We’re engineers on the team testing the RS-25 engine that will power the core stage of NASA’s Space Launch System rocket. Ask us anything about the engine and it’s role in NASA’s Journey to Mars!

I am an incoming colleges freshman pursuing aerospace engineering. In an effort to be prepared for the real world when I graduate, I'm looking to learn how to use software that is prevalent in the industry. What kinds of software do you use to design and simulate at NASA?

Answered a similar question above: C++ seems to be the most prevalent programming language. For programming real-time systems, understanding how a computer works is also a key element. You also might want to get experience using a Real-Time Operating System (RTOS) for developing various applications.

Hi NASA
The RS-25 Engines are now close to 35 years old.
What is the reason that we use the same engines for the SLS as we did for the Space Shuttles? (Permormance? Fuel consumption? Power? or something else?)
Did the RS-25 Engine ever undergo any big changes in this time and what were these changes? (Reason?)
Thanks for the AMA :)

Hi this is Walt. When NASA designed the engines for the Space Shuttle, they used state of the art technology. The laws of physics haven't changed. This is still the highest performing engine cycle and the RS-25 is the only stage combustion rocket engine that is human rated built in the US. Yes we have continuously upgraded the engine over the life of the shuttle program. Improving the reliability and maintainability with each upgrade. Adapting the engines for the SLS we are upgrading the electronics, as an example of upgrades.

Kathryn - The Space Shuttle Main Engine(SSME) Program is 35 years old, but much of the original technology was upgraded throughout the Shuttle program. It was never really a static design - everything from injectors to turbopumps were redesigned and incorporated into the later versions of the SSME as late as the 2000s. We are using the RS-25 for a couple of different reasons. It is still the most fuel efficient engine on the market, and have 16 in stock and ready to go for the first 4 SLS flights. On the performance side, the efficiency helps us have more lift margin, and from a solely logistics side it means we can get off the ground sooner. As far as changes between SSME and RS-25, the biggest change we have made for the set of 16 is a new modernized controller. Also, because the engine will be integrated into a new vehicle, the operating conditions for the engine are slightly different, but the original engineers who designed this beautiful machine did it so well that it didn't actually require us to make any major mechanical changes to meet our mission goals.

Hi this is Walt. When NASA designed the engines for the Space Shuttle, they used state of the art technology. The laws of physics haven't changed. This is still the highest performing engine cycle and the RS-25 is the only stage combustion rocket engine that is human rated built in the US. Yes we have continuously upgraded the engine over the life of the shuttle program. Improving the reliability and maintainability with each upgrade. Adapting the engines for the SLS we are upgrading the electronics, as an example of upgrades.

The SLS looks like it will greatly contribute to science and space exploration, and will probably become famous along the lines of the Saturn V and Space Shuttle. The engine is obviously a key part to this. Assuming you believe in such a thing, do you feel that your contributions to this project (and by extension, science and humanity itself) fulfill wholly or in part your life's purpose?

This is Kathryn - For me, it is a huge part of my life's purpose. It is an opportunity to make a positive impact to the entirety of mankind, while doing something that I also just really enjoy. Having that sort of purpose for my life is priceless, and incredible rewarding and something that I wouldn't trade for anything. NASA has definitely become part of my own personal identity.

How much room is left for chemical rockets like the SLS to improve? Could you build something that goes twice as fast or carries twice as much if you were given more money? Or are we approaching a limit on what we can do, given the constraints of atmosphere and Earth's gravity?

Hi, this is Walt. We are always looking for ways to make the rockets more efficient and powerful. New technologies are always improving our ability to make them more affordable and we will be using those in the next generation of rocket engines.

What are some of the biggest engineering challenges you face when designing these shuttle engines?

Finding suitable materials to meet the requirements, because temperatures range from cryogenic to the melting point of iron. - P.B.

Hi, this is Walt from Aerojet Rocketdyne. One of the biggest challenges is managing 1500 gallons of propellant into the combustion chamber and controlling the combustion process.

Would you ever trust a commercial company to take you to space and back safely?

Sure. It's all about design maturity and reliability. In the 1920s or 30s, who would have thought we'd have reliable air travel from the U.S. to Europe in just a couple hours? - P.B.

If you met a nice martian lad, what sort of human activities would you spend the day doing? (Preferably in montage form around New York City)

Hi this is Walt. I'd take him to Bourbon Street. He'd feel most at home there.

