# Rockets should land on their tails, as God and Heinlein intended **URL:** \ **Category:** Miscellaneous and Personal Stuff I Must Share\ **Created:** [May 2, 2014, 7:11am UTC](https://boards.straightdope.com/t/rockets-should-land-on-their-tails-as-god-and-heinlein-intended/687490 "2014-05-02T07:11:25Z")\ **Posts on this page:** 9\ **Page:** 4
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [August 19, 2014, 5:41am UTC](https://boards.straightdope.com/t/rockets-should-land-on-their-tails-as-god-and-heinlein-intended/687490/61 "2014-08-19T05:41:09Z")
Missed this one… > [@buddy431](#): > > The advantages of crossfeed in real life are less than they are in Kerbal space program. In KSP, the engines have much lower thrust to mass ratios and the tanks have proportionally higher masses compared to the amount of fuel they containe, compared to real life. This makes it a huge advantage in KSP to dump engines and empty tanks as soon as you don’t need them, but the advantage is muted for real life rockets. Yep. Another thing is that the Heavy won’t have full-fledged crossfeed. Only the outer three engines on each side of the center core will pull propellant from the booster cores, but early in the flight all the engines are going. At some point, they’ll probably shut down the inner three engines to reduce thrust, and then they’ll have full crossfeed. I was thinking a while back that they could achieve some of the same results with half the complexity by only pulling oxidizer from the side cores. You would change the RP-1/LOX ratio so that it all runs out at the same time. There are some issues with the idea but if plumbing became a bottleneck it might work out. > [@buddy431](#): > > For real life numbers, [SpaceX claims](http://www.spacex.com/falcon-heavy) a high-end payload of about 53 tonnes to LEO with a falcon heavy with crossfeed, compared to 45 tonnes without. Well, that’s pretty good for a “free” technology (nothing is ever truly free, but normally that kind of performance boost would require extra cores, solid boosters, or a fuel change). > [@buddy431](#): > > Does anyone know what the mass of the payload of the first falcon heavy flight is going to be? SpaceX apparently doesn’t really like to talk about the details of their missions. No idea. The first launch of the FH is supposed to have–perhaps among other things–the [Green Propellant Infusion Mission](http://www.satellitetoday.com/technology/2014/08/15/aerojet-rocketdyne-exec-says-smallsats-likely-to-be-first-adopters-of-green-fuel/) (GPIM) demonstrator. I presume most of the payload will be a dummy, though. I read the John D. Clark book _Ignition!_ recently. It’s basically a history of rocket propellant development in the US. Though it’s a bit sad–almost every single thing they came up with was ultimately thrown out, and only the “obvious” propellants such as LOX/hydrocarbon survived. The Russians had a much less involved program and yet we both came up with the same set of propellants (main difference is that we had much more hydrogen experience, but that was still one of the obvious propellants). The book doesn’t specifically mention the hydroxylammonium nitrate that is to be used in the GPIM mission, though it does say that they tried attaching many things to an ammonium nitrate base (giving an ionic liquid) and that they didn’t work very well, at least compared to hydrazine. At any rate, it’s an interesting and amusing read. ---
**Author:** ![Stranger\_On\_A\_Train](https://avatars.discourse-cdn.com/v4/letter/s/13edae/32.png) [@Stranger\_On\_A\_Train](https://boards.straightdope.com/u/Stranger_On_A_Train)\ **Post date:** [August 19, 2014, 5:47am UTC](https://boards.straightdope.com/t/rockets-should-land-on-their-tails-as-god-and-heinlein-intended/687490/62 "2014-08-19T05:47:30Z")
> [@Dr.Strangelove](#): > > I looked it up, and at least some Falcon 9 missions have used the Northrop Grumman [LN-200](http://www.northropgrumman.com/Capabilities/LN200FOG/Pages/imu_spacex.aspx) IMU on both the Dragon capsule and Falcon rocket. It’s certainly a high quality IMU, but there’s nothing extraordinary about it in terms of specifications. It uses MEMS accelerometers and fiber optic gyros. It’s qualified for deep space missions of up to 6 years and has been actually used for missions of over 10 years. It’s rated for shock/vibration/etc. loads well beyond what a rocket would experience. > > So