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.


WISE Launching!

Posted in Launches, Observatories on 12/14/2009 by clairdeluna

Go check out the launch on NASA TV! (Now that the launch is complete, this link is no longer showing the WISE launch. However, you can watch the replay below.)

Update: Once again, despite my constant expectations, the rocket didn’t blow up! Don’t believe me? Here’s the replay…

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!)

WISE Launch Slips to Saturday

Posted in Launches, Mission Operations, Observatories on 12/11/2009 by clairdeluna

Note: As palmerin mentions below, the launch is now scheduled for Monday. Enjoy the free weekend! I bet the mission team is frustrated!

The WISE launch, originally scheduled for yesterday, has slipped to Saturday morning. The current WISE mission launch page indicates that the reason for the slip is a “problem with a booster steering engine.” Such a problem could range from relatively minor (two sensors giving incompatible readings) to serious (some hardware failure could make the booster unsteerable). But with a 24 hour hold, it’s probably a minor issue.

When your launch window is less than 14 minutes (6:09:33 – 6:23:51 a.m. PST for Saturday), you only get one shot at launching. The typical Delta-II recycle time (how long it takes to get a second try) is about 20 minutes. I’ve seen it done in 17 minutes, and that was very fast. If the problem requires more than a few moments to figure out, the launch is usually scrubbed (postponed) and rescheduled for a later time. How long they wait depends on the severity of the problem. The minimum time is however long it takes to get to the next launch window that will put the spacecraft into the proper orbit. For a mission like WISE, that time is probably 24 hours, hence the 24-hour delay.

Sadly, that means those of us who are launch junkies have to actually drag ourselves out of bed on Saturday morning if we want to watch the launch. But hey, at least we aren’t gonna have to work the weekend!

But the Aliens were Cooler!

Posted in Launch Failures, Launches, Test on 12/10/2009 by clairdeluna

Update: The failed launch has been confirmed as a Russian Bulava missile test.

This morning I posted about the Aliens Hypnotizing Norway. Via Spaceweather and Bad Astronomy come some useful updates.

First, there was a navigation warning issued for the area indicating that no one (let me repeat NO ONE) was trying to hide the fact that there was a launch test being performed. Second, another picture has emerged of the event:

The inset clearly shows a perfectly normal launch trail leading up to the more bizarre imagery that we have seen. This white trail has begun to dissipate, with differing winds at different altitudes pushing the trail around until it looks irregularly jagged. Anyone who has seen a successful launch on a clear day will recognize that trail.

At the top of the trail, there is a brighter spot and the trail itself ends abruptly. I suspect this is where the anomaly likely occurred. Having seen this picture, I will revise my earlier statements and say that the upward thrust probably ceased at this point. The remainder of the upward motion was simply from the rocket’s momentum. The blue trail is likely fuel venting from the first stage.

At the end of the blue trail, something changes. It could be that a second stage on the rocket ignited autonomously (most launches are run by an onboard script…you don’t want to have to try to command something that complex by hand), or simply that the torque got too large and created venting as I suggested before. Either way, the rotation had already been imparted to the system. Once there was some sort of thrust (engine firing, massive venting) the spiral started to form.

Anyone who says the system should have fallen to earth at that point needs to play with those spiral fireworks for a little bit. The push given by the tangential thrust may very well have kept the system rotating in what is a nearly stationary position. From some of the images the cloud is obviously three-dimensional. However it is clear that by the time the spiral started, very little of the rocket’s original forward momentum remained.

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.