How many physical engines do you go through before you build the one that will be used for a mission?

This is Steve.

We have 16 flight engines in hand from the Shuttle program that will be used for the first four flights of SLS. We also have two designated ground test engines that we'll use to certify the RS-25 for flight on SLS. We're not expecting to expend any engines in that process.

How many launches per year do you expect when SLS becomes fully operational?

Our launch rate will largely depend on funding, which comes from Congress. Our current plan is to have the first test launch of the integrated system, Exploration Mission-1 (EM-1) in the 2018 time frame with the first launch with crew about three years later. -MS

How much of the SLS is reusable?

Steve here - as an affordability measure, SLS is being designed as an expendable vehicle. This allows us to avoid the costs of infrastructure and turnaround activities associated with reusability, not to mention the costs and challenges of designing and building a re-entry vehicle to recover those systems. I will mention however, that the crew vehicle, Orion, is reusable. As are the people who will be flying it. :)

How has 3D printing changed the rocket design process? Is it opening up more options simply due to production style?

Is is the SLS able to take on more stages? Addition for farther delivery?

Kathryn - This actually a huge area of emphasis for us right now! Once we progress beyond the first 16 engines, we will be flying brand new engines that we are kicking off production on now. Part of what we are incorporating into these new engines are 3D printed parts. At the moment, we are looking at pretty much every piece part of the engine, and identifying components that it makes sense for us to transition to advanced manufacturing methods. For the areas we are implementing it, it saves us an incredible amount of processing time and tooling which means that we will be able to make these engines cheaper in the future. There are still more places we could use it if someone can just go solve that troublesome surface finish problem (get busy Reddit - I know you guys are smart). As far as more stages - it's possible but that's more of a launch vehicle design trade off - it isn't just constrained by our engine.

Thanks for doing this! What are your favorite pizza toppings?

BACON and sausage. -P.B.

My favorite is mushroom and pepperoni. -MS

Steve here - mushrooms and hamburger. Accompanied by a bowl of thin gruel, preferably.

Hi, this is Walt, thanks for the question. I'm a veggie pizza lover!

White cheese. Just lots and lots of cheese with tomatoes and mushrooms - Kathryn

I don't like pizza. -Ronnie

In an average launch, how long will it take for the SLS to reach Mach 1?

About 45 seconds. Thanks for your question! - P.B.

I am an incoming colleges freshman pursuing aerospace engineering. In an effort to be prepared for the real world when I graduate, I'm looking to learn how to use software that is prevalent in the industry. What kinds of software do you use to design and simulate at NASA?

C++ seems to be the most prevalent programming language. For programming real-time systems, understanding how a computer works is also a key element. You also might want to get experience using a Real-Time Operating System (RTOS) for developing various applications.

What is the air speed velocity of a latent swallow?

Steve here - what do you mean? African or European?

Are there any differences in the physical challenges caused by the gender of the astronaut? Like do they make the suit a different way or is there different gear?

In general, no. We have suits of various sizes for people of different builds. They all use common facilities and have identical sleep stations. There are some differences in allowable radiation exposure for women of child-bearing age. Because women are more prone to osteoporosis as they age, their exercise regimen while in space requires additional strength training. For reasons we don't yet understand, men seem to experience more vision loss. We're currently studying all these factors on the space station, especially with astronaut Scott Kelly in the midst of his one-year mission. -MS

How do you calculate exactly how much fuel is needed, seeing as weight is an issue, do you only use enough to get the job done or do you ever have a surplus of fuel just to make sure that you are able to reach where you are going?

We carry extra propellant to cover variations in engine and vehicle performance, but minimize it so that we're not carrying an unreasonable amount. - P.B.

We know the size of the tanks and set a goal on where we want to fly and how long we want to fly. From there, we can figure out whether we've got enough fuel to accomplish the mission or if we need to change our goals. In some cases, we can also bring extra fuel or fuel tanks. -MS

Thanks for taking time to answer questions!

What are the short and long-term goals for this project? To send humans to Mars faster, to send probes farther, to push boundaries on rocket-propulsion technologies for the future, etc?

Thanks!

