Archive for the Word Meanings Category

Words have meaning: Delta-V

Posted in Mission Operations, Word Meanings on 12/14/2009 by clairdeluna

Over at PlaneTalking, Ben Sandilands reports on “Delta V,” a joint venture between Virgin Blue and Delta Air Lines. This caught my eye, because “delta-v” means something completely different in mission operations.

For spacecraft in orbit, there are several forces that can alter the trajectory over time. Atmospheric drag slows the spacecraft, causing the orbit to decay. Pressure from the solar wind can push the spacecraft to lower altitudes. Even the gravitational effects from other bodies (most notably the Moon, Sun, and Jupiter) can affect spacecraft orbits. At times it is necessary to modify that orbit to compensate for these effects. This is accomplished by firing the thrusters to change either the altitude or inclination of the orbit. A change in altitude is accomplished by changing the velocity of the spacecraft, hence the term delta-v. Delta is the mathematical symbol for “change in,” and v stands for “velocity.”

Delta-v maneuvers are a regular part of operations management for certain low-earth orbiting satellites. In fact, any spacecraft that requires a particular orientation with respect to the body it is orbiting (like geosynchronous satellites), or that needs to maintain formation flying with other nearby satellites, will need delta-v maneuvers from time to time. The need for regular delta-v maneuvers increases the payload size by necessitating the inclusion of a propulsion subsystem, and increasing the propellent load the satellite must carry. Recent safe-ocean disposal requirements have made propulsion a must on new NASA spacecraft to allow for a controlled de-orbit (as was performed several years ago with TRMM).

Such maneuvers are also used to help guide interplanetary missions to their targets. Small changes in velocity early in the trajectory translate into large shifts in position at the target location. Even a mission with a single destination (like MRO) requires multiple delta-v maneuvers and a skilled navigation engineer. The more complex the flight plan, the more maneuvers will be required. Luckily, when dealing with such large distances, the amount of propellant needed for these adjustments is reasonably small.

In unmanned space missions, a common propellant is hydrazine, which was used in all three missions I have launched. This highly volatile substance ignites immediately upon release, and does not require an oxidizer to produce thrust. So, while you often hear such maneuvers referred to as “burns,” this particular fuel is not actually burning (oxidizing). For missions where delta-v maneuvers are needed solely to de-orbit the spacecraft, the propulsion system is often left “dry,” remaining untested until the mission is complete. This reduces risk that a failure of the system could terminate the mission prematurely.


Words have meaning: Hold (Hold Hold)

Posted in Launches, Mission Operations, Observatories, Word Meanings on 12/13/2009 by clairdeluna

The last few minutes of a launch are very critical. It is at this time that systems on the launch vehichle, and (for some missions) the payload, are switched from using an external power source to using their internal batteries. For Delta launches, this switch usually occurs four minutes before launch, and gives the final insight into the complete health of the system. At this point, if any data indicates a possible problem it is necessary to quickly stop the launch.

In mission operations, words that indicate a specific, time-critical action must be very precise. In this situation, the words used by an engineer to the launch director to stop the launch are:

Preparing to launch a multi-million to multi-billion dollar observatory requires practice. So the last six months prior to launch are filled with practices. “Mission rehearsals” that exercise the post-launch and activation process are very complicated, and are run repeatedly by the mission operations team. “Launch rehearsals” that practice the pre-launch process are performed jointly with the launch vehicle operations team several times before launch. During launch rehearsals, one of the hardest things for me to learn was never to say the word “hold.”

Saying “hold hold hold” only has consequences if you say it on the particular communications channel (voice loop) that is connected to the launch operations team, and I never had any reason to be on that loop. Even so, I spend the last few months finding alternative words to use. “Please carry this for me,” rather than “Please hold this.” “Can you wait a minute,” rather than “Can you hold on.” And the list goes on…

It seems like such an innocuous word, hold, until you realize that with that one word you have the power to stop a launch. And right before launch, even a one day delay is a LOT of manpower, and money. But that’s not nearly as painful as the investigation that would follow a failed launch.

You hope you never have to say “hold hold hold.” But if something’s not right, it’s absolutely the right thing to do…

Words have meaning: (Launch) Window

Posted in Mission Operations, Word Meanings on 12/11/2009 by clairdeluna

Word today that WISE launch is now scheduled for Monday, December 14, 2009 (6:09:33 – 6:23:51 a.m. PST ). For those of you paying attention, you may notice that while the date changes, the time of the launch is still the same. This time range is the “launch window.”

The timing for the launch window can be as short as minutes, and as long as weeks. It all depends on where you are trying to put your spacecraft. For any mission that must operate in a particular orientation with respect to the Earth (this would be pretty much anything that directly orbits the Earth), the window is on the order of minutes. That’s because the Earth turns relatively quickly, rotating 15 degrees per hour. The more precise the needed orbit, the shorter the window becomes. It’s hard to compensate for an orbital offset once you’re in space.

Missions with more distant targets, like the moon, other planets, comets, or one of the Lagrangian points, have a better chance of compensating for changes in launch time. Small perturbations of the path early on translate into much larger shifts in position after they’ve travelled millions of miles. This means the timing of the launch window is typically a function of the relative positions of bodies in the solar system. In fact, more important than knowing where the planets/moons/etc. are now, is knowing where they WILL be at the time you want the spacecraft to rendezvous with them.

After all, if they miss, there’s no turning back…

(Hat tip to palmerin for the ability to sleep in!)