I highly doubt your claim that the IMU requires bench calibration after each flight. In fact, I doubt it requires any kind of recalibration at all–it is entirely a solid-state unit and there aren’t too many progressive degradation mechanisms. The brochure makes a specific point of the long lifetime. As you note, replacement would only offer the opportunity for misinstallation. I am intimately familiar with the Litton (now Northrop Grumman) LN-200 series IMU, which isn’t just used by SpaceX but by a wide range of large targets, space launch vehicles, and spacecraft, as well as more mundane applications such as aircraft and tactical missiles. This is actually a medium precision commercial IMU that has been adapted for launch and space vehicle use. The fact that it uses FOGs instead of mechanical gyroscopes is not the point; in order to obtain enough precision for launch vehicle applications (which are several orders of magnitude higher than typical aircraft or tactical missile applicants) the unit has to be calibrated and the unit’s particular drift rates and bias inputed as guidance parameters, and these will change during high rate operation requiring recalibration. Stranger ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [August 19, 2014, 6:54am UTC](https://boards.straightdope.com/t/rockets-should-land-on-their-tails-as-god-and-heinlein-intended/687490/63 "2014-08-19T06:54:34Z")
> [@Stranger\_On\_A\_Train](#): > > The fact that it uses FOGs instead of mechanical gyroscopes is not the point; in order to obtain enough precision for launch vehicle applications (which are several orders of magnitude higher than typical aircraft or tactical missile applicants) the unit has to be calibrated and the unit’s particular drift rates and bias inputed as guidance parameters, and these will change during high rate operation requiring recalibration. _What_ high rate operation? The unit is rated at over 11,000°/sec. If your rocket is spinning at even, say, 100°/sec you are having a bad problem and _[you will not go to space today](http://xkcd.com/1133/)_. I am not familiar with the LN-200 specifically, but I do have direct experience with MEMS accelerometers, and they don’t behave as you say. They absolutely need to be calibrated individually–once. And of course they need to be characterized across a temperature and perhaps voltage range. But they don’t degrade in a way that requires periodic recalibration the way mechanical or non-solid-state units might. You imply that some extraordinary level of precision is needed while acknowledging that the LN-200S is only a medium precision unit. If SpaceX actually needed high precision, they would have used something like the Honeywell MIMU, which [actually does have orders of magnitude better performance](http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20100005703.pdf) than the LN-200S. The LN-200S is used on various Mars rovers. If it can survive a launch, a Martian injection (which generally includes a spinup operation far in excess of what the rocket would experience), a rather bumpy descent to the Martian surface, and then finally go around for many years without having to fly a technician out to check on things, then I think it can handle a single flight. ---
**Author:** ![Stranger\_On\_A\_Train](https://avatars.discourse-cdn.com/v4/letter/s/13edae/32.png) [@Stranger\_On\_A\_Train](https://boards.straightdope.com/u/Stranger_On_A_Train)\ **Post date:** [August 19, 2014, 7:46am UTC](https://boards.straightdope.com/t/rockets-should-land-on-their-tails-as-god-and-heinlein-intended/687490/64 "2014-08-19T07:46:16Z")
> [@Dr.Strangelove](#): > > The LN-200S is used on various Mars rovers. If it can survive a launch, a Martian injection (which generally includes a spinup operation far in excess of what the rocket would experience), a rather bumpy descent to the Martian surface, and then finally go around for many years without having to fly a technician out to check on things, then I think it can handle a single flight. The Mars rovers require only enough precision over a travel rate of a few scores of feet a day, experiences almost negligible instantaneous angular rates, and requires only enough precision to know where it is relative to the location of the relay satellite which given the short distance it travels in a day is less than the spread of the communications link. This is very