For the Journey to Mars, we need a heavy-lift rocket and a crew capsule to carry our astronauts, food, water, and supplies into deep space. Once we've demonstrated the ability to do that safely by flying missions around the moon, we'll prove the technologies to take us farther into deep space to enable us to visit any number of destinations. -MS

Hello from Huntsville! Hopefully there won't be any massive engine explosions that rock the city (and surrounding areas...damn was that impressive!) this year. :)

What are your favorite NASA missions from the past?

Steve here - my favorite NASA missions from the past are the Apollo moon landings, which inspired me when I was a kid. I'm also partial to STS-128, where I had the honor of being a primary Mission Management Team member for the Space Shuttle Main Engine Project.

The moon landings were phenomenal accomplishments for our nation and the agency. Wish I had been older when they occurred to have been more a part of them. - P.B.

For Ronnie: What are your major challenges as a test engineer for a shuttle engine?

In my opinion, our major challenge has been adapting the test facility to simulate launch vehicle requirements. For example, while remaining at a sea level position, providing various fuel and liquid oxygen pressure and temperatures to simulate actual flight conditions. - Ronnie

As far as I know the design of the Shuttle (changing center of mass etc) made it necessary to have the possibility to gimbal the thrust up to 10° (I dont know the exact number)...will there be changes in that regard (smaller maximum gimbal angle) with the SLS since the center of mass would always be directly in front of the engine?

Kathryn - We actually already in work on this, and the gimbal angle requirement will be relaxed which gives us some pretty valuable margin in other areas of engine design for the new engines

Where will you guys be launching the SLS from? KSC?

Yup - KSC! - P.B.

To the Huntsville folks: For rocket science in particular, Huntsville is a fine place, but for even related technology jobs, it is very hard to convince someone from elsewhere to move to ahem Alabama. What do you say to those considering a move? What ideas do you have to make it easier to recruit people to come to Huntsville?

I was born and raised in Alabama, and proud to live here and work at NASA. Huntsville is a developing, thriving technology-based town that has a lot to offer. Don't knock it 'til you've tried it! - P.B.

I'm currently studying computational/aerospace engineering. Do you have the impression that it's hard to find a job in the industry? Do you have any advice for a student wanting to work there after graduating/doing his Ph.D.? EDIT: I'm especially talking about the space technology industry of course.

Hi, this is Walt. It's important to get work experience into your resume and co-ops or internships are great ways to get exposure to the work environment and give visibility to your skills and talents. The SLS is going to need a lot of good engineers in the future. Good luck!

What's your favorite video game?

Hi, this is Walt, My favorite video game is World of Warcraft.

This is Steve. Ummmmm, Pong?

Horde or Alliance?

Alliance. -- Walt

I'm into the classics: Donkey Kong, Dig Dug, Intrepid, PacMan and the occasional Candy Crush. -MS

Hello NASA folks. What upcoming technology, that is currently in development, intrigues you the most currently?

I really like additive manufacturing, specifically selective laser melting where you take a metal powder and a high-powered laser to manufacture 3-D parts. I also like laser communications where you use light waves over open space to transmit voice and data. -MS

Do you believe in extraterrestrial life that possesses similar or superior intellectual capacities to human beings? Why or why not?

I think we're naive to think the Earth is the only place with life. There's so much out there that we know so little about, which is why we need to continue to explore. -MS

What is your favorite dinosaur?

Hi - Mine is Indominus Rex - P.B.

Hi this is Walt. My favorite dinosaur is pterodactyl. It's gotta fly!!!!

Steve here - I'm a T-Rex man myself.

Me. I'm a Ron-a-saurus. :) - Ronnie

Hey, thanks for doing this AMA! I had heard that NASA has opened up a competition for students to help rename the SLS. If you had a say in the matter, do you happen to have a particular name you'd want to call the rocket? Let's face it, SLS is a bit cold and unfeeling, compared to, say, the Saturn V!

We haven't opened a competition to name the SLS. We like that "Space Launch System" is perfectly descriptive of what it does. -MS

I notice you use imperial units when you answer questions here. Is that the universal system used at NASA?

Steve here - most of the industrial companies we deal with are American, so English units are the standard for compatibility. Other NASA programs (mostly science missions) have used metric units.

What's the biggest piece of evidence that has led you to believe in extraterrestrial life?

Hi, this is Walt. Space exploration is all about exploring for signs of life. Finding intelligent life elsewhere would be amazing. SLS will be one tool that we could use to find life in the solar system.

