Sideways Engineering the SpaceX Dragon

[josh_simonson]

The motor for the Apollo LES had a fuel fraction of only 66%. And that's just the main motor, not including the other hardware. In total the Apollo LES had a fuel fraction of 40%. It is not worth spending high ISP second stage fuel to get a motor with a 40% fuel fraction to orbit. The additional tankage for an additional pound of fuel in the existing OMS/RCS system would have a fuel fraction well over 90% - and it's more efficient, safer fuel.<br /><br />Relative to the weight of the fully fueled second stage + dragon, the LES will have little mass penalty in the boost phase of launch, roughly 1/6 the weight of the LAS. So you can either have the LES with a fuel fraction of 40% in orbit, or you can add 83% of the weight of the LES in OMS fuel and tankage. It's better to just drop the thing after the boost phase unless the fuel fraction can be more than doubled over the apollo version.

 

[mrmorris]

<font color="yellow">"The motor for the Apollo LES had a fuel fraction of only 66%. And that's just the main motor, not including the other hardware."</font><br /><br />Do you have a source for that number? I did a Google and checked Astronautix, but neither gave me specs at that level. I don't doubt it -- the engine was over 30 feet long according to Astronautix and burned in about four seconds. That implies that it had to burn at ~15ft/sec -- which would likely mean a fairly open star-core to maximize surface area for a high thrust and fast burn rate. This would reduce the overall density of the propellant and therefore the propellant-to-mass ratio. Nevertheless, part of the reason I do this is to locate valuable data sources, so I'd like to know where the figure came from.<br /><br /><br /><font color="yellow">"In total the Apollo LES had a fuel fraction of 40%."</font><br /><br />Given the method by which the Apollo LES was implemented -- this isn't suprising. The photo below shows just how little of the hardware involved in the Apollo launch escape tower *was* the actual SRM. However, a launch-tower system isn't what I've advocated, but rather a cluster of SRMs mounted in the cargo section of Dragon. If you look at the mass fractions of various Solids in the Thiokol STAR line, you'll find they are all upward of 90%. SRMs in the size/thrust class I've proposed will be a bit lower -- after all there's a smaller fraction of propellant to thruster/casing, but I would still expect a fraction of greater than 85%. Because of the change in location -- the tower and boost protective covers wouldn't be required and mounting for a 'push' rather than a 'pull' requires less hardware. In all -- 'more than doubled' would be a reasonable expectation over a 40% mass fraction.

 

[josh_simonson]

http://www.apollosaturn.com/asnr/escape.htm<br /><br />I doubt the fuel fraction can be improved much, especially in a low overhead environment. Chop that 30' LES motor into 15 2' tall motors with individual nozzles and the fuel fraction goes down, not up. Considering the difference in ISP, trading the weight of a SRB LES for 83% of it's weight in a different fuel would be a good deal. SpaceX's kestrel engine is more than 17% more efficient the the shuttle SRB for instance. <br /><br />Remember also that the most pressing need for the LES is at launch, with full atmospheric pressure, so the nozzle will be optimized for sea level thrust, not vacuum ISP.<br /><br />The only reason to cart the LES to orbit would be to re-use it, but the refurbishing costs for solid motors are about the same as buying them new. Bringing such motors all the way to orbit is simply less efficient, less safe, and more expensive when you consider the opportunity cost in payload against the slim savings on recycling the SRMs. This is why I suggested a re-useable hybrid system that is discarded at stage separation. That would be cheaper and easier to re-use than solids (no cracking or delamination issues), be simpler and safer, and not incur as much of a payload penalty as bringing an LES motor all the way to orbit.

 

[mrmorris]

