Tuesday, February 14, 2006

Why Merlin 2?

SpaceX does not need to design and qualify a third, much bigger, engine in order to become a profitable launch company, or to take over payloads intended for the Shuttle, or to fly people to either ISS or a Bigelow hotel. SpaceX should concentrate on execution, development of parallel staging (Falcon 9S9), and on a regeneratively cooled Merlin. Execution includes things like recovery of first stages and getting to one launch per month, with at least one or two Falcon 9 launches each year.

In this context, a regenerative Merlin 1 (hereafter refered to as Merlin 1x) may make sense because it may improve safety, reliability, and operational costs, and the cost of those improvements may be realistically amortized over dozens of launches. As the engine cycle is more efficient, a small increase in Isp and perhaps thrust is likely.

So why Merlin 2?

Elon keeps saying that Merlin 2 will be the biggest single thrust chamber around, but smaller than the F-1. Presumably, he is implying that it will have less total thrust than existing multiple thrust chamber engines, in particular the RS-180 (2 chambers, 4152 kN). That puts a fairly tight bound on the size. Here's a list of the biggest current engines, by thrust per chamber.

Enginethrust per chamberIspVehicle
F-17740 kN265 - 304Saturn V
RS-683312 kN365 - 420Delta IV
RS-242278 kN363 - 453Space shuttle
RD-1802076 kN311 - 338Atlas V
RD-1711976 kN309 - 337Zenit
NK-331638 kN297 - 331N-1/Kistler
Vulcain 21300 kN318 - 434Ariane 5

Taking Elon at his word, Merlin 2 will have between 3312 and 4152 kN of thrust. Call it 4000 kN, 17% more than the Falcon 9 first stage.

What can SpaceX do with this engine that cannot be done with the Merlin 1B?

Improve Falcon 9 LEO lift to 11000 kg. That's not even close to lifting stranded Space Shuttle payloads, which is the largest obvious market in terms of LEO mass in the next four years. I'm not sure what payloads are enabled by a 9300 to 11000 kg capability improvement.

Cost-reduce the Falcon 9. I can't see how this can pay for development, unless the Falcon 9 is launching monthly by 2010. And a further (fractional) price decrease doesn't seem like it would stimulate the market any more in the near-term than the existing, bold, price statement.

Improve Falcon 9S9 LEO lift to 30000 kg. The promised Falcon 9S9 lift capacity (24750 kg to LEO) is almost exactly that of the Shuttle (24400 kg to LEO), but the shuttle payload bay supports its payload better. The additional weight of a frame inside the 9S9 fairing might make some Shuttle cargoes too heavy to lift. A Merlin 2 based first stage for the Falcon 9S9 would fix this. The trouble is, a modest increase in thrust from the Merlin 1x would also fix the same problem. And it seems the latter change would have to be less costly than a whole new engine.

If the Falcon 9S9, improved with either Merlin 1x or Merlin 2s in the first stage, took over a dozen Shuttle ISS launches, I can imagine that would be a high enough flight rate to pay for Merlin 2 development. The trouble is that if I were paying to lift a billion-dollar ISS segment, I'd prefer to go on a machine with engine-out capability at launch. Three Merlin 2s don't give you that capability, and 4 implies some kind of vehicle (and its development cost) other than a Falcon 9S9 with single Merlin 2s on the first stages.

Build a 100,000+ kg to LEO launcher. This is what you get if you want engine-out capability with Merlin 2s. This kind of capacity is not necessary for exploring the solar system with robot probes, or even for building big orbital telescopes. It makes sense only if you want to send people a long way for a long time. Elon Musk passionately wants to build this rocket, that's why it's called the BFR.

The only organization with any credibility talking about using such heavy launches is NASA, for use in sending people to the Moon and maybe Mars. The BFR is Elon Musk's statement that he wants to take over the U.S. manned space program's launches. The business case for Merlin 2 and BFR must fundamentally rely on the U.S. government privatizing a critical, and the most public, portion of the manned space program. A program which from its outset has been about national pride.

A more likely scenario is that NASA will spend billions developing its own HLLV in competition with SpaceX, in the process abandoning the Space Station and strangling SpaceX, and will end up being able to afford just two or three launches to the Moon before abandoning VSE for the next thing. The history of heavy launchers is not reassuring. The Saturn V (118,000 kg to LEO, $2.2B per launch in 2004 dollars) was launched 13 times. Energia (85,000 kg to LEO, $1.4B per launch) was launched twice.

If all this sounds dire, a more reassuring comparison can be made by considering what SpaceX intends the Merlin 2-based vehicle to cost. Mr. Musk has stated several times that the point is to get costs below $1000/kg. A 100,000-kg-to-LEO vehicle priced at $500/kg would be $50M per launch. The history of $50M launchers is much more attractive. The various Delta incarnations (~1300/kg to LEO, ~$50M per launch) were launched hundreds of times. Soyuz (7200/kg to LEO, $45M per launch) has been launched 714 times.

A big rocket at such prices would clamp Falcon 9S9 prices to $30M and also reduce the 9S9 flight rate, at which point the parallel-staged rocket might cost too much to fly profitably. So SpaceX faces a fork in the road: develop either the Merlin 2 or parallel staging, but not both. Parallel staging will cost less to develop, implement, and support, and the BFR will cost more but enable people to get out of low earth orbit, and perhaps convert the VSE from a boondoggle into a success.

And thus we get to the vision thing. Elon has at various times stated that SpaceX is out to make money, and at other times stated that the goal of SpaceX is to enable the colonization of space. Let me point out that you can make money first, and enable colonization second, but not the other way around.


  1. it would almost be cheaper for elon to pick restart the rd-270. not very green, but if you are launching from an atoll, nobody is there to breathe the fumes,

    only kidding.