Hi NASA!

How strong is the SLS compared to the Saturn V and the Space Shuttle?

Are you currently ahead of schedule, or are you slower than schedule?

Thanks!

The Space Launch System will be more powerful than both the shuttle and Saturn V. SLS will have 8.4 million pounds of thrust at liftoff; the space shuttle had 7.8, and Saturn V had 7.5.

We're on track to be ready for launch in 2018. Right now, we're building the Orion crew capsule for that mission, and the core stage for the Space Launch System. The programs are doing their critical design reviews, and we'll set a launch date after those are complete. -MS

What scientific breakthrough could have the most impact on a faster/lighter/more powerful engine, that hasn't been invented/discovered yet?

Warp drive would be awesome. In the meantime, we are beginning to implement adaptive manufacturing techniques to expedite design and manufacturing, plus reduce costs. - P.B.

Hello, I don't really have a question about your engine but I have a question about how you guys got into the position you are in today. Well, actually about college degrees. If I want to be an aerospace engineer and hopefully work for NASA in the future, would it be better to get my undergrad in mechanical engineering and then my graduate degree in aerospace engineering? Or would it be better to just get my undergrad in aerospace and hope for the best? I'm sorry that this question is a little off topic. Thank you very much for doing this AMA!

Steve here - ME or AE are both great for this business. I don't see a huge advantage of one or the other in terms of preparation--they both have strengths that would qualify you. It's more a question of what you make of it after you get it.

I understand that the RS25 has a lower outlet pressure than any other atmospheric engine, allowing it to be more efficient above ~7000 meters, this is done by creating a shell of normal (for a rocket engine) pressure around the main stream.

My questions are:

1 What am I getting wrong?

2 Why is this only the RS25, and not other atmospheric engines that get above 7km in flight?

Hi this is Walt. The RS-25 was optimized for the Space Shuttle flight profile and now these are existing engines being adapted for SLS. The geometry of the engine doesn't change so we will get the same performance. The vehicle is designed with that in mind.

Who would you say has had the largest influence on this project?

The American aerospace workforce who show up every day in 49 of the 50 states (South Dakota is the only state not working on the Space Launch System or Orion crew capsule). -MS

Hello! Did the Orion crew capsule undergo any necessary changes after the test flight? How many of these capsules do you have and plan on making?

Thanks!

As a result of building the capsule for the first flight test, we learned how we could make the structure lighter so we can carry more. We're also adding deep space navigation and communications hardware. As we go through the flight test program, we're going to continue to apply lessons learned to improve the safety and capability so our astronauts can go farther and faster. We'll have a new capsule for each mission. -MS

This is so awesome to see! What do you think each of you would be doing if you were not scientists or engineers? I'm particularly interested in your response, Mr. Benefield.

Hard to say ... It would have to be something technical; maybe an architect. Worst case, a professional soccer player. - P.B.

The first few launches of SLS will use engines originally built for the space shuttle, I'm guessing some time in the 2000`s. What's the process like to store them and get them ready again for a flight? I'm guessing you can't just put a highly complex rocket engine on a shelf and dust it off for a launch 10 years later (or can you!?)

Steve here - the shelf life is really, really long actually. Provided you keep them in sealed containers and purge them with inert gasses (which we're doing).

Actually, you more or less can! The engines left over from Shuttle have been stored at Stennis Space Center since 2011 -- in an environmentally controlled facility, of course. Some of our engines will have been "on the shelf" for 10 years before they fly on SLS. - P.B.

What do you personally think will power the SLS after the first four flights use up the 16 available engines?

More RS-25 engines. We are working now to restart production of the RS-25s, and plan to have them ready for Flight #5. - P.B.

Are they really the best engine for the job if affordability is the prime driver? RS-25s are pretty complex (i.e. expensive) compared to other engines from what I hear.

Is there budget to support a major redesign to adapt from Shuttle to SLS?

Steve here - they're absolutely the best engine for the job! They are the most reliable, most efficient rocket engine in their thrust class with a tremendous proven record of flight reliability. We made 5 block upgrades over the years in the Shuttle program specifically to improve the safety and reliability of the RS-25.

But we never made a block upgrade for affordability. That's what we're working on now. We'll have the same great reliability and safety, but a more affordable engine.

