Letter From Tom Stafford to NASA Regarding Crewed SpaceX Falcon 9 Fueling Issues
“There is a unanimous, and strong, feeling by the committee that scheduling the crew to be on board Dragon spacecraft prior to loading oxidizer into the rocket is contrary to booster safety criteria that has beenin place for over 50 years, both in this country and internationally. Historically, neither the crew nor any other personnel have ever been allowed in or near the booster during fueling. Only after the booster is folly fueled and stabilized are the few essential people allowed near it. Furthermore, in addition to the personnel risk, there is the risk of operating the engines outside their design input conditions. As an experienced “Prop” guy you know the problem here as well as anyone. Pump-fed chemical engines require a sufficient and consistent input pressure to reduce the likelihood of cavitation or unsteady flow operations. We are concerned that there may be insufficient precooling of the tank and plumbing with the current planned oxidizer fill scenario, and without recirculation there may be stratification of oxidizer temperature that will cause a variation in the input conditions to the oxidizer pump.”
Mr. Stafford is one hundred percent correct. With limited experience with densified propellant with rockets why would one risk the lives or the Astronauts?
Just recall that video of the Falcon 9 explosion where that Dragon capsule falls down in the fireball and think of that as being on LC39A with Astronauts inside.
SpaceX should just stick to using the densified propellant loading process with non-crewed missions.
Now, SpaceX is rushing to launch before the year ends. They will be using LC39A. Can you imagine how that will put the Commercial Crew Program behind if they have another accident at LC39A.
Stafford assumes that there’s less risk bring in the crew after the main fuelling. But that’s based on previous launchers, not on a separate risk assessment. (Ie, it’s “how I was taught to do things”.)
The F9 explosion occurred after the main LOx fuelling had finished. Precisely when the crew would be approaching the pad, give or take ten minutes.
Densified propellants may mean that the safest time to load the crew is before fuelling starts, when the rocket is inert. That’s the danger with trying to apply old lessons onto new systems.
Errr, I’m not sure what video you watched, but it wasn’t a Dragon flight, so there was no capsule.
He was referring to the earlier flight in 2015 to the ISS that failed:
http://www.parabolicarc.com…
Bob Clark
I don’t believe LOX loading was complete when the accident occurred which is part of the problem. According to Spaceflight 101 list <http: spaceflight101.com=”” falcon-9-ft-countdown-timeline=””/> fuel load starts about 35 min prior to launch and 2nd stage LOX is complete around 2 minutes before launch.
Is there something magical about LC39A? You seem to emphasize it, but I’d think the same arguments would apply to any crew launch.
NASA crew launches will be from LC- 39A.
Can someone explain to this novice exactly why SpaceX wants to, or must, load fuel with crew onboard
The Full Thrust version of the Falcon 9 uses supercooled liquid oxygen. At the lower temperature, the density of liquid oxygen is higher, so the can get more in the tanks. That imposes a fairly short time between fueling and launch, so they’d like the fueling to be one of the last things done before launch. I’m not sure if they could go back to the Falcon v 1.1, which uses “normal” temperature liquid oxygen. It would certainly be a performance hit and might affect recovery of the first stage.
Got it. Thank you
I wonder just what delta they get from supercooled- and I’ve not been able to learn just how much colder they’ve made the two liquids, either.
Not sure what Space X has achieved. The goal for liquid oxygen is cooling to its triple point. Slush LOX was ruled out for Shuttle due to too low a bang for your buck from a GLOW perspective..
SpaceX does not use slush LOX. The triple point of Oxygen is -218 C, they use approximately -207 C Oxygen.
For comparison, Shuttle used LOX just below the boiling point, but the Antares and the Soyuz 2-1v used LOX at lower temperatures.
Soyuz 2-1v (-192°C)
Antares (–196°C)
Density of LOX at various temperatures
LOX at -184 C = 1141.7 kg/m3 (just below boiling point)
LOX at -192 C = 1185.4 kg/m3
LOX at -196 C = 1204.3 kg/m3
LOX at -207 C = 1254.9 kg/m3
LOX at -217 C = 1299.0 kg/m3 (just above freezing point)
While I can understand Mr. Stafford’s point of view, I do not find it convincing. I am not the first to point this out, but the most hazardous part of the prelaunch process is crew boarding. With the hatch open and personnel on the access arm, there is no immediate way to escape. Once the hatch is closed and the LAS armed, the crew is protected from minor hazards and can escape major ones.
Moreover, failure modes must be understood and the design or procedures corrected to eliminate them. Launch vehicle mishaps are almost never random events. They are deterministic, almost certain to happen under specific circumstances. If the cause is determined now and the hardware or process changed to eliminate the failure mode, it won’t happen again. SpaceX believes the failure occurred because the temperature in the COPV was allowed to drop below the freezing point of the surrounding LOX, resulting in sharp-edged LOX crystals forming around the tank. I think it is also necessary to test a sample of the COPVs through multiple warming and cooling cycles to ensure there is no intrusion of the cryogenic liquid into the composite laminations around the pressure vessel, and testing of a sample of the tanks to their actual burst pressure while immersed in LOX.
Based on previous history, the most hazardous periods are vehicle fueling and liftoff. I don’t recall an accident occurring during crew loading. Typically the crew is loaded during a time when the vehicle is stable. Loading fuel and oxygen takes some time (30 to 60 min?) and is not a stable period. During that whole time, the crew would be required to be ready for an abort.