Looking at the link you provided, there's no figure that I can locate indicating a propellant to mass ratio, so I assume you calculated it yourself. Pulling the relevant statements from the document regarding mass and dimensions, we get:<br /><br /><i>"The subsystem looks like a large rocket connected to the command module by a lattice-work tower. It is 33 feet long and weighs about 8,000 pounds.<br /><br />Launch Escape Motor (Lockheed Propulsion Co.) Solid rocket motor about 15-1/2 feet long and 26 inches in diameter in steel case; weighs 4700 pounds. <br /><br />Pitch Control Motor (Lockheed Propulsion Co.) Solid rocket motor 2 feet long and 9 inches in diameter in steel case; weighs 50 pounds. <br /><br />Tower Jettison Motor (Thiokol Chemical Corp.) -- Solid rocket motor a little over 4-1/2 feet long, about 26 inches in diameter, and weighs about 525<br />pounds.<br /><br />Launch Escape Tower -- Welded titanium tubing truss structure, 10 feet long, about 3 feet square at the top and 4 feet square at the based where it is connected to the command module. It weighs about 500 pounds including all attachments, wiring and insulation. <br /><br />Boost Protective Cover -- It is 11 feet tall, 13 feet in diameter, and weighs about 700 pounds. "</i><br /><br />Unfortunately, the first part of the document indicates the entire subsystem massed about 8000 pounds, but the sum of all the major elements that it lists is only 6,475 pounds. If you match the elements we have mass broken out for to the picture above -- it's obvious that there simply isn't 1,500 pounds <b>left</b> after all you eliminate them. Ergo the 8,000 pound figure can't be right -- or one of the others is wrong. Looking at the LES entry from Astronautix, it lists 5,500 pounds for the whole system. That doesn't account for the values above, and so is no help.<br /><br />I located a NASA technical note Ap

 

[mrmorris]

I was reading through a paper today that did actually quantify drag and gravity losses. It gives both calculations and typical values. The calculations are specific to the flight profile flown, so I can't really work them specifically for Dragon. However, given an LEO velocity of ~25,600 fps, they give typical gravity losses from 3,500 fps to 5,000 fps (12.0% to 16.3%), and atmospheric drag losses of about 500 fps (1.9%).

 

[josh_simonson]

>The launch escape motor is the largest of the three rockets in the subsystem. It is approximately 15-1/2 feet long (including nozzles), 26 inches in diameter, an weighs 4,700 pounds. **********Two-thirds of this weight is in its polysulfide solid propellant.*********** The motor produces about 147,000 pounds of thrust in its 3.2 seconds of burning, enough to lift the 13,000-pound command module and carry it a mile away from the launch vehicle.<br /><br />Your ISP numbers probably aren't off, since some of the thrust is wasted by directing the nozzles to the side the effective ISP falls. Polysulfide has an ISP of about 220.<br /><br />Thrust is proportional to chamber pressure, but is also proportional to the size of the nozzle. If you break 1 big motor into a bunch of smaller motors with proportionally smaller nozzles, you need the same chamber pressure to get the same thrust out of that configuration. ISP also goes up with chamber pressure.<br /><br />Hybrid rockets have several useful safety features. They can be handled relatively roughly without concern for damaging the fuel grain (potentially allowing retrieval and re-use). They cannot fire uninententionally, so they are much safer to work around, and build. One could flick a cigarette down one's nozzle and nothing would happen. Hybrids are pressure fed with NO2, which means that if the chamber pressure rises for some reason, the backpressure will reduce the flow of oxidizer and reduce the rate of combustion, which will result in the pressure going down. Even if the grain has significant defects that result in a faster burn rate, this feedback mechanism keeps the hybrid from exploding. Last, hybrids are easy to reload. Pump the NO2 tank full again and slide a new fuel grain into the chamber and it's ready to go - this is how SS1 was reloaded. SpaceDev claims to be closing in on hybrid designs with ISPs aroud 300, which is the best that solids can do too.

 

[mrmorris]

<font color="yellow">"If you break 1 big motor into a bunch of smaller motors with proportionally smaller nozzles, you need the same chamber pressure to get the same thrust out of that configuration."</font><br /><br />So what you're saying is that one thruster which produces 100,000 newtons of force will have the exact same chamber pressure as ten thrusters that each produce 10,000 newtons?<br /><br /><font color="yellow">"One could flick a cigarette down one's nozzle and nothing would happen."</font><br /><br />While I wouldn't want to do that around a liquid booster, I'd be shocked to find a cigarette to provide enough heat to ignite a solid either. They're like explosives -- more stable than the average person thinks, albeit less stable than the experts would care for. This is the nature of materials with huge energy potentials -- the more emergy, the bigger the potential boom. As a powder, the solid propellant would be highly flammable and explosive. Once formed into a solid pack in the casing -- it's <b>not</b> going to ignite by accident.<br /><br />I'll grant that the hybrid has <b>no</b> possibility of igniting in that situation, wheres a solid has a .0001% chance of igniting. For a safety feature to be meaningful, there has to be a reasonable chance that the event it's saving you from will happen.<br /><br />I do appreciate you addressing the safety issue, but none of them provide a reason why there would be any concern about <i>'...light(ing) off a bunch of solids 1 meter from your body and adjacent to your heat shield while in orbit'</i>. Understand -- I am <b>not</b> saying that hybrids don't have safety features that solids cannot. What I am saying is that in the specific implementation that we are talking about -- namely a thrister or set of thrusters that fires for under five seconds -- I do not see where any of these apply. If the safety features of hybrids do not apply in this implementation, then it's not a factor in making a d