Words have meaning: Anomaly

Posted in Launch Failures, Mission Operations, Word Meanings on 12/09/2009 by clairdeluna

In these posts, I typically like to highlight words with very specific meanings that are used far too generally or improperly by the average person. However, the popularity of my Norway Spiral post has led me to another word, one which is used as a catchall by engineers. And a word that seems to imply much, while actually giving nearly zero information. I mean, of course:

In engineer-speak, an “anomaly” is anything that didn’t go the way you expected it to go. While we test this hardware over and over on the ground, and rehearse the expected launch and activation sequences repeatedly, sometimes things just don’t behave as you expect. This can be due to something catastrophic, like a mission-ending hardware failure, or something as simple as not getting telemetry when you expected. (This can happen due to a scheduling conflict giving access to the communication satellite you were planning to use to a different, higher priority, user.)

When things don’t work the way you expect, it must ALWAYS be documented (an issue/discrepancy/anomaly/pick-a-word report should be filed). If the reason for the anomaly is simple, this step is a formality. But good engineering practices means it should be done anyway. On the other hand, some anomalies, especially those that result in mission loss, will be investigated by a number of experts for as long as it takes to either 1) find the root cause and communicate it to the rest of the space industry, or 2) determine that the cause is unknowable due to lack of sufficient information.

The Columbia tragedy started out as an anomaly and was investigated for years, leading to significantly improved processes for monitoring orbiter health post-launch. And you can bet the Norway Spiral was a launch vehicle anomaly. We watch this video and chuckle at the network callout that “we have just had an anomaly…” True, that may seem like a gross understatement. But in an industry where words have very specific meanings, no other word can be used without implying more than simply “something didn’t behave as expected.”

So next time you hear about some “anomaly” occurring on your favorite spacecraft, remember: it could be anything…or nothing at all.

Words have meaning: Exploded View

Posted in Mission Operations, Word Meanings on 12/09/2009 by clairdeluna

Working on a space mission, you get used to seeing a LOT of technical drawings. These include schematics, interface diagrams, flow charts, and, of course, the

Exploded View

To those of you who are unfamiliar with the nomenclature for technical drawings, these are the graphics that depict the hardware spread apart in pieces. If you were to move the pieces into place next to the adjacent pieces, you would see what the finished product looked like intact.

These drawings are excellent for understanding how a particular item (for example the bearings in your wheel mechanism) fit into the larger structure. They are a very useful tool in engineering almost any type of technical hardware. Say the phrase “exploded view” to anyone who does space mission operations, and you see a rather interesting reaction. They cringe.

One of the last promotional products sent out before a mission launches is an “exploded view” of the observatory sitting on top of the launch vehicle. I love receiving these, because they are hard evidence of the mission in its final form. I have three of these posters, all framed and mounted like hunting trophies on my wall. However, I refuse to call these diagrams by their proper name: Launch vehicle, exploded view.

Place an observatory, with 5-15 years of many people’s lives invested in it, place it on top of a ticking time bomb filled with liquid explosives, and then send out a lovely graphic called an “exploded view” and see what kind of response you get. In the Mission Operations Center (MOC) we tend to call these graphics the “launch vehicle poster.” It’s descriptive enough to know what you’re referring to, without using the dreaded “E” word.

My rockets may always blow up. But that doesn’t mean I WANT them to…

Words have meaning: Satellite, Spacecraft, Observatory, Payload

Posted in Mission Operations, Observatories, Word Meanings on 12/07/2009 by clairdeluna

Ever look up in the sky and see a star moving slowly across the sky, passing the other stars around it? What you are seeing is a satellite, an object orbiting another object, reflecting sunlight off its surface.

In this case, the satellite is man-made. However, the moon is also a satellite, just like that little dot you saw. It is an object orbiting the Earth. The Earth itself is a satellite, since it orbits the Sun. And there are man-made satellites that also orbit Mars (e.g. Mars Odyssey), Saturn (Cassini), the Sun (SOHO) and many other solar system bodies. Even those observatories that are no longer functioning (like Mars Global Surveyor) are still satellites, they still are in orbit. I find that many people think of satellites only as man-made objects, even though nature made them first. And we tend to forget that the word satellite has little meaning without knowing what body the object is orbiting.

If you watch NASA TV (and who doesn’t, right?) and listen to those engineers and scientists talking about their satellite it can seem like they are using a lot of different words for what seems like a simple subject. However, in the space industry there are certain very specific terms that we use that have specific meanings. So while it may seem that we are spouting lots of words to just sound extra-super-duper-smart, in reality we are using those terms in order to be precise. Take, for example, the terms “spacecraft,” “observatory,” and “payload.”

A spacecraft is a support structure. It contains the hardware necessary to power, command, and control the “payload” that is connected to it. Often the term “spacecraft bus” is used to indicate the fact that this structure is the support for the payload. When the spacecraft and payload have been mated together, the whole unit (at least for a science mission) is the “observatory.”

Hardware that has been connected to support hardware is a “payload.” The instruments (scientific sensors) are the spacecraft’s payload. (For commercial satellites, the payload may be an antenna/transmitter for relaying cell phone signals, or digital TV.) The observatory is the launch vehicle’s payload. A spacecraft bus without its payload is like a person’s brain with only rudimentary sensory input. It can function, and even receive and respond to commands, but it has a hard time interacting with the world around it and will not be very productive. Likewise, it is unusual to expend a launch vehicle without a payload (though this situation does happen sometimes in testing a new launcher).

So, the next time you see that small speck of light moving among the stars, remember: that observatory in space, that satellite of the Earth, is a complex system made up of multiple functional parts, each with their own precise terms. And it has gone through many phases to become what may seem to you to be a simple “satellite.”