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Aug 5, 2015
From a Million Miles Away, NASA Camera Shows Moon Crossing Face of Earth
A NASA camera aboard the Deep Space Climate Observatory (DSCOVR) satellite captured a unique view of the moon as it moved in front of the sunlit side of Earth last month. The series of test images shows the fully illuminated “dark side” of the moon that is never visible from Earth.
The images were captured by NASA’s Earth Polychromatic Imaging Camera (EPIC), a four megapixel CCD camera and telescope on the DSCOVR satellite orbiting 1 million miles from Earth. From its position between the sun and Earth, DSCOVR conducts its primary mission of real-time solar wind monitoring for the National Oceanic and Atmospheric Administration (NOAA).
It's probable that the impact object comes from a Chinese rocket launched in 2014. //
It was engineer Jon Giorgini at NASA's Jet Propulsion Laboratory who realized this object was not, in fact, the upper stage of a Falcon 9 rocket. He wrote to Gray on Saturday morning explaining that the DSCOVR spacecraft's trajectory did not go particularly close to the Moon. The second stage would, therefore, be extremely unlikely to strike the Moon. This prompted Gray to dig back into his data and identify other potential candidates.
He soon found one: the Chinese Chang'e 5-T1 mission launched in October 2014 on a Long March 3C rocket. This lunar mission sent a small spacecraft to the Moon as a precursor test for an eventual lunar-sample return mission. The launch time and lunar trajectory are almost an exact match for the orbit of the object that will hit the Moon in March.
"In a sense, this remains 'circumstantial' evidence," Gray wrote. "But I would regard it as fairly convincing evidence. So I am persuaded that the object about to hit the moon on 2022 Mar 4 at 12:25 UTC is actually the Chang'e 5-T1 rocket stage."
As SpaceX charges forward with full and rapid rocket reuse, the company's stretch goal is to fly each "ship" every six to eight hours. These "ships" are the Starship launch system's upper stage, which is 50 meters tall and designed to carry payloads into orbit or be refilled there to fly to the Moon or Mars. The first-stage "booster" could fly even more frequently, as much as once an hour, he predicted. The first stage makes a six-minute flight to space and back and is intended to be loaded with propellant on the ground in just 30 minutes. //
SpaceX has unquestionably come a long way since 2016, when Musk first revealed the full scope of his plans to build a launch system that could establish a self-sustaining settlement on Mars. By his own estimates, such a venture would require 1 million tons of food, water, and construction materials. The settlers will need to build an entire industrial base to mine the red planet, and manufacturing consumer products will require a huge infrastructure base to refine and shape materials.
This is an incredible logistical challenge. Consider that throughout the last five decades, during the entirety of its Mars exploration program, NASA has landed a grand total of a couple of tons on the surface of Mars.
For his settlement plan, therefore, Musk proposed an unprecedented rocket and spacecraft. During a 90-minute speech in Guadalajara, Mexico, five years ago, Musk spoke of his “Interplanetary Transportation System,” or ITS. This was a huge and fully reusable launch system with a second-stage spaceship that could be fueled in low Earth orbit and then flown to Mars fully laden with supplies or dozens of settlers. Eventually, after more name changes, the ship would be christened Starship.
The 2016 speech was striking in its candor. Musk laid bare his entire vision for the first time for all the world to see. It was easy to criticize, and many did. The general viewpoint among the established space community at the time was that such a vision was preposterous.
And who could blame the critics? Only four weeks before Musk gave his speech, SpaceX had blown up its second Falcon 9 rocket in a year, losing the Amos-6 satellite on the launch pad on September 1. The company was also going to be years late delivering a Crew Dragon capability to NASA and its astronauts. And for all the talk of reusable rockets, SpaceX had not yet re-flown a single Falcon 9 rocket. Critics watched the Guadalajara speech and saw Musk the Charlatan—over-promising, grasping for government money, and spewing lies about the future when he couldn’t deliver in the present.
But in the five and a half years since Musk’s first Mars moment, the billionaire has answered those critics. SpaceX has not lost a single rocket since Amos-6. In fact, the Falcon 9 booster recently set the record for the longest streak of successful launches by any rocket ever. SpaceX also has become a reliable provider of crew transportation services to NASA, years ahead of its competitor Boeing, which NASA paid 60 percent more for the same service to low Earth orbit. And Falcon 9 rocket first stages have now flown 11 times, with no end in sight.