 

[mrmorris]

The shuttle underwent an expensive retrofit after Columbia -- adding sensors, cameras, etc. in order to be able to locate potential problems with the orbiters. Dragon needs a comprehensive Health Monitoring System to be designed into it from the outset. Given the relative sizes and complexities -- designing a system to monitor the health of Dragon <b>should</b> be a couple of orders of magnitude easier than it was for the shuttle. The problem, of course, is figuring out <b>everything</b> that needs to be monitored... *before* it causes a failure. Doubtless I'll miss some things, but I'm going to give a shot at working up some of the elements that would be required to give me something to use in the mass and power requirements for the system. We're going to want:<br /><br />MEMS accelerometers/impact sensors at multiple locations around the shell of the craft.<br />MEMS temperature gauges around the shell and in the aft equipment section -- especially on the RCS system.<br />I see a camera in the cargo section pointed aft to provide video of second-stage separation and aft-view from then on.<br /><br />We also need pressure, temperature, and CO2 monitors for the cabin. These are more a function of the ECLSS than the HMS though.<br /><br />It would also be very valuable to get video of the exterior of the capsule, to look for impact damage, provide close-in video of the ISS arm attachment to the capsule, etc. The shuttle, of course, does this now using the robot arm. Obviously this isn't practical for Dragon. However -- mounting a camera externally has lots of impact in terms of aerodynamics, TPS, etc. If a camera is mounted internally (perhaps in the forward equipment bay) than there must be a hatch for the camera, plus a mechanism to move it outside, and possibly pan as well. This can get complex and has serious re-entry implications if the exterior hatch fails to close. In addition, one camera would not be sufficient to monitor the entire craft. I see the prac

 

[larper]

Hmm. Instead of a camera that is fixed to the capsule, why not simply deploy one of those micro-sats things that they have played with on the station. A simple ball that contains a camera and a nitrogen propulsion system. It is deployed once on orbit, can "dock" with the capsule when not in use, and it transmits its video to the capsule.<br /><br />If it fails or is lost, not big loss. You throw it away before reentry anyway.<br /><br />This would give you the ability to completely survey the outside of the capsule without having to design a complex mechanism that is attached to the capsule and that has to pierce the capsule hull. <div class="Discussion_UserSignature"> <p><strong><font color="#ff0000">Vote </font><font color="#3366ff">Libertarian</font></strong></p> </div>

 

[mrmorris]

<font color="yellow">"...why not simply deploy one of those micro-sats things that they have played with..."</font><br /><br />If those ever get developed to the point they're reliable, cheap, lightweight, and tiny... sure. Until that time, Dragon needs to be designed with equipment that either exists, or can be made to exist with minimal development.

 

[larper]

I think the tech has moved along to the point where developing a working model would be fairly cheap and straight forward. Right now, it is a "solution looking for a problem." <div class="Discussion_UserSignature"> <p><strong><font color="#ff0000">Vote </font><font color="#3366ff">Libertarian</font></strong></p> </div>

 

[larper]

One other thing that a free-flying monitor would give you: Dramatic images and video in-flight. Like it or not, spaceflight needs to be sold to the American people, and dramatic pictures and video are a very effective way of getting people excited about space travel.<br /><br />The monitor-ball would be able to dock with a fixture on the outside of the capsule to:<br /><br />1) Allow the capsule to manuever without endangering the monitor and without having to have the monitor "keep up".<br /><br />2) Allow the capsule to dock to the ISS without the monitor endangering the ISS.<br /><br />3) Possibly allow the monitor to be recovered for reflight if the fixture is in a TPS protected recess.<br /><br />4) Allow the monitor to recharge its batteries. Flight time of the ball would be limited to propellent, not power. <div class="Discussion_UserSignature"> <p><strong><font color="#ff0000">Vote </font><font color="#3366ff">Libertarian</font></strong></p> </div>

 

[mrmorris]

<font color="yellow">"...developing a working model would be fairly cheap and straight forward..."</font><br /><br />Our definitions of 'cheap' and 'straightforward' are so far apart that further debate is contraindicated. Right now the project I'm pretty sure you're referring to is at the very very early stages of experimentation. <br /><br />I'm sorry Obi Wan -- I simply don't have that much faith in the force. I'm <b>not</b> going to put the blast shield down and start swinging.

