Great Summary. I enjoyed the presentation and was floored not only by his speed of development, but also that he has a paying customer for the initial launch.

I'm a mere enthusiast, not an expert, so forgive me a somewhat elementary question:

What does "SL" stand for in the Isp rating and why is Isp higher (and thrust higher) in vacuum?

SL = 'Sea Level'
The optimal shape for the bell nozzle depends on the outside air pressure. At a different presure, the Isp will be less than the maximum possible so the shape ends up being a comprimise with peak Isp set for a specific altitude. This is one of the things an aerospike engine doesn't suffer from since the exhaust shape automatically adjusts to the external pressure.

Would the Saturn V class rocket have a single engine powering it's first stage or will it have five engines like the original Saturn V?

If it is an F-1A-class engine, and the vehicle is to have a Saturn V-class payload, it's likely. Note that the "Saturn V-class payload" and "need an F-1A engine" comments were in separate parts of the presentation, and didn't imply to me at the time that a SpaceX HLLV would resemble Saturn V.

Just speculation, though -- I don't work for SpaceX, so all I know about Falcon and future plans is what you see above.

Is there any chance thay anyone will build a single chamber rocket engine of about 7.5 Million pounds of sea level thrust?

Hi All

There was a design from the early sixties with a much bigger thrust than that even... the Sea Dragon. Here's a link...

http://www.astronautix.com/lvs/searagon.htm

...its main engine would have had 80 million pounds thrust. That meant 1,000,000 lbs to LEO - three ISS at once!

Adam

Oh, THAT would be fun. Doesn't seem like it would work, but TRW and NASA (of the 1960's) seemed okay with the idea.

Please email Tom Mueller at tom.mueller@spacex.com and/or Elon Musk at elon.musk@spacex.com to find out wether or not SpaceX plans to develop their Saturn V class rocket with a single engine per stage. Using the minimum number of engines per stage increases reliabiliy of the launch vehicle as well as giving the best possible sound quality on liftoff for a given payload class. When you have found out, please email me at rocket04pledger@yahoo.co.uk.

T.L. James,

Why did you say that a 7.5 Million pound sea level thrust single chamber engine wouldn't work?

I should have been more specific -- I found the Sea Dragon concept in general a bit fanciful. I wasn't referring to the engine specifically.

The concept seems somewhat impractical to me for several reasons. First, the vehicle is floating on its side in seawater, which makes corrosion of systems and structures a serious concern, and is going to result in the thing turning into a popsicle as soon as the LOx tanks are loaded.

Second, the need to erect a fully-loaded vehicle of that size by cantilevering it from its base, without a strongback to help take some of the load, is going to make the structure inefficient. Erecting the vehicle by first sinking the base (think SRB retrieval in reverse) might be a way around this problem. Of course, if you have a million pounds of payload capacity, whats a few tens of thousands of pounds of structural inefficiency?

Third, there is the "iceberg" problem. Will it ignite with its first stage engines and tankage beneath the surface? If not, how big does the submersible base have to be to lift the entire vehicle, standing, above the water? Then there is the location of the center of gravity -- a huge mass standing above the water will require a fairly large submerged base mass (or a sophisticated active control system) to remain vertical.

Just a few things off the top of my head, related to the vehicle in general. I'm not saying that it can't be done, or that there aren't creative solutions to each of these potential problems, just that the program (at least as described in the Astronautix article) leaves open a number of serious questions.

As for the engine...it would be entertaining to watch a single huge engine like that fly, but what about engine-out capability? And how would one build such a large, single-chamber engine in the first place? And could it be done without the thing tearing itself apart from the acoustics alone? Again, I'm not saying that it can't be done, just that it would be one heck of a challenge.

Give me the development money, and I'll be happy to give it a whirl.

T.L. James,

If a launch vehicle has a large number of smaller engines instead of one huge one, there is a much greater risk of a catastrophic faliure of one of the engines. The Russian N-1 rocket had 30 first stage engines and all four that were launched failed! This is because when one engine explodes, it takes out the adjacent engines leading to a chain reaction which ultimately results in the destruction of the rocket. If the Keep It Simple Stupid (KISS) doctrine is followed by keeping the rocket as simple as possible, then the reliability of the rocket is as high as possible.

The acoustics could be solved by the use of sound absorbent materials in the launch pad which would reduce the intensity of the shockwaves that are reflected upwards to the vehicle. With modern materials, the launch vehicle could be made strong enough to be able to withstand the shockwaves generated by it's engine without making it too heavy.

One thing that confuses me-why didn't he mention x-cor as a possible competitor? Their progress in engine development is every bit as impressive as spacex's, because they've done it on a shoe-string budget. Also, I would think some of their innovations would be of interest to spacex as possible cost-saving measures, specifically their fuel piston-pump(same space-saving advantages as Spacex's turbo-pump, plus it's simpler, cheaper, and more scalable)and their clear demonstration that a regeneratively cooled engine CAN be developed both fast and innexpensively. Is he really overlooking this company, and if so, why?

Yes, Dominic, I'm aware of that. The optimum number for a manned vehicle is typically from three to five, as one or even two engines may not give you the engine-out capability you need, while more than five is pressing your luck on reliability.

T. L. James,

To give you engine-out capability, the engines need to share a common propellant supply with interconnecting propellant feed lines. If the engines have independent propellant tanks like in the Delta IV Heavy which has three separate common core boosters (CCB's). The failure of the engine of one of the boosters is likely to be non-survivable. This is because the failed CCB would become dead mass and the remaining operational CCB's would have insufficient thrust to take the vehicle safetly into orbit.

T. L. James,

A manned launch vehicle can be built with one engine per stage without compromising saftey if pintle injector engines are used. Pintle injector engines have been tested with 25 different propellant combinations with thrusts from 5 pounds to 650,000 pounds. There has never been a failure of a pintle injector engine! This is due to the inherent combustion stability of pintle injector engines.