I actually had an interesting conversation with Elon Musk about this. The question was, what would it take to build a self-sustaining settlement on the surface of Mars? It would take one million metric tons of stuff, propellant, the 3D printers, the stock for the 3D printers, food, agriculture, domes, wherever you're gonna live, all of it, to get to the point where you could have all that on Mars and those people could then survive without intervention from Earth. So one million metric tons, if you think about that, it takes a very large rocket and a whole sort of sophisticated spacecraft. The Curiosity mission was what, a couple billion dollar mission to get to Mars? And that Rover was one ton. That's the challenge we're talking about, sending one million Perseverances or Curiosities worth of mass to Mars. It's an enormous challenge. //
Right now we're focused on stepping one foot on Mars and then 10 and then a hundred, slowly building up to have a permanent human presence. There's no physics reason preventing us from inhabiting Mars, it's a matter of technology and engineering and patience, and most importantly, money. But there's no reason why we can't eventually be on Mars. Humanity will have a presence on Mars. Well, I'm not going.
SpaceX has been launching Falcon 9 rockets thick and fast of late. With 10 launches since the beginning of December, the company has flown rockets at a rate greater than one mission a week. And another launch could happen as soon as today, shortly after noon (18:13 UTC), with a Starlink satellite launch planned from Florida.
Lost amid the flurry of activity are some pretty significant milestones for the Falcon 9 rocket, which made its debut a little more than a decade ago. //
The Falcon 9 rocket has now launched a total of 139 times. Of those, one mission failed, the launch of an International Space Station supply mission for NASA, in June 2015. Not included in this launch tally is the pre-flight failure of a Falcon 9 rocket and its Amos-6 satellite during a static fire test in September 2016.
Since the year 2020, the Falcon 9 has been the most experienced, active rocket in the United States, when it surpassed the Atlas V rocket in total launches. Globally, the still-flying Russian Soyuz and Proton rockets have more experience than the Falcon 9 fleet. The Soyuz, of course, remains the king of all rockets. It has more than 1,900 launches across about a dozen variants of the booster dating back to 1957, with more than 100 failures.
The Falcon 9 reached a notable US milestone in January, equaling and then exceeding the tally of space shuttle launches. During its more than three decades in service, NASA's space shuttle launched 135 times, with 133 successes. To put the Falcon 9's flight rate into perspective, it surpassed the larger shuttle in flights in about one-third of the time.
There is no way to know how many missions the Falcon 9 will ultimately fly. At its current rate, the rocket could reach 500 flights before the end of this decade. However, SpaceX is also actively working to put its own booster out of business. The success of the company's Starship project will probably ultimately determine how long the Falcon 9 will remain a workhorse. //
Speaking of safety, this is where the Falcon 9 rocket has really shone of late. Since the Amos-6 failure during its static fire test, SpaceX has completed a record-setting run of 111 successful Falcon 9 missions in a row. It probably will be 112 after Thursday.
There are only two other rockets with a string of successful flights comparable to the Falcon 9. One is the Soyuz-U variant of the Russian rocket, which launched 786 times from 1973 to 2017. The other is the American Delta II rocket, which recently retired. (Eventually, the Atlas V rocket could also exceed 100 consecutive successes before its retirement later this decade.)
The 20th-century was marked by competition between two Cold War adversaries, the Soviet Union (USSR) and the United States, to achieve superior spaceflight capability.
The space race led to great technological advances, but these innovations came at a high cost. For instance, during the 1960s NASA spent $28 billion to land astronauts on the moon, a cost today equating to about $288 billion in inflation-adjusted dollars.
In the last two decades, space startup companies have demonstrated they can compete against heavyweight aerospace contractors as Boeing and Lockheed Martin. Today, a SpaceX rocket launching can be 97% cheaper than a Russian Soyuz ride cost in the ’60s.
The key to increasing cost efficiency?
SpaceX rocket boosters usually return to Earth in good enough condition that they’re able to be refurbished, which saves money and helps the company undercut competitors’ prices.