 

[larper]

Umm....<br /><br />It was just a suggestion. You asked for comments and ideas. <br /><br />I would say that the development of SPHERES is at about the same stage as that of Dragon itself. Probably more advanced, since they have actually flight tested some technology.<br /><br />Also, note that I said "I think". <div class="Discussion_UserSignature"> <p><strong><font color="#ff0000">Vote </font><font color="#3366ff">Libertarian</font></strong></p> </div>

 

[mrmorris]

<font color="yellow">"It was just a suggestion. You asked for comments and ideas."</font><br /><br />My apologies, I didn't handle that well. Unfortunately, my alternate post that went into exhaustive detail was perhaps worse. <br /><br /><font color="yellow">"I would say that the development of SPHERES is at about the same stage as that of Dragon itself."</font><br /><br />I have to disagree for two reasons. First -- SPHERES is a laboratory experiment at this time. They are at the point of seeing if they <b>can</b> make a floating doobobby that can maintain station, locate a docking port, fly in formation, etc. It's only after (and if) they have determined that this platform is buildable that they will begin actually trying to develop practical applications for it. By contrast, Dragon is a reverse-engineered technologically upgraded version of capsules that flew in the sixties. The basic capsule technology is well developed, the instrumentation will be almost entirely COTS, it's the engineering and practical application that is being done now. Put in software terms, Dragon is release two or possibly three of an application designed years previous. SPHERES isn't even at alpha stage yet.<br /><br />Second, while there is still development to be done on Dragon, that development is <b>required</b> by definition. However, SpaceX is not likewise required to put in the research and development to make SPHERES into a platform capable of performing the tasks you envision. It would be better for them to do without external camera views than to throw hundreds of thousands at least, and more likely millions of development dollars into such a project. Keep in mind -- the only way to really <b>test</b> that platform is in zero-G. Zero-G is expensive to attain even if all of the other development requirements were cheap... which they wouldn't be.

 

[josh_simonson]

You'd only need cameras/microsat if the dragon went somewhere other than the ISS.<br /><br />The microsat may not have to be mobile other than gyros to maintain attitude. Dragon could drop it, back off and use it's RCS to rotate around a few axis, then pick up the microsat again when done. It'd probably still be expensive though.

 

[larper]

<blockquote><font class="small">In reply to:</font><hr /><p>It would be better for them to do without external camera views<p><hr /></p></p></blockquote><br />That I completely agree with. Figure out if an external visual scanning system is required in the first place. If it isn't, don't bother with the added complexity, not matter what the solution.<br /><br />SPHERES has flown on the ISS, just not outside. It is definitely years away from completion, but ONCE IT IS DONE should be considered as a visual inspection instrument if new spacecraft require such a system.<br /><br />I would think you need some sort of monitoring system for the heat shield, visual or otherwise. The Apollo 13 incident showed that, while such a system wasn't necessary, it sure would have been nice to have. Mercury 1 could have used something as well.<br /><br />As has been noted, however, no system is required if docking to the ISS. <div class="Discussion_UserSignature"> <p><strong><font color="#ff0000">Vote </font><font color="#3366ff">Libertarian</font></strong></p> </div>

 

[mrmorris]

<font color="yellow">"You'd only need cameras/microsat if the dragon went somewhere other than the ISS."</font><br /><br />I agree that the ISS can photograph the capsule on arrival, and then it can photograph it again prior to departure. However, this can't compare to the ability to have continuous telemetry from the time of orbital insertion until just before re-entry. Given the system I described, the Dragon would have the ability to take pictures of the exterior just before departure from the ISS, compare those photos to ones taken earlier and be able to scan for impacts. Since the photos would be essentially identical -- I'm sure the process could even be set up to use computerized matching techniques to point out areas of particular concern.