How do you plan to navigate through the asteroid belt?

Very carefully. -MS

If you have read it, what did you think of The Martian, by Andy Weir? It has a bunch of stuff about what rockets they would need to get to and from Mars, which I knew nothing about and found pretty interesting.

Hi this is Walt. I read the Martian and I thought it was great. I'm looking forward to the movie coming out.

Hi this is Walt. I read the Martian and I thought it was great. I'm looking forward to the movie coming out.

Hello, thank you for answering our questions. In regards to present day technology, do you believe our engines could ferry the necessary payloads to Mars without, say, refueling on low-Earth orbit? I understand probes are ok, but life support, food etc can be pretty heavy.

Hi this is Walt. Yes, the ultimate goal for the SLS is to build a rocket capable of lifting 130 metric tons. The reason for that size is that is allows trips to Mars without a lot individual launches. Fewer launches is more affordable and more reliable.

Question 3, has a decision been made on the RS25E or RS25F yet once the shuttle engines are used up?

Steve here - we're not really using the -E and -F terminology anymore. Once the 16 existing engines have been flown, we'll be using new production RS-25 engines, which we're starting the ball rolling on production restart now. The new production engines will incorporate numerous minor changes to take advantage of expendability vs. reusability, as well as changes and improvements for producibility and affordability. Lots of exciting stuff for these engines, including manufacturing advances like additive manufacturing.

Hi, thanks for taking your time to stop by!

A big issue with the Shuttle program throughout its lifetime seemed to be how dangerous or prone to danger it could be (I can't claim to be knowledgeable but I've read a few astronaut memoirs).

What's the process behind identifying failure modes and the proper response for the SLS?

Also how much rocket is too much rocket?

Kathryn - This engine is not any more or less dangerous than any other engine, but it is more complex. Despite that, in all the flights we flew under Shuttle, we never had a mission failure due to a liquid engine. There were a few engines lost during testing, but that is why we test - to identify problems before we fly. Most of the test failures we had were during the original program activation, before we figured out all the things that make running this engine very different than running any other engine out there. Also, because we have so very much history behind this engine now (30 years worth of detailed analysis, and failure mode mitigation) I believe it is a very very safe engine, as far as rocket engines go.

I'm a huge fan of you guys and I'm currently a chemical engineering student from the University of Sasktchewan.

Do you have any good books that you can recommend me read that will help me build better character as a future rocket engineer? Also what rocket books (if any) can you reccomend me and other inspiring rocket engineers to read?

I recall a rocket propulsion textbook - "Rocket Propulsion Elements" by George P. Sutton - was a good book for my college days. Also, "The Saturn V F-1 Engine" by Anthony Young. - P.B.

One of my favorites as a child was an autobiography by Chuck Yeager, the first man to fly faster than the speed of sound in the Bell X-1 Rocketplane. I really enjoyed reading Neil Armstrong's autobiography "First Man," and if you are really into technical reading, I highly recommend the four-volume series called "Rockets and People" by Boris Chertok, who was Sergei Korolev's deputy in the Soviet space program. All of them encountered challenges few people could understand let alone overcome. -MS

Did you spend a lot of time blowing things up as kids?

Steve here -- hmmm. Occasionally actually. In a safe manner of course.

Don't tell my mom.

I did. I learned to hunt and fire a rifle and shotgun at a young age. I built Estes rockets and launched them. And some other stuff I won't tell you about. -MS

Can I go to Mars? Please?

Hi this is Walt. Study hard, enter the astronaut program and you too could be on your way to Mars. We are working hard to get you there and back safely using SLS.

Work on your engineering degree and prepare to contend with some very competitive candidates to be an astronaut. We're going to need the best and brightest. -MS

I hear the Space Shuttle had very old 90s computers. What kind of computers / OS are being used in the SLS?

The space shuttle had very old 70s flight computers - the IBM AP101. -MS

Hi this is Walt. The Space Shuttle main engines used the computers similar to the original Apple Mac. This is why our main goal is to update the electronics on the RS-25 for use on SLS. No more dial up.

On the Shuttle, the RS-25s were frequently reused, but on the SLS they will be expendable. What kind of changes, if any, go into the engine design/manufacturing/etc... when reusability isn't part of the plan?

