it’s the solid fueled rockets that are the really nasty ozone polluters. I’m sure liquid fueled rockets may still have some impact, but it would take a lot of them to make up for even one Shuttle SRB flight. I’d like to actually see more data on propellants that any RLV is likely to use.

~Jon

24. Pete Lynn says:

I doubt a propellant specific RLV is desired until the market becomes somewhat mature and ripe for specialization. It will depend on the ratios of provisions to propellants to people, however I suspect that spare tank/payload/propellant capacity might be used to transport a significant proportion of desired propellant. With high flight rate, standard flights might be regularly scheduled, and if not full with people/provisions, they might carry extra propellant.

As for an RLV market – the market for space settlement is space settlement. The more interesting questions to me are what might the economic foundations of initial space settlement be? And thereby, what type of space transport will the initial space settlement market favor?

25. Eric Collins says:

I just had a slightly out-of-the-box thought. While thinking about your assertion that the principal near-term markets for the RLV will be people, propellant, and provisions, I was suddenly struck by an idea to launch the crew in an aqueous environment. The crew is probably going to be in pressure suits anyway, why not go ahead and flood the crew cabin with water? This would make excellent use of the available volume and could potentially address all three markets simultaneously (since water is both a provision and can also be processed into propellant). After the vehicle makes it to orbit, the water can be pumped out into the second stage tanks, and the crew can continue the trip in a shirt sleeve environment.

I’m a little wary about posting this without thinking the consequences through a little bit more, but it seemed like such a novel idea that I wanted to share it.

26. “”””””1-Drop tanks mean you are dropping something that youâ€™re not controlling (and probably not recovering/reusing). “”””””

A tank dropped after the boost phase — like the Shuttle SRBs can be recovered, again like the SRBs. It does indeed create a limitation as to where you can launch, I concede that.

“”””””2-Drop tanks can actually increase the development costâ€“if youâ€™re developing your RLV the right way.””””””

Perhaps. You have more expertise with developing R&D cost business models than I do.

“””””Drop tanks drive up vehicle complexity. You now have to deal with more â€œstagingâ€ events, more complex aerodynamics, loadings, etcâ€¦.”””””

Less complex than TSTO, since I’m arguing a “virtual” SSTO. All the expensive stuff stays with, and is recovered with, the orbiter in one unit, like SSTO, but the reduced size of the orbiter due to the use of a drop tank is a reduction in complexity — in my view. My whole philosophy here is that — given propulsion system constraints — a rocket-powered SSTO will always run into a weight-penalty issue due to trying to take the entire structural mass into LEO. Weight penalties are inevitable because of aneed to make the system robust, so — in my view — is that the simplest way to solve the structural weigh-penalty problem is to drop some structural mass via an external tank.

27. Two other thoughts: “JATO”-style boosters, and game-changing technologies.

If you have short-duration boosters to help get the vehicle off the pad in what is otherwise a viable near-SSTO RLV, that is another option. I’m talking about using small strap-on boosters as a “virtual catapult.”

We’ve all heard of, and even discussed, things like rocket sled or carrier-type catapult launch systems to start a launch sequence. The thing about such systems is they limit launches to wherever such a system has been built. Using short-duration, high-acceleration boosters would mean 1) the “catapult effect” becomes portable, and 2) that the boosters would travel a relatively short distance before separation, thus facilitating recovery and simplifying operational issues. For instance, a 20-second burn at 4 Gs might mean staging at about 5 miles and at about Mach 2. Very rough guess (perhaps optimistic on the Mach numbers, but, you get the concept), but recovering from 5 miles up, and perhaps 10-15 miles downrange is less complicated than a large, fly-back booster. One of you folks did an entry on a runway rocket sled that is another option. With land-based speed records now exceeding Mach 1, that may have merit. The “booster” never leaver the ground, but doesn’t need rails, just a runway. HOTOL flirted with that idea years ago.

Technologicasl game-changers: I imagine that we’ll see the first aircraft fuselages built primarily with carbon nanotube meterials within a decade. This could make a dramatic diffrence in RLV design, and possibly allow for SSTOs with conventional propulsion.

More potent, denser hydrocarbon propellants, like a mixture of Quadricyclane and JP-10, instead of RP-1. Combined with advanced composite vehicle bodies, it could alter the equation for RLVs dramatically.

I haven’t said anything about air-launch, as that is indeed a whole ‘nother discussion handled at length on your blog in the past.