A Washington-state based aerospace company has exited stealth mode by announcing plans to develop one of the holy grails of spaceflight—a single-stage-to-orbit space plane. Radian Aerospace said it is deep into the design of an airplane-like vehicle that could take off from a runway, ignite its rocket engines, spend time in orbit, and then return to Earth and land on a runway. //
The current design of Radian One calls for taking up to five people and 5,000 pounds of cargo into orbit. The vehicle would have a down-mass capability of about 10,000 pounds and be powered by three liquid-fueled engines. The idea would be to get as close to airline operations as possible, by flying, landing, re-fueling, and flying again. //
If Radian can succeed technologically, large markets would likely open. A vehicle like Radian One would be well suited to fly people to commercial space stations in low Earth orbit, which NASA seeks to foster development of by 2030. These planes could also perform Earth observation work and play a role in bringing back space-manufactured goods. There is also the potential for point-to-point travel on Earth.
There can be no question that this is a hugely challenging endeavor that many people have tried before. Will Radian find the right stuff, at the right moment in time? We'd like to think so.
The Dark Ars Scholae Palatinae
ColdWetDog wrote:
But they're not that expensive either. In the grand schema of boats being holes in the water that you dump money into, a couple of modified barges and a general purpose Gulf workboat or two is pretty meh. All depends on how much money there is in boosting payloads a bit further.
And, of course, SpaceX is looking to offshore launches in the future so as not to annoy everyone in a fifty mile circle. That will take some marine capability and having some experience in that field may be useful.
The estimate I saw from someone in the shipbuilding industry was $10 million to build and outfit the barge, $60 thousand per month in berthing fees, and $120 thousand per launch in operating costs, not including insurance. Use one twice per month over ten years, and you're looking at an uninsured cost of around $200,000 per launch. Reduce the cadence to once per month, and it's over $260,000 per launch. For smaller launchers, it might not make economic sense the way it does for Falcon 9.
Edit to add: the $200k per launch is roughly the minimum possible even if launch cadence increases, because the cycle time for barge operations means you'll have to add more barges to consistently get more than 2 landings per month.
With clearing skies and moderate winds, SpaceX's Falcon 9 rocket rideshare mission safely launched into space on Thursday. The first stage then sent its upper stage and a payload with 105 small satellites on its way into low Earth orbit. The Falcon 9 first stage made a smooth landing back near its launch site.
Remarkably, this single Falcon 9 rocket first stage has now launched 550 satellites into orbit, as well as one Cargo Dragon and one Crew Dragon. It has flown, on average, every two months since its first launch. It would seem that rocket re-use is more than a fad. //
niwax Ars Tribunus Militum et Subscriptor
Hispalensis wrote:
Quote:
Upon launch, it will become the third Falcon 9 first stage that SpaceX has flown 10 times.
First reaction: hmm, that's interesting...
Delayed reaction: 10 times? That is insane!
It is funny how our brain quickly accepts the extraordinary as the new normal
This booster has delivered two Dragon capsules to the ISS, first with two astronauts then with 3t of supplies, plus a GEO commsat, 295 LEO commsats, 9 traffic monitoring satellites, 48 earth observation satellites, an in-orbit data transfer demonstration constellation, a space tug with 18 payloads, a synthetic aperture radar and an optical spectrum observatory and 10-15 random other cubesats.
That is before todays launch of some 105 new satellites.
I'll repeat what I said way back on SSO-A: The unbelievable projections of the small sat industry have come true, only to be gobbled up by a workhorse F9 on it's 10th flight.
How on Earth do you patch the software on a computer orbiting the Moon? Very carefully.
FRANK O’BRIEN - 1/30/2020, 12:30 PM
In the afternoon of January 31, 1971, the flight thundered away from the Kennedy Space Center on its Saturn V launch vehicle after only a brief 40 minute hold for weather. After restarting the S-IVB third stage for trans-lunar injection (TLI), the command module Kitty Hawk and her crew were on their way to the Moon. //
However, less than four hours before the scheduled landing, controllers noticed that according to the indications on their consoles in Mission Control, the LM's Abort pushbutton appeared to have been pressed. When asked via radio, Shepard confirmed that no one on board Antares had pressed the Abort button—which meant there was a short-circuit or other electrical issue somewhere inside the LM's complicated guts.
This was potentially a mission-ending problem: if the button was pressed and the engine was firing, the LM would immediately begin its abort procedure as soon as the lunar descent started, making a landing impossible.
Under hard time pressure, the ground had to quickly figure out what was wrong and devise a workaround. What they came up with was the most brilliant computer hack of the entire Apollo program, and possibly in the entire history of electronic computing.