unlike rocket launch vehicles which require high precision over very long distances through a duration of a few hundred seconds with high instananeous angular rates. And no, that rate isn’t a constant body rotation rate but the dynamic rotation experienced by the IMU during flight while it is strapped down directly to vehicle structure; that means that every tiny rotation the vehicle (or even the mounting structure that the IMU is strapped down to) goes through is integrated which compounds any drift and bias errors as it goes. It is clear you really don’t understand how the IMU functions in a space launch vehicle or why such precision (requiring accurately characterizing the drift rates and bias in the gyros) is necessary in order to get sufficiently accurate position and velocity states. You continue to persist on asserting that it just must not be of any concern, all to justify the continued insistent that a “bathtub curve” used to characterize population reliability of long life, steady operation devices that experience minimal wear must also necessarily apply to rocket launch systems which experience highly dynamic loads and operate close to the basic limits of material capability with continual wear and fatigue throughout their very limited life. Regardless that you think this, it just isn’t so. Stranger ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [August 19, 2014, 9:32am UTC](https://boards.straightdope.com/t/rockets-should-land-on-their-tails-as-god-and-heinlein-intended/687490/65 "2014-08-19T09:32:51Z")
> [@Stranger\_On\_A\_Train](#): > > The Mars rovers require only enough precision over a travel rate of a few scores of feet a day, experiences almost negligible instantaneous angular rates, and requires only enough precision to know where it is relative to the location of the relay satellite which given the short distance it travels in a day is less than the spread of the communications link. Funny; the slow angular rate sounds precisely like the time when you would need very low drift rates. Far more so, in fact, than a rocket stage that completes its mission in 20 minutes. Although it’s not the only means of navigation, the gyro is surely also used for pointing the rover in a new direction to move in. Rovers do everything quite deliberately and the mission planners would obviously want to know any discrepancies between wheel turn and actual direction change. If the turn takes the better part of an hour, they might be annoyed if the gyro has drifted significantly in that time. > [@Stranger\_On\_A\_Train](#): > > And no, that rate isn’t a constant body rotation rate but the dynamic rotation experienced by the IMU during flight while it is strapped down directly to vehicle structure; that means that every tiny rotation the vehicle (or even the mounting structure that the IMU is strapped down to) goes through is integrated which compounds any drift and bias errors as it goes. The vehicle is not making tiny rotations at thousands of degrees per second. It’s ridiculous–that would imply accelerations on the circumference of hundreds of gees. Vibrations being coupled into the mounting, sure. But I think SpaceX can figure out how to add isolation to their mounting hardware. A rubber band worked wonders on the IMU for my quadcopter, but I’ll bet SpaceX has something fancy. Maybe one of those $10,000 rubber bands. > [@Stranger\_On\_A\_Train](#): > > It is clear you really don’t understand how the IMU functions in a space launch vehicle or why such precision (requiring accurately characterizing the drift rates and bias in the gyros) is necessary in order to get sufficiently accurate position and velocity states. Where did I say that characterization isn’t necessary? Oh right; I said the exact opposite. My only claim is that a robust and relatively low-performance unit like the LN-200 doesn’t need a full bench calibration after 20 minutes of relatively gentle use. I should add that any programmer that treats drift rate as a _guidance parameter_ should be taken out back and shot. Ideally, calibration parameters such as these are stored in firmware within the instrument and the adjustment is invisible externally. Under duress, the adjustment can be done in the IMU code, ideally as early as possible and as cleanly separated from the rest of the code as is reasonable. But a guidance parameter? Unless we have very different notions of what that phrase means, I really hope no one is plugging calibration values into that bit of the code (even if they are recalibrating every time). ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [August 23, 2014, 2:01am UTC](https://boards.straightdope.com/t/rockets-should-land-on-their-tails-as-god-and-heinlein-intended/687490/66 "2014-08-23T02:01:08Z")