28. Gary C Hudson says:

Just a quick follow up to anonymous 17. The Roton ATV did have a flight weight and worthy structure including the fuel tanks. (We built the kerosene tank gores but didn’t assemble or install them since that volume was occupied by the peroxide propellant for the ATV test phase.)

And to echo other comments: excellent post, Jon.

29. MG says:

Re: people, propellant, and provisions.

The big difference is that the first of those three has the least tolerance of failure, and the least payload density.

Perhaps a workable development program (to the extent it exists) would be to build and test the bejesus out of a propellant delivery vehicle that has the same geometry and payload capacity as a people mover. It would be suboptimal as a propellant-only carrier, but would wring out a lot of risk.

Assuming more than one stage (and referring to Jon’s earlier post on stage recovery), the first stage could be suboptimal for the orbital mission mass, but be better tailored to the recover / relaunch role.

One of the things that attract me to suborbital field is that a lot of the constraints that have driven launcher design (e.g. maximize mass ratios) are relaxed… and sometimes considerably.

30. Anonymous says:

A quick follow-up to the Gary 27 post.

If one could buy SpaceX Merlin engines for \$1 Million each, how much would it cost to build an SSTO out of composites, and how long would it take, if re-usability was not a goal? Would this SSTO look something like the Roton ATV, and would you have Scaled Composites build your shell? The payload requirement would be 1,000 lbs to a 200 km reference orbit.

31. Chris says:

Merlins have really craptacular isp. You could probably build a non-reusable SSTO with one, at great expense, but the question really is why. Noone other than the education market (cubesats) wants a rocket that can only take a tiny payload to a VERY low LEO.

You’re better off spending a little more on NK-33’s or even taking a one year development detour, calling the flometrics guys, and atleast seeing if you can make their system work. I think their system is going to be heavier than they say it will, but you can’t argue with their price.

32. Gary C Hudson says:

30 Anon, I don’t think the Merlin alone can do the job. I know one could build something like you ask for using the NK-33; we looked at just that for the Roton as a backup, but unfortunately the engine size was mismatched to the vehicle GLOW. (You want a smaller engine and more of them for a VTOL due to ascent abort considerations, or you end up with a very large vehicle.)

You could possibly build a “mixed mode” VTOL using Merlins plus RL-10s. Just guessing, I’d figure such a vehicle would have a GLOW in the 400K lb. range to match the derated thrust of 6 Merlins, with perhaps 3 RL-10s for the “second stage”. The cost of the RL-10s would kill the project, however, since they are in the double-digit million range, now. The project would end up at a few hundred millions to be a useful demonstrator.

FYI, Gwynne told me a few years ago that the “sale” price (if they were ever to sell, which is questionable to me) would be \$5M per Merlin. So right there, the engines would cost you about \$50-60M for one shipset.

33. I alos don’t see how a Falcon 9 first stage could become a viable SSTO RLV. The post above that first mentions this does say that it wasn’t SpaceX proposing, but an independent poster imagining that possibility.

One option to make either the Merlin or NK-33 more efficent for possible SSTO use would be carbon-carbon nozzle inserts inside upper-stage configured nozzles for the engines. These inserts would configure the nozzles for boost-phase effciency, then be ejected so the nozzles would be reconfigured for the rest of the flight. Still a long shot though. Until carbon nanotube derived materials are available for major fuselage cmponents, I don’t see how conventional engines could power an SSTO RLV, even with using a drop-tank to lose structural weight.

34. “”””””You could possibly build a â€œmixed modeâ€ VTOL using Merlins plus RL-10s. Just guessing, Iâ€™d figure such a vehicle would have a GLOW in the 400K lb. range to match the derated thrust of 6 Merlins, with perhaps 3 RL-10s for the â€œsecond stageâ€.””””””

Sounds like a job for Aerojet’s TAN (Thrust Augmented Nozzle). See the Selenian post at http://selenianboondocks.com/2007/11/random-thought-thrust-augmented-aj26-60/

and also:

http://selenianboondocks.com/2007/11/thrust-augmented-nozzles/

35. Pete Lynn says:

Speaking of Merlin engines – why do they have such low ISP? I had thought due to the low expansion ratio, but then I note that the sea level ISP is not that great either. I further note that they have a high expansion Merlin for the Falcon IX second stage – wonder what performance it gets.

Pete.