Pluto Probe Delayed

setirw writes "Due to high wind conditions at the launching site, the launch of the NASA's probe to Pluto has been delayed for 24 hours. "The wind limit at the pad is 33 knots [and] we have exceeded that limit several times today," said NASA spokesman Bruce Buckingham." From the article: "Glitches with an Atlas 5 vent valve, a ground tracking station in Antigua and NASA's Deep Space Network also led to launch delays, though the wind concerns were omnipresent throughout those issues."

Comments

Re:Huh?

[Anonymous Coward]

And? Couldn't a few feet difference here mean thousands of miles later on, because of an altered trajectory?

wind limits on rocket lauches

[mstrcat]

Wind limits on rocket lauches are a combination of several things, just as most complex engineering problems.
        The structural forces placed on the structure from side winds are negligible when compared to the acceleration forces due to lauch.
        For the most part, it's not the final trajectory of the payload that sets them, as secondary burns and mid-course corrections are more than adequate to correct any small variation in the launch vector.
        More importantly is stablity of the rocket under side forces. Because of a rocket's tall slender build, the center of mass is far away from point of thrust (the engine nozzles). Any small horizontal motion of the center of mass with respect to the point of thrust can quickly lead to tumbling. It's this control problem that really determines the launch limits. A rocket is inherently unstable and requires dynamic control, typically small engines around the periphery of the main nozzle that can swivel to provide righting moment. As with any real control system there are limits to the perturbation it can handle, and this is translated into wind speed limits.

Re:wind limits on rocket lauches

[DerekLyons]

Wind limits on rocket lauches are a combination of several things, just as most complex engineering problems.

The structural forces placed on the structure from side winds are negligible when compared to the acceleration forces due to lauch.

Negligible compared to acceleration forces is not the the same as trivial. (Especially since the wind forces manifest themselves in a direction other than that where the rocket is strongest - along the vertical axis.) Side loads can and do induce body bending and a variety of vibratory modes.

For the most part, it's not the final trajectory of the payload that sets them, as secondary burns and mid-course corrections are more than adequate to correct any small variation in the launch vector.

Assuming of course the variation is small, (defined as within the excess performance available). If the winds are strong enough, and the margin small enough, then they can in fact affect the final trajectory.

All that aside, the winds that were the problem today were ground winds, not altitude winds. Right after launch the rockets ability to correct for errors is the least, leading to the possibility of impacting the tower. Another problem from ground winds is ensuring they are blowing in a direction that will carry any gases released (from the launch or from a failed launch) in the proper direction.

For the rocket scientists out there....

[Starker_Kull]

...If I read the mission description correctly, this probe is scheduled to be in the vicinity of Pluto and Charon only for a day - in other words, it is doing a fly-by. Why not try something more ambitious, like enter orbit around Pluto? I understand the heat/technical problems with actually landing on Pluto or Charon; but is the energy requirement to enter orbit rather than just flyby that large? I know the probe is getting a gravitational boost from Jupiter, so it shaves a few years off of the flight time, but if it didn't get that boost, wouldn't it arrive in Pluto's vicinity with less energy and thus be easier to put into orbit? Or is the extended time in space that much more likely to lead to failure? I don't have to explain the obvious payoff in terms of scientific benefits of a long-term orbit versus a one day fly-by...

Re:For the rocket scientists out there....

[Tango42]

They want it to explore the Kuiper belt afterwards, so they would have to enter orbit and then leave it and build up the speed again, which would take a lot of fuel.

It will be in the vicinity of Pluto for longer than a day - it won't be very close for that time, but it will be more than close enough for the work they want to do.

Re:For the rocket scientists out there....

[StupendousMan]

Quoth the parent:

> is the energy requirement to enter orbit rather than just flyby that large?

In brief, yes.

(Warning: back-of-the-envelope calculations follow)

In order to reach Pluto in a reasonable number of years, the probe must move very fast. Let's see ... very roughly, it goes 40 AU in nine years. That's about 4.5 AU per year, or a cruising speed of about 21,000 m/s.

If you wanted to put it in orbit around Pluto, you'd have to decrease its speed to the orbital speed of Pluto, which would be a few hundred m/s. That means you'd have to decrease the speed by roughly 20,000 m/s ... or, to a good approximation, you'd have to remove all the velocity you had added to the probe in the first place. The mass of the probe is roughly 1,000 kg, so its momentum must be decreased from about (20,000 m/s) * (1,000 kg) = about 20 million kg*m/s to zero.

To reduce the momentum, you fire an engine pointing backwards: the engine throws exhaust products forward at some speed, and the momentum they carry away reduces the remaining momentum of the probe. Chemical rockets have typical exhaust speeds of around 2,000 m/s, so to remove 20 million kg*m/s, you'd have to throw around 10,000 kg of mass out of your engine. (Yes, yes, it's more complicated than this, but for the purpose of illustration, it's close enough).

But, wait a minute: the probe's mass is only 1,000 kg. It can't carry 10,000 kg of fuel and oxidizer, too. So it cannot slow itself down enough to enter orbit around Pluto. If you wanted to design a probe which could enter orbit, you'd have to make it carry huge amounts of fuel for this burn when it reaches Pluto ... but then you'd need an enormous rocket to accelerate the fuel and probe to 20,000 m/s in the first place.

It just isn't practical. Sorry.

Launch scrubbed for Wednesday as well

[PianoComp81]

The launch has been scrubbed for Wednesday as well, because of a power outage in Maryland at the New Horizons control center

(see http://www.floridatoday.com/floridatoday/blogs/plu tolaunch/)