I think you are using “hazardous” in different ways. He’s factoring in the probability of the crew surviving a serious incident. That’s 100% before they get near the pad, and (in theory) in the high nineties once they are in the capsule and the escape system is armed. Therefore, the time while they are boarding is the most “hazardous”, since that is the time when they are most vulnerable.
Strictly speaking the incident was caused by helium pressurization rather than fuel loading. COPV ruptures are rare but difficult to predict, and can occur when a system is apparently in a static condition. To the extent that the failure mode has been identified, it will be prevented in the future by keeping the helium above the LOX freezing point. If there are other unforseen COPV failure modes, they could occur at any point after COPV pressurization. The Soyuz pad incident did not occur during fueling per se, but at the point when the fueling umbilical was disconnected close to launch.
Reading the extended comments, it appears that Stafford’s focus is less on astronaut loading than it is on the use of a helium tank inside the oxygen tank, drawing as some have a parallel to Apollo 13.
I agree but I believe the Saturn V did use helium tanks inside the LOX tanks, although they were metal rather than composite. The problem with Apollo 13 was a poorly designed heater in the crygenic LOX dewar for the fuel cell power supply.
In the long term I believe SpaceX plans to shift to methane as a second stage propellant, and to use “self-pressurization”, i.e. pressurize the LOX tank with gaseous oxygen and the methane tank with gaseous methane.
A crewed Dragon (any Dragon, really) should have its abort escape system ready during stage fueling. It would be an exciting but safe ride.
On a previous thread I have stated that I see crew loading before fueling as opposed to after fueling as a case of weighing up pros and cons and the best people to do that are in my opinion the Space-X engineers who know their LV better than anyone else and the Astronauts who will in the end sit atop the vehicle when the blue touch paper is lit. As much as I have great admiration for Tom Stafford and his knowledge this really sound as if it comes straight from the ULA PR department, and without wishing to cast any aspersions one has to wonder how much influence via friendships and associations etc. formed in the “Old Space” days the likes of ULA and other old space companies have on this “advisory” committee.
I see this as more of a traditionally-minded rather than (deliberately) pro-ULA statement. It looks like the logic is: There is nothing wrong or unprofessional about the way we (NASA) do things. Therefore, we do things in the right way. We don’t do things that way. Therefore, that is the wrong way to do things.
Stafford’s review is an independent review which is much better than just a SpaceX review. Dismissing Stafford and his team’s immense experience as just a competitor’s PR is rather cavalier. SpaceX should be fully capable of defending their process and procedures with facts instead of innuendo.
As I said I have the utmost respect for Tom Stafford’s experience and knowledge, however it is extremely “cavalier” for an advisory committee to assume that Space-X engineers are incapable of making considered decisions where the conclusions of those decisions differ from those set out at the beginning of the Space Age. Space-X should not have to “defend” or justify every single working decision it makes but merely show that it’s decision making processes clearly put safety as a priority.
What do we actually know about the scope and level of detail in the Stafford review? If they simply compared SpaceX practices with the practices NASA would follow, noted the differences, and said “bad because we wouldn’t do it that way.” the review would be pointless and worthless. If they did a detailed analysis of every difference between SpaceX and NASA practices, it would have required much more time and resources than they had. So what did they actually do? For any technical issue, you can’t base a rational decision on the executive summary of the report.
This is General Stafford of the aborted Gemini 6A launch?
Yes.
never mind
Ok, so the old school way is to board the crew after fueling is complete. In other words, let’s expose the crew and their support team directly to an instantaneously exploding rocket while climbing up and into the capsule. Who says a fully fueled rocket is stable? It’s not, especially when some tank gets over-pressured and explodes like the Falcon 9 on September 1st. By definition, when President Obama walked up to a Falcon 9 sitting on the launch pad with Elon Musk several years ago, there was no fuel to be found anywhere which means there was zero risk of it exploding with fuel onboard.
The new school way is to board the crew before fueling it. Since no fuel is present, there is absolutely no danger to fuel exploding in over-pressurized tanks. When fueling commences and completes, there are no humans standing on the gang plank exposing themselves directly to exploding tanks. The only humans around are those secured inside the capsule ready to be rocketed a safe distance away in a matter of seconds. If I were a crew, I would want to be inside the capsule than outside of it when the rocket explodes. It becomes a no-brainer for the crew support helping the crew ingress. They would do it either way since their job depends on them agreeing to expose themselves to the risk of the rocket exploding while helping the crew get in their spacecraft. If they don’t agree, someone who is willing to risk that for money will.
You do realize that all this has been thought of many times before? The consensus is that it’s safer to load the crew after fueling. The SpaceX explosion happened during the act of fueling, apparently due to a procedural error.
Well, yes and no; SpaceX’ materials (and methods) differ sufficiently from traditional thinking that the General’s comments, while apparently not fully thought out, still point to a subject not completely resolved.
I tend to agree with you because past performance is an indication of future results. The point is who is in charge here? Elon (Mr. Innovation) or NASA (Ms. Risk Averse Stagnation). It is an arm wrestle that never stops until one opponent relinquishes their control. Telling Elon what to do is a mistake in my opinion because it squashes innovation. If you run him off, he is likely to go do it his way somewhere else with a whole new set of people willing to follow him and accept the risk. NASA told him it was not safe to launch his rocket a long time ago and he did it anyway. At some point, Elon won’t need to rely on NASA to slow things down and stagnate into the risk averse pool of mediocrity that is the primary goal of promoting a jobs program.