 

[mrmorris]

<font color="yellow">"...ONCE IT IS DONE should be considered ..."</font><br /><br />Pretty much what I said on my first reply to the notion... <img src="/images/icons/smile.gif" /><br /><br /><font color="yellow">"As has been noted, however, no system is required if docking to the ISS. "</font><br /><br />I've already argued that point. I'll go so far as to say that Dragon Cargo doesn't really require it for ISS visits. However, Dragon crew <b>should</b> have such a system, even if the chances it will catch something dangerous are sub 1%. Given that -- it should be developed and tested before it's needed for the crewed version. Essentially the best way of testing it is to put it on Dragon Cargo... which brings us full circle. <img src="/images/icons/smile.gif" />

 

[rocketman5000]

Other possible sensors could be ultrasonics. Using ultrasonic sensors would allow detection of cracks, or mircometor damage without the use of hatches and camaras using visual inspection. The sensors are small and could be perminantly attached to the inside of the aeroshell

 

[mrmorris]

<font color="yellow">"Using ultrasonic sensors would allow detection..."</font><br /><br />I did some Googling on the subject but wasn't able to come up with much. The most relevant hit I got was regarding ultrasonic detection of leaks in a spacecraft. Two-sensor ultrasonic spacecraft leak detection using structure-borne noise is an interesting article if you really like reading material full of phrases like: 'propagation of the incident wave', 'circular synthetic aperature analysis' and ''use the Lanczos inverse, modified with exponential eigenvalue rolloff'. <img src="/images/icons/smile.gif" /><br /><br />In terms of finding structural cracks, etc., I did find this:<br /><br /><i>"Imperium, Inc. <br />Robert Lasser <br />Ultrasound camera for real-time simultaneous C-scan & A-scan subsurface aerospace material imaging<br />Under several current and previous government contracts, Imperium has developed and patented a novel handheld ultrasound camera capable of quickly finding subsurface defects in aircraft structures. The output of the device is both a real time ultrasound C-scan over an area, and a full waveform A-scan at a region of interest. In this paper, our presentation includes the technical development of the sensor, its evolution and its NDE applications for composites and metals, including real time imagery of delaminations, voids, and cracking. A multi-angle camera, built for the US Navy, will be reported on to describe the technique for shear wave imaging. The multi-angle camera can be easily switched between longitudinal and shear modes. Video imagery will be presented of relevant subsurface defects in metals and composites with the latest technique with the adjustable camera head."</i><br /><br />However, in that application, at least, it apparently involves an external camera being

 

[rocketman5000]

Ultra sonics are used quite often for non destructive testing of the thickness of parts in manufacturing. While in college I was given a tour of a lab that was doing testing for the rail industry to use ultrasonics to test for cracks in rails and cutdown on the inspection costs. If you know the time period you are supposed to receive a signal return and you get something different you'll know something is wrong.<br /><br />think of the ultrasonics like Britian's homechain of radar station in WW2. Cracks would be analogous to "boogies" Each sensor could tell you how faraway a crack or hole is from the sensor. By trangulations the location could be ascertained. <br /><br />The second paragraph is just of my thoughts and conjectures of what should be therotically possible. I doubt there is such a working system as I can't think of an application at the moment.

 

[mrmorris]

<font color="yellow">"Ultra sonics are used quite often for non destructive testing of the thickness of parts in manufacturing. "</font><br /><br />I've heard of this -- it's analogous to the italicized paragraph in my response. However, as I indicated ion the post, it requires an external camera to take the pictures. Even after the pictures are taken, it requires considerable analysis to locate any problems (i.e. more analysis than could be done onboard autonomously). From your post, I thought you might be aware of something that could actually be emplaced <b>in</b> the hull/struture/etc. that could be used for ongoing diagnostics.<br /><br />Unless something COTS or near-COTS exists, I think this is another tech that SpaceX will have to wait on NASA to front the R&D for.

 

[rocketman5000]

the systems I am speaking of use surface contact. I had never heard of a camara system

 

[mdodson]

Is heat shield material too spongy for ultrasonics? Its damping might be too high to propagate much of a wave.<br /><br />Eddy-current testing is something I've seen used on aluminum aircraft parts.<br /><br />I wonder if some on-orbit testing will be specified for the Orion heat shield?