To explain exactly what the hack was, how it functioned, and the issues facing the developers during its creation, we need to dig deep into how the Apollo Guidance Computer worked. Hold onto your hats, Ars readers—we're going in. //
Once again the LM’s orbit carried it behind the Moon and out of communications, leaving the crew with just a smattering of procedures and few options. The normal work of finishing the system configurations continued, and the crew maneuvered to the descent attitude, tidied up the cabin, and put on their helmets and gloves. In the meantime, Don Eyles’ team was feverishly working to find a better solution to the Abort bit issue.
Working the problem involved unraveling a complex, daisy-chained series of events. The main landing program, P63, does not perform all of the landing computations itself. Rather, it orchestrates a large number of Jobs and Waitlist Tasks, each performing a necessary part of the effort. Another Job running concurrently was the SERVICER, which sampled attitudes and accelerations that fed into the guidance equations. SERVICER, in turn, scheduled Routine R11 as a Waitlist Task, running every 0.25 seconds. R11 first checked whether aborts are enabled (via the LETABBIT flag), and if so, it then checked the status of the Abort bit. With aborts allowed, and the abort signal set (presumably because the crew pressed the Abort pushbutton), P63 is terminated, the AGC's Major Mode switches to P70, and the abort process begins. //
This was the breakthrough. If R11 could be spoofed into believing that an abort was already in progress, then it didn’t matter if the Abort button was pressed or not—the button's state would be ignored.
But how did R11 actually inform itself about whether or not an abort was executing? The answer was in plain sight on the DSKY: The Major Mode display, under the label “PROG”. //
In less than two minutes after the descent to the Moon had started, the Abort pushbutton had been successfully disabled and the computer was happily managing the descent. All indications were that the next lunar landing would be successfully accomplished in eight more minutes. //
As Antares passed through 32,000 feet (about 9,700 meters), Mitchell became concerned and informed controllers that the radar hadn’t locked on. Houston replied with a suggestion to pull the circuit breaker for the radar, and then power the system back on, which did the trick. Solid radar data began flowing into the computer, and the crew quickly agreed to accept it. Just a few minutes later, Shepard made a smooth and on-target touchdown at the Fra Mauro highlands.
After the mission, when asked if he would have attempted to land without the radar, the notoriously hard-charging Shepard reportedly replied, “You’ll never know.” In Gene Kranz’s Failure is Not an Option autobio, Kranz recounts that Flight Director Jerry Griffin was convinced that Shepard would indeed make an attempt to land without radar, and would just as certainly have had to abort when fuel ran out. //
The idea that a single errant switch could derail a lunar landing attempt was unacceptable. After the mission, a new variable in the AGC code was introduced that allowed the crew to "mask out" (that is, to ignore) the Abort and Abort Stage pushbuttons. The scenario assumed that a failing switch would be recognized well before the descent began, and commands could be entered in time to prevent an inadvertent abort. Like the fix used for Apollo 14, this would make initiating an abort through a pushbutton impossible, and any urgent situation would have to be performed on the Abort Guidance System. //
The recovery from Apollo 14’s Abort switch failure can only be described as brilliant and heroic. But the most important enabler of this effort was that the software, while fiendishly complex, could be understood by a small team of developers. Modern hardware and software, with its extensive protection schemes, virtualization and dynamic program management simply would make such a simple hack impossible. Faced with a comparable problem today, even if the fix were trivial, the solution likely would require large amounts of code to be recompiled, tested and uploaded to the spacecraft. This may not be possible given the short timeframe necessary to save the mission.
In the end, Apollo 14’s fix truly represented the “Spirit of Apollo," where talented teams made the impossible happen.
The other piece of news, less well-covered but still important, emerged during a news conference on Saturday. NASA's Mission Systems Engineer for the Webb telescope, Mike Menzel, said the agency had completed its analysis of how much "extra" fuel remained on board the telescope. Roughly speaking, Menzel said, Webb has enough propellant on board for 20 years of life.
This is twice the conservative pre-launch estimate for Webb's lifetime of a decade, and it largely comes down to the performance of the European Ariane 5 rocket that launched Webb on a precise trajectory on Christmas Day.