  2. Useful, interesting post. I view the Merlin 2 as something that can take in several of your scenarios. The goal would be to remove all of the remaining excuses for not using SpaceX's rockets.

    Besides, Musk now has an experienced engine development team assembled at considerable cost and care, so why not use it? Would he be able to avoid parrallel staging altogether on the Falcon 9, if engine development is successful and on time?

    Dan Schmelzer

  3. rd-170 and rd-180, not rs :)

  4. I would not worry too much about this. Obviously the merlin 2 is a long term project.

    For the text two years, the spacex engine design team will be very busy designing and building a regeneratively cooled version of the merlin 1, a high expansion ratio version of the merlin 1 that will double as a heat shield (remember the falcon 9 upper stage is supposed to be reusable), and maybe trying to increase the combustion efficiency of the merlin 1.

    Only when all this is done will they start seriously designing the merlin 2. And there is no alternative to that since elon needs to keep his engine design team together.

    An experienced engine design team is a very valuable asset if he wants to do an IPO. And big plans for the future are also quite nice for an IPO.

  5. Largest active engine is the RD-170 at 1.7 million pounds thrust at 4 chambers. For SpaceX to have the largest single-chamber thruster that is not the F-1, then Merlin 2 should have greater thrust than RS-68, but less thrust than F-1. Using the RD-180 at 850,000 lbs thrust as the upper limit for Merlin 2 thrust is probably not correct. Musk's public comments probably indicate they are thinking of Merlin 2 as something between the 650,000 lbs of RS-68 and the 1.5 million lbs of F-1.

    The TR107 was a pintle-injector LOX/RP engine with thrust rated at 1.1 million pounds. Northrop received about $40 million in NASA contracts up to FY05 to develop this engine for the now cancelled SLI. Northrop maybe thought that SpaceX had borrowed their Merlin 1 engine design from the TR107 engine.

    I am just guessing, but SpaceX could continue the design work of the TR107, and call it the Merlin 2. This Merlin 2 would look very similar to both the Merlin 1 and the TR107, and they could ask for NASA/DOD contracts to develop it.

    A Merlin 2 with 1.1 million lbs thrust, like the TR107, would give 3.6 meter diameter Falcon 9 vehicle a 33,000 lbs to LEO performance - similiar to Sea Launch Zenit or proposed Soyuz 3. They would price this between the $35 million Falcon 9 with 5 meter fairing and the $51 million version that does 33,000 lbs to LEO. This rocket would eliminate the need to learn parallel staging on the S5 version of the Falcon 9. There is probably not a large market for the 50,000 lb payload version of the Falcon 9 triple, but it looks nice in brochures and Air Force/NRO EELV customer briefings.

    The 1.1 million lb thrust Merlin 2, similiar to the TR107, could be later integrated into a 7.2 meter or 8 meter diameter vehicle that offers 132,000 lbs to 330,000 lbs payload to LEO, with engine out capability on a 5-engine or 9-engine first-stage. For this new rocket, SpaceX would have to learn how to double the diameter of their Falcon 9 vehicle, to 7.2 meters or greater, just as they doubled the diameter of the Falcon 1 to become the Falcon 9. If you re-evaluate the Saturn V vehicle with a LOX/RP second-stage powered by a Merlin 2 (or TR107), an 8-meter diameter, a 5 Merlin 2 engine first-stage (i.e. 5.5 million lbs thrust), and no thrird-stage then you probably get a payload number around 176,000 lbs (i.e. 80 Metric tons).

    Who is the customer? What is the market? The customer could just be a development contract for Merlin-2 engine development. For payloads, the customer for a 33,000 lb version of Falcon 9 is the same customer-base for Sea Launch and Ariane V - commercial launch of 10,000 lb class satellites to GEO, which the proposed Falcon 9 single can't do right now. The customer for a 60 metric ton to 130 metric ton launcher is the Air Force or anyone wanting to fly to the Moon or Mars. Space Adventures will fly a minimum of 2 paying passengers to the moon for $100 million each, which is a minimum of $200 million. I would think that many countries would pay $200 million to fly around the Moon, Venus, or Mars. I would guess that SpaceX could find a way to profitably launch a much more capable payload than Space Adventures is planning for well under $200 million if SpaceX ad this vehicle.

    The cost of Merlin 2 engine development should be under $100 million, because SpaceX has the TR107 as a baseline. If it costs SpaceX $200 million total to develop an 80 metric ton payload vehicle, and a Merlin 2 engine to propel it, SpaceX would make a $50 million profit if the U.S. Air Force gave SpaceX a $250 million demo launch contract similar to the $250 million launch contract they gave to Boeing for the partially successful Delta IV Heavy launch demo (with only 25 ton capability). If SpaceX gets NASA or the Air Force to pay for the development on cost-plus contracts, then the economics are really in their favor.

    If the Merlin 2 is the TR107, then the risk to return economics are not as bad as they might initially appear. For an investment of $100 million to $200 million in the Merlin 2, SpaceX might double their competitveness and the market of payloads that they are qualified to launch.

    What do you think?

  6. This is in response to the last, detailed, informative comment.

    You're right. I goofed. I had thought that the RD-170 was not an actively utilized engine, but of course it is used in the SeaLaunch and Zenit rockets. That puts a much larger upper bound on the thrust of Merlin 2. You suggest a 5,000 kN thrust instead of the 4,000 kN thrust that I supposed. That doesn't really change the lift options I was suggesting.

    You point out a payload that I hadn't thought about: the heavy satellite to GEO. A 5,000 kN Merlin 2 would make Falcon 9S9 able to lift more to GEO, but it can already lift a reasonable amount there, and a lot of those launches are currently double satellite launches. I don't see a large increase in possible payloads here.