Sunday, October 14, 2012

Re: On the lasting importance of the SpaceX accomplishment.

Copyright 2012 Robert Clark 

Congratulations to SpaceX on their second successful flight to the ISS. However, it is disturbing that there have been engine anomalies on all the flights, the last being the most serious:

 It is reassuring that the mission was able to be completed even with one engine shut down. However, I don't think that would be an acceptable state of affairs for manned flights to have an expectation that during any flight at least one engine would malfunction and need to be shut down, including to the extent that that engine would be destroyed, shedding debris in the process.
 I think SpaceX should investigate the possibility of producing a larger version of the Merlin to reduce the number of engines required. It's been reported also that NASA is not too sanguine on the possibility of using so many engines on a manned vehicle.
 There was a discussion of this possibility on the forum:

Should SpaceX aim for a 330,000 lbs engine rather than am F1 class engine?

 The idea was generally disparaged on that forum, but I think it is a good idea. SpaceX was considering building a 1.5+ million pound thrust engine referred to as the Merlin 2 as part of a proposal to NASA for a heavy lift vehicle. They estimated a $1 billion development cost for the engine. Based on thrust size, we might estimate the development cost for this smaller upgrade at 1/5th of this so only $200 million. Given the billion dollar contracts SpaceX already has for NASA and commercial satellite launches, there is little doubt that SpaceX could again get private financing for the development of this engine.
 SpaceX has shown that it is able to cut development costs when it follows a private financing path. I think that would be the ideal approach to follow in this case as well.
 If they did develop the 330 klb. engine, that would still require 5 engines for the Falcon 9 v1.1 first stage. My preferred solution then to minimize the number of engines at an affordable cost would be to go for a 500,000 pound thrust engine. Again estimating based on thrust size, this would be a ca. $300 million development cost, not too much more than the 330 klb case. But in this case you would only need three engines.

   Bob Clark


  1. Robert, the NK-33 is already available at that thrust capacity with fairly good thrust to weight ratios. The problem is reusability. How do you land that stage? This is a non-negotiable concept for Elon Musk as it should be for you. If you can find another way to land the stage then single of few engine stages are completely reasonable, otherwise they are not because you will not be able to compete with SpaceX and they will take away your business. Another problem is the roll control. With the new Soyuz they solve that problem with a combined four nozzle roll control and thrust enhancing upper stage engine in the first stage, which if you were alert could also be used as an upper stage engine, deorbing engine and a landing engine, if and only if it was throttleable, which unfortunately it is not. These are very simple concepts, I'm surprised at your naivete with regards to these issues.

    1. There is an ongoing discussion over on the Nasaspaceflight forum in regards to how SpaceX would achieve a controlled landing with the Falcon 9 first stage. The main problem being discussed is that you can't hover when the thrust of the engine is so much higher than the dry weight plus a little bit of reserve fuel for the landing.
      However it's been pointed out that by making repeated short burns you can cancel out the downward velocity to allow a controlled landing, even though you would not be hovering. The same would work for the NK-33 engines. The main problem with using the NK-33 is that SpaceX would want to use their own engines. That's why I was estimating costs for developing these larger size Merlin versions.
      Still, I myself for the increased safety and response to unexpected conditions such as winds, I like the ability to hover. This can be solved rather easily actually. Since this will only be used for the dry weight plus low fuel reserve case you would only need small extra engines for hovering.
      For dense fuel such as kerolox, the T/W is in the range of 100 to 1. Using an estimate of 13,000 kg for the dry weight of the first stage, plus only a fraction of this as fuel reserve, we can estimate the extra weight of the engines as ca. 130 kg, a minimal subtraction from the payload. These engines could also be used for roll control.
      The size of these engines would also be small as measured by nozzle diameter, since being used only near sea level you could use short nozzles.

      Bob Clark

    2. However it's been pointed out that by making repeated short burns you can cancel out the downward velocity to allow a controlled landing, even though you would not be hovering.

      First of all the atmosphere provides a very convenient way to cancel your downward (and indeed forward) velocity and there are easy ways to enhance that drag effect.

      I'm not sure who proposed a series of short burns but that's an 'idiotic' concept for engines as powerful and difficult to start as the Merlin or the NK-33. That would work for specialized hypergolic designs (pulse engines) only such as what was done on the Mars Polar Lander.

      You are just making things up here, Robert. Either you have to throttle down according to the gross mass ratio of the stage, which varies from 30 to 1 for hydrocarbons and 10 to 1 for hydrogen, or you have to use a smaller engine, or you have to use a completely separate landing system such as the Dracos and super Dracos. And it also requires an engine restart, which is not trivial. I'm all for innovation, which is why I have tested these concepts in simulation. You would be well advised to do the same, remembering that engineering reality is vastly different.