Hi this is Walt. Yes, we are proposing to do a lot of things to make it more affordable: modern manufacturing tools, new inspection capabilities, modern materials, new electronics, and current engineering capabilities. The goal is to make them more affordable but just as reliable as they always have been.

Steve here - we've got an entire program devoted to that. We're working very hard on it and drawing on two main sources. First, we're drawing on 40 years' worth of manufacturing lessons learned on this engine--we know where the cost drivers and difficult fabrication operations are and we're systematically attacking those using Value Stream Mapping and other techniques. Second, we'll be taking advantage of tremendous advances in manufacturing technology that have occurred since RS-25 was first developed. Things like additive manufacturing, structured light scanning, tremendous advances in forging and casting technologies, and materials advances. Like you mentioned, we can also take advantage of expendability--the new engines only need a certified life of 6-10 starts, versus 60 starts on Shuttle. We'll have the same reliable, safe engine, but it will faster, easier, and less costly to produce.

Hi NASA!

I've been intrigued and interested by space exploration and colonization since a young age. I've often dreamed of leading colonies onto Mars, but as I grow older and learn more about the harshness of space and especially the harshness of Mars, I've become skeptical that we can actually set up a colony on Mars, at least within this generation. Is there any possibility that I could be wrong?

Thanks for the AMA (:

We're working on technologies that will allow us to recycle oxygen and water from the atmosphere we live in as well as in situ resource utilization to harvest from orbiting bodies to produce water, oxygen, and propellent. We're working on technologies to reduce the effects of radiation and countermeasures for microgravity through exercise. Some of those technologies can be applied to life on Earth. Are you looking at the glass half empty or half full? -MS

What's it like getting to work with rocket engines everyday? Being as complex and as mission-critical as they are is it more tense than some other aspects of SLS or is all the more exciting? And with the RS-25 being an older piece of hardware does it ever pose any quirky challenges? And lastly, how did you get involved with the propulsion engineering side of spaceflight? Thank you!

Hi this is Walt. Working with rocket engines every day is really exciting. I work with some of the most talented, educated and passionate people in the industry. We know this engine very very well. 135 successful flights and a million seconds of test time. This engine is very reliable. To adapt these engines, we are upgrading the electronics but the engines are sound and ready to use for SLS.

Working with rocket engines on a daily basis promises new experiences. Working in the test environment, our objectives are often to push the engines to their limits which provides interesting challenges. The basic design of the RS-25 engine has been around since the mid 70s, however some of the engines in our inventory have never been hot-fired. - Ronnie

Hi from Spain NASA! I'm a huge fan of your work and I'm looking forward to see some bootprints on Mars. With the launch in 2020 of the new rover, will the manned mission to Mars be delayed? Thanks for answering questions guys! Good luck with the tests!

Hola! Thanks for being a fan. The Space Launch System and Orion crew capsule are on schedule to take us back into deep space. When we have the capability to fly humans in space for 1,000 days, we'll be ready to go to Mars. -MS

Have you considered the option of printing food in space? Maybe I'm wrong but if you can get all components of said food in compressed ways you could have more of them stored. Is it feasible?

If someone could figure out how to use protein powder and a 3-D printer, you could print your own shrimp, steak, or surf 'n turf. -MS

I'll be attending the NASA social event in person, so hopefully I'll get to meet some of you answer-ers!

What part of your job on this project has made you feel most like a superhero? (Because you are, this is awesome)

Steve here - great! Thanks for the compliments. I look forward to meeting you.

I'll be the one wearing glasses.

Things that have made me feel like a superhero: splashdown of the Orion capsule in December 2014, and (previously) shaking the hands of our astronauts getting off the space shuttle on the runway after landing. -MS

Since they were designed for the Shuttle, what changes are necessary to adapt RS25 to SLS?

Steve here - we're developing a new controller and software for the RS-25s, since the old ones from Shuttle were electrically incompatible with SLS. Second, we're certifying the engines to be able to handle the different propellant inlet conditions on SLS versus Shuttle--the lox tank on SLS is much taller and thus has a higher head pressure at the bottom. Third, we're developing a new ablative insulation system to protect the exteriors of the nozzles from the different heating environment on SLS--the engines are coplanar with the boosters now, so we have a higher radiant heat load on them.

The biggest change for the engine is the control unit. We have to develop a new one to interface with the new rocket's computers and power systems. - P.B.