Prior to launch, the telescope was fueled with 240 liters of hydrazine fuel and dinitrogen tetroxide oxidizer. Some of this fuel was needed for course adjustments along the journey to the point in space, about 1.5 million km from Earth, where Webb will conduct science observations. The remainder will be used at Webb's final orbit around the L2 Lagrange point for station-keeping and to maintain its orbit.
So every kilogram of fuel saved on Webb's journey to the Lagrange point could be used to extend its life there. Because ten years seemed like a fairly short operational period for such an expensive and capable space telescope, NASA had already been contemplating a costly and risky robotic refueling mission. But now that should not be necessary, as Webb has at least two decades of life.
A lot of this comes down to the performance of the venerable Ariane 5 rocket. NASA and the European Space Agency reached an agreement more than a decade ago by which Europe would use its reliable Ariane 5 rocket to lift the telescope into space, and in exchange, European scientists would get time to use the telescope. //
The Ariane 5 program also selected the best components for Webb based upon pre-flight testing. For example, for the Webb-designated rocket, the program used a main engine that had been especially precise during testing. "It was one of the best Vulcain engines that we've ever built," Albat said. "It has very precise performance. It would have been criminal not to do it." //
Albat admitted that the days prior to launch were exhausting and nerve-wracking. But soon after the launch, Albat said he and the entire European space community could take pride as Webb took flight and began to unfurl its wings. Now, he said, "I feel totally relaxed." The same can be said for a lot of scientists who have been watching Webb's development for two decades.
The temperatures measured where the oxidizer enters the vehicle at an umbilical connection were about four degrees Rankine or Fahrenheit too high. “The requirement is about 169.1 Rankine, that’s -290.57 degrees Fahrenheit, so we didn’t quite get there on Monday,” Bassler explained. “What we saw at the temperature for the interface to the Core Stage was 173 Rankine, which is -286.67 degrees Fahrenheit.”
A slow flow rate of propellant chills down the facility and vehicle lines to start the loading process, so they aren’t shocked by the sharp, several-hundred degree drop in their temperature. Normally after the chilldown phase, the loading transitions to a slow fill phase, which is at a higher flow rate, and then a fast fill phase to load the propellant tank to the top.
The concern with the warmer propellant in the early phases of loading is geysering. “The thing that the team is trying to protect in the [temperature] limits that got tripped are relative to the concern of developing a gas bubble in [a] feedline,” SLS Program Manager John Honeycutt said. “[A bubble] could end up collapsing and then you could have quite a bit of energy released when the liquid oxygen above that bubble could release and fall back down the feedline.”
Would love a detailed breakdown from Ars on the impact of these launches on the climate and environment. Thank you!
Everday Astronaut has an excellent article on this: https://everydayastronaut.com/rocket-pollution/
Summary: some exotics suck, but modern rockets are inconsequential at current volumes.
The impact of rocket pollution is mostly symbolic, especially tourist flights. They're seen as the most conspicuous consumption by much of the general public. Why should an average Joe who is struggling to get by sacrifice to combat climate change while billionaires are dumping hundreds of tons of carbon into the air to fly to space?
There are already good answers to that question, but they are nuanced, and the answer could be quite clear. Bezos' rocket already runs on Hydrogen, he should be paying a little extra for green Hydrogen, just for PR reasons. Musk has already committed to using synthetic methane on Starship. Branson doesn't have an easy answer, but he's mostly irrelevant in the symbolism arena. //
Would love a detailed breakdown from Ars on the impact of these launches on the climate and environment. Thank you!
~16.25 billion gallons of jet fuel burned per year.
One Falcon 9 launch, 25,000 gallons of Kerosene in the 1st stage (the 2nd stage is effectively burning it above the atmosphere, so not sure you can count that).
30 launches in 2021. 750,000 gallons of kerosene.
Total around the world launches of Rockets in 2021 was, what? 60 ish? Many smaller rockets. Let's just double that though and say 1.5 million gallons.
That is ~3.33% of all of the jet fuel burned...in one day. For an entire YEAR of launches at the current rate.
Metholox will produce somewhat lower emissions per joule of energy released to launch a rocket.
So basically, you are talking less than 1/10th of 1% of the entire aviation industry. It would be nice if it was zero emissions. Also of note, SpaceX is looking to do carbon capture and generate methane for launch at some point (though not soon, they will be using in situ wells at their launch facility for the methane).