      I like the ability to hover.

      I like a lot of things. I'm particularly fond of reality, although there isn't a great deal I can do about it. Hovering capacity in a commercial launch vehicle is way down on the list of things I'd like to have. Mostly, I just want the stage back in one piece and I don't particularly care where it ends up as long as it's not on top of an innocent bystander or their property. These are things that can be accomplished with no fuel at all and very little weight penalty. Landing engines are only required to go from terminal velocity to zero velocity, and that terminal velocity can be made very low indeed, compared to say, hydrocarbon booster release velocity.

    3. That being said if Mr. Musk were to develop some BFEs - with a Merlin landing engine he'd be good to go. But even then geometry still gives him seven at the least, and either by serendipity or design that also gives him nine just by bumping the edge engines out a bit, so he doesn't lose much. I'm kind of interested in if he can pack them into his existing tank diameter. I think his original rationale was that he couldn't build a big engine when he started and he decided more thrust is better and nine gives you more than seven. But it's still basic geometry if your intent is to maximize the thrust of the booster.

  2. Well, men flew on the Saturn-V 3rd flight, after both of the first 2 flights had very serious anomalies of one kind or another. Of course, that was the more adventurous NASA of the 1960's.

    Seems like at least one of the Apollo missions had an engine-out and press-on incident with the first or second stage. I'm pretty sure -13 did, not so sure about any of the rest. Both the first and second stages had 5 engines. We were lucky with the S-IVB 3rd stage, which only had one engine. No backup there.

    What you say about too many small engines may be true, but is dependent upon the individual reliability of the engines. I hadn't heard about any anomalies with the Merlin on previous flights, but then I had not looked, either.

    The Russian N-1 moon rocket failed because of too many engines, of too low an individual reliability. If the reliability is not high enough, then you essentially guarantee failure in large clusters. We saw that same effect in the early days of multi-engine aircraft, too.

    What I liked about the Merlin engine-out incident was the automatic detection of the problem, and shutdown, all at superhuman speed. A pressure loss like that reported means a leak somewhere upstream of the throat. You have a small fraction of a second to react, before that kind of a leak goes completely catastrophic. Their system reacted properly, and in-time.

    Kudos to Spacex for that. It's a safer rocket to ride, precisely because of that capability, than many before it that actually did carry men. Atlas-D, for example.

    But, why was there a leak? That will have to be answered and addressed.


    1. Thanks for the informative response. I don't think SpaceX has acknowledged the existence of a leak, though you very well might be right. If it was a fuel or oxidizer leak that potentially would be a very serious problem that would need to be rectified.
      That one of the engines was not only shut down but likely destroyed on a vehicle intended for manned flights would be a very serious concern to NASA.

      Bob Clark

    2. Bob:

      I think it was a leak in the combustion pressure vessel.


    3. Thanks for that. Calling it the combustion chamber makes it more clear, and makes it more disturbing. In this article it's called a breach in the "fuel dome":

      Dragon enjoying ISS stay, despite minor issues – Falcon 9 investigation begins.
      October 19th, 2012 by Chris Bergin

      The image on this site shows the area that would have been breached:

      A breach of the combustion chamber is very serious because the combustion chamber is where both the pressures and heat are highest. Note that this would be at the top where most of the complex and delicate engine components are located. Quite likely hot combustion products shooting out at high pressure in this area would have produced shrapnel from this part of the engine.
      This is very serious because these lead directly into the propellant tanks. This means likely if the engine had not been shut down it would have led to an explosion. This is a different scenario from an engine just being shutdown because it is giving anomalous readings but the engine remains intact.

      Bob Clark

  3. Other Falcon 9 engine anomalies are discussed here:

    Falcon 9 Drops Orbcomm Satellite in Wrong Orbit
    Posted by Amy Svitak 2:17 PM on Oct 08, 2012

    Bob Clark

  4. To be completely factual, the combustion chamber is NOT where the pressures are the highest in a rocket engine. The outlet from the turbopump is necessarily higher than the combustion chamber or the propellants wouldn't flow into the chamber. Thus, the combustion chamber has the highest temperature with the downstream components steadily losing pressure and temperature as theheat and pressure in the combustion chamber are converted into kinetic energy.

    Though, from that wireframe, the fuel dome =/= the combustion chamber as it's where the propellants are piped through to be injected into the chamber.