Boom! Well, not all tests go entirely according to plan. SpaceX’s latest test with their F9R-Dev1 vehicle [auto-terminated](http://www.kwtx.com/home/headlines/Rocket-Explodes-at-Space-X-272370541.html). Musk’s [response](https://twitter.com/elonmusk/status/502974683864518657): _Three engine F9R Dev1 vehicle auto-terminated during test flight. No injuries or near injuries. Rockets are tricky …_ Of course the media reports that the rocket “exploded,” which is ostensibly true, but the explosion was a byproduct of the failure, not the reason. From the video it’s clear that something caused the rocket to tip over, at which point the range safety officer hit the big red button. It’ll be interesting to see what the actual reason is. The engines have been through a fair number of cycles already, so maybe something failed on one of them. I didn’t see any fire coming out of the end (well, any more than usual), so I’d guess against combustion chamber burn-through or the like, but maybe the fueldraulics system was damaged. Or it could just flat out be a software bug in their guidance system. Who knows, maybe they forgot to calibrate their LN-200’s before the flight :). ---
**Author:** ![Skywatcher](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/skywatcher/32/254_2.png) [@Skywatcher](https://boards.straightdope.com/u/Skywatcher)\ **Post date:** [August 23, 2014, 2:14am UTC](https://boards.straightdope.com/t/rockets-should-land-on-their-tails-as-god-and-heinlein-intended/687490/67 "2014-08-23T02:14:10Z")
So … the OP has gone from bombs to rockets? ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [August 23, 2014, 2:31am UTC](https://boards.straightdope.com/t/rockets-should-land-on-their-tails-as-god-and-heinlein-intended/687490/68 "2014-08-23T02:31:11Z")
> [@Skywatcher](#): > > So … the OP has gone from bombs to rockets? Not so farfetched. My associates say I bear a striking resemblance to Dr. Werner von Braun. ---
**Author:** ![Irishman](https://avatars.discourse-cdn.com/v4/letter/i/b487fb/32.png) [@Irishman](https://boards.straightdope.com/u/Irishman)\ **Post date:** [August 24, 2014, 9:32pm UTC](https://boards.straightdope.com/t/rockets-should-land-on-their-tails-as-god-and-heinlein-intended/687490/69 "2014-08-24T21:32:15Z")
> [@Dr.Strangelove](#): > > Fair enough, but that wasn’t really my point. The military is traditionally conservative–once they’ve qualified a vehicle, they don’t seem to like changes. I’ve heard that SpaceX’s small fixes from one vehicle to next pose a problem in this respect. To me, this implies that the AF cares less about the legs themselves than they do about having a vehicle that will fly in the exact configuration as it was certified with. NASA works similarly. Very little tinkering is allowed before something has to be recertified. There are lots of fine rules about how much and what you are allowed to tweak before it no longer is equivalent and the identity rolled to the next dash number. Even on crit III hardware. It’s all about knowing what you have and what you certified. > [@Stranger\_On\_A\_Train](#): > > First of all, misassembly of components such as the IMU that was installed backwards in the Proton-M that crashed in June 2013 is not an example of a latent failure; this is a process and inspection failure (and possibly a design failure as well insofar as the part could be installed improperly and still function). Latent failures are failures of a component or system which are not detectable or inspectable by any feasible method. Agreed. > [@Dr.Strangelove](#): > > I don’t see how classifying the type of failure excludes it from contributing to the bathtub curve. Significant design failures tend to get weeded out early on for mass produced devices, though even that’s not always the case. I suppose if you are trying to extend the bathtub curb to the vehicle as a whole, though I’m not sure that’s appropriate. [Previous page](https://boards.straightdope.com/t/rockets-should-land-on-their-tails-as-god-and-heinlein-intended/687490.md?page=3)