Until such a point as rocket launching maybe approaches >1% of aviation emissions I think we can safely consider it a rounding error.
edit actually the above should be less than 1/100th of 1%. //
Would love a detailed breakdown from Ars on the impact of these launches on the climate and environment. Thank you!
A very large percentage of the information about those hurricanes comes from satellites. So there’s a pretty big impact from rocket launches.
SpinLaunch is playing with a different, electric model for mass launching to orbit. It is trying to throw mass into space, but there are challenges.
What’s novel about it? Well, the launcher is a giant solid sling inside a vacuum chamber. It has a big counterweight on a short arm at one end, and a long end that holds the payload at the other. Over 90 minutes or so, it uses electricity to bring the rotating arm with the dart on it up to absurd revolutions per second, about 10,000 gravities of centripetal force.
Then, at exactly the right microsecond, they let the dart go. It goes up through a tube with a light plastic sheet keeping the vacuum in and air out, and continues upward under its own inertia for 10 kilometers right now.
Their goal is to get the device up to the 200 kilogram range and throw satellites with final stage rockets into orbit. //
The parts I have concerns about are the following:
First, while the demonstrator is amazing, as a prototype it’s well below the rule of thumb of quarter-scale by volume for mechanical system prototypes. They assert that it’s a 3rd scale, but that’s by diameter, not 3-dimensionally. As such, it’s a great demonstrator of the principal and as impressive as any piece of awesome engineering that cost $38 million to build, but doesn’t derisk nearly enough of the major technical challenges in my opinion. This is a fairly constant challenge in aerospace, as actual quarter-scale prototypes are wickedly expensive. //
The second challenge is that the sabot, enclosed orbital vehicle, and payload have to be able to survive not only 10,000 G lateral forces, but the orbital vehicle and payload have to manage the rocket forces when they kick in. The sabot is shed by that point, but it’s much easier to build something that will survive extreme forces in one direction than something that will survive extreme forces at right angles to one another.
The payload has to be able to survive both as well, which means that the engineering and packaging of the payload has just become harder. We’re not going to throw iron bars into space for processing with orbital solar smelters. Non-compressible liquids are possible, but liquids like to slosh, so the sudden change of forces would be really difficult to dampen. //
Third, the gripping component of the spinning arm has to be able to support the sabot at 10,000 gs and also release it in a microsecond without causing any wobble. That’s an extreme engineering edge case by itself. //
Just preventing the sabot from crumpling under the stress at the attachment point, or even folding in half is also seriously non-trivial engineering. 10,000 gs at what is necessarily a small set of attachment points around the center of gravity of the sabot leaves dangling sabot under serious strain at either end. The more gs you pile on, the more attachment points you need, and the less ability you have to release them instantly.
A 1,000 kg total package for a 200 kg payload at 10,000 gs is equivalent to 10 million kg of weight on earth. Electromagnets are absurdly strong, but a 3 Tesla magnet only puts out 522 psi, and the strongest electromagnet is 35 Tesla. That degree of magnetic field will also fry a lot of things. It’s unclear to me what their attachment solution is intended to be, but it’s expected to do an absurd job.
Fourth, the rotating arm’s moment of inertia is going to change radically and instantly at release. The buildup of velocity takes 90 minutes, so it’s easy to balance, but the release is instant, with a couple of tons of mass at 10,000 gs disappearing at the long end of the arm. //
Fifth, atmospheric buffeting at release will be non-linear. Hypersonic speeds in the bottom parts of Earth’s atmosphere are non-trivial, which is engineering speak for really hard. To hit orbit, it will be at serious multiples of Mach speed at ground level. So, also very, very noisy. Not a good neighbor.
My intuition — and it’s only a somewhat informed guess — suggests that the combination might not be surmountable on Earth. However, on the Moon or Mars, a lot of things become much simpler. No atmosphere or an incredibly thin atmosphere both eliminate or reduce the need to create a vacuum chamber in the first place, and make the hypersonic sabot’s interaction with the atmosphere immaterial. Much lower orbital velocity requirements mean that the issues related to 10,000 gs aren’t there, just a smaller but still absurd number of gs.
To acheive full thrust of the first stage F-1 engines of the Saturn V only liquid oxygen should be pumped into the combustion chambers. A mix of gaseous and liquid oxygen would reduce the desired mass flow of oxygen to the engines and may damage or even destroy the oxygen pumps. A reduced mass flow of oxygen would reduce thrust and endanger a successful liftoff.
But the LOX lines through the fuel tank had a bad ratio of surface to volume, much worse than the large oxygen tank. The heat flow to the LOX lines would cause the LOX inside to boil heavily.
By bubbling of cold gaseous helium through the LOX lines and tank the LOX was cooled before launch below the boiling temperature. The gaseous helium and oxygen at the top of the tank are exhausted through umbilicals and the evaporated LOX is replaced by toping of more LOX from the large tanks at the launch pad.
Just before ignition the helium bubbling is finished and the very cold LOX within the lines and tank does not boil as long as its temperature is below the boiling point. So the oxygen pumps and combustion chambers could be fed with pure liquid oxygen free from bubbles.
At a pressure of 1 bar, the temperature of liquid boiling oxygen stabilizes at 90 K. For sub-cooling of LOX, the temperature should be lower. It is possible to cool LOX by forced evaporation by a pressure lower than 1 bar. But the LOX tank in a rocket should be as light as possible. If the pressure inside the tank is substantially lower than outside, extra strength and weight is necessary. But according to these papers: (1) (2) and (3) there is another method.
Cold helium gas is injected at the bottom of the tank and the bubbles raise in the LOX. At the surface of the bubbles, LOX evaporates into the bubble and cools the remaining LOX. But extra space is needed for the bubbles in the LOX and for the gas mixture of helium and oxygen above the liquid level. For topping off, the injection of helium is stopped and the remaining space is filled with LOX. Figure 8 of the first paper shows the effect of different helium gas temperatures. The cooling works best with helium at 85 K, but even helium at 150 K cools the LOX.
A bubble injected into the LOX consists of 100 % helium and 0 % oxygen at first. The LOX around this bubble would boil just like in a vaccum because the partial pressure of oxygen in this bubble is zero. Even a bubble consisting of 50 % helium and 50 % oxygen is able to cool LOX at 90 K. Without sub cooling in a tank with boiling LOX at 90 K, the gas above the liquid is 100 % oxygen and the partial pressure of oxygen is 1 bar. If the partial pressure of oxygen is lower than 1 bar in the gas above the liquid or inside the bubbles, the LOX is cooled by evaporation.
At the launch pad the LOX may be precooled using a heat exchanger with ground suplied liquid nitrogen boiling at 77.355 K. To save weight of the rocket, this heat exchanger should be outside the rocket but close to it. Liquid nitrogen and oxygen should not be mixed to avoid solving of nitrogen within the LOX. Cooling with helium bubbles may be used within the rocket LOX tank.
The same question could well be asked of the LM's descent engine and the main engine on the Apollo service module, however, which did both need to fire in free-fall. In those cases, the smaller RCS thrusters on the LM or CSM were fired first, to "settle" the tankage and separate the fuel from the helium. In the LM case, this "ullage burn" was about 7.5 seconds. The first couple of service module burns -- typically for mid-course correction while en route to the moon -- generally didn't need an ullage burn prior, as the tanks would be full of propellant with little or no volume of helium. SPS burns later in the mission did require ullage burns. The RCS thrusters produced about 100 lbs of thrust each, and four would be used for the ullage burn, yielding roughly 1/200 g acceleration.
The same RCS ullage burn technique would also apply to a situation where the descent engine failed and the ascent engine needed to be used for abort from free-fall, or in flight testing of the ascent engine.
That, in turn, raises the question of how helium ingestion was avoided in the RCS thrusters, since they were also helium pressurized. In those cases, the helium was separated from the propellants by a teflon bladder, so the helium didn't mix with the propellants. This was more practical to do on the smaller scale of the RCS propellant tanks than it would have been for the larger engines.
NASA's Space Transportation System (STS) vehicle, better known as the Space Shuttle, used two single engine Solid Rocket Boosters (SRB) as Stage 0, an engineless external tank providing propellant for the three Space Shuttle Main Engines (SSME) on the orbiter as stage 1, and additional two Orbital Maneuvering System (OMS) hypergolic liquid-propellant rocket engines on the Space Shuttle orbiter as stage 2.
The two solid rocket boosters used roughly 500,000 kg (1.1 Mlb) of a 11-star perforated solid propellant cake of Ammonium Perchlorate Composite Propellant (APCP - a mixture of of ammonium perchlorate, aluminium, iron oxide, PBAN or HTPB polymers, and an epoxy curing agent) each, that provided 124 seconds of burn time with a specific impulse (Isp) of 269 s that provided 12.5 MN of thrust per SRB and the external tank that came in three different configurations (mostly progressively reducing tank's own weight) capacity was 629,340 kg (1,387,457 lb) of cryogenic liquid oxygen (LOX) as the oxidizer and 106,261 kg (234,265 lb) of cryogenic liquid hydrogen (LH2) as the fuel components of the bipropellant LOX/LH2 that provided 480 seconds of burn time with specific impulse of 455 seconds, resulting in 5.45 MN of thrust at sea-level (for the Super Lightweight Tank or SLWT, the last and most advanced of the three versions used with STS).
So to answer your question directly, not counting the OMS propellant as per the specifics of your question, the total mass of all propellants of the SRBs (stage 0) and the external tank (stage 1) was at launch of the STS 1,735,601 kg (3,821,722 lb). The solid rocket boosters provided roughly 83% of liftoff thrust for the Space Shuttle and were the largest, most powerful solid-propellant motors flown to date.
One might imagine that 60+ years of development must have produced large gains, but chemical rocket performance is fundamentally limited by the amount of energy in the chemical fuels, and the 1960s engines were already getting at least 2/3 of the maximum theoretically possible performance (see comparison table below). //
The usual primary metric is specific impulse.
But specific impulse is a somewhat unintuitive quantity to understand, so let's start with effective exhaust velocity, which is the average speed of an exhaust particle (in the backward direction). For example, the Rocketdyne F-1 engines used in the first stage of the Saturn V (the Apollo rocket) have an effective exhaust velocity of 2.58 km/s at sea level.
What does 2.58 km/s mean in terms of rocket performance? It means if you build a rocket whose weight is about 63% fuel, and you fire the engine in deep space until the fuel runs out, the rocket will now be going 2.58 km/s faster in whatever direction it was pointing: //
So, what is change in velocity, Δv, good for? In the solar system there are two main uses for Δv: launching from the surface to achieve orbit, and transferring from one orbit to another. The article Delta-v budget has some examples, but the most relevant to Apollo is the Δv to get into low Earth orbit from a sea level launch, which is (very roughly) around 10 km/s. That breaks down as about 8 km/s of required velocity to stay in orbit (any slower and you'll come back down) and 2 km/s spent lifting the rocket against gravity and pushing through the air on the way up. //
So let's take a quick comparison of ve for the F-1 and the SpaceX Merlin engine. This is a relatively fair comparison because both burn RP-1 (refined kerosene) and liquid oxygen in a gas-generator cycle. These characteristics are good for a first stage due to high energy density per unit volume and high thrust, although other fuels have better ve
F-1 2.58 km/s (sea level)
Merlin 2.77 km/s (sea level)
F-1 2.98 km/s (vacuum) 65% of max
Merlin 3.05 km/s (vacuum) 66% of max
Theoretical max 4.61 km/s (vacuum)
The theoretical maximum is based on the total chemical energy in the fuel. //
Finally then, what is specific impulse? It's obtained from ve
by dividing by the gravitational acceleration on Earth:
Isp=veg
where g is usually standard gravity, or about 9.81ms2. The resulting quantity has units of seconds. For example, for the F-1 at sea level, Isp=263s
What is the physical significance of Isp?
Well, consider our rocket from before with 63% fuel by mass. Suppose we start the rocket while it is sitting on the pad, let it just barely lift off, then hover just off the pad until it runs out of fuel (this assumes we can arbitrarily throttle the engine without affecting its performance, which is not realistic, but ignore that). Isp is how long it will hover. That is because, for every second of hovering, we consume 9.81 m/s of Δv in order to overcome gravitational acceleration accumulated during that second. After Isp seconds, all of our Δv is gone.
michaeltherobot Seniorius Lurkius et Subscriptor
NOV 22, 2021 2:05 PM
Printzer wrote:
My back-of-the-envelope math shows Minotaur IV having a TWR of ~2.6 at liftoff. And to be fair, those things scoot when they head out.
Which means it rises as fast as if it were falling in 1.6x Earth gravity! TWR was always an abstract number to me until I learned to subtract 1 to imagine the gravity under which it is falling upside down