New launch schedule: Rescheduled to lift off at 3:31 a.m. EDT on Aug. 12, 2018.
Scrub Announcement Video: "Parker Solar Probe Launch Postponed".
There will be live coverage on the NASA Kenedy YouTube Channel.
Current live stream: "NASA Live: Official Stream of NASA TV's Media Channel".
To me, this makes it sound like the margins for the energy budget will be very small.
The margins will be small enough to not waste fuel, by carrying unnecessary fuel.
But there will be extra fuel to allow for error and to permit some wiggle room.
A Goldilocks fuel budget.
So why is the launch window over a week wide?
There's a setup time to prepare for launching, it's easier to figure out a way to launch every day for a week than to be able to launch a dozen times in a year (which would not be possible for an interplanetary launch) since people would have to pack up and leave, then return early to get setup again - or not leave until it's launched, with a huge wait between windows (but that's not how interplanetary windows work, where you are chasing a moving target, as opposed to simply orbiting Earth).
Whether you leave at the start or end of the window on day one, or a different day, changes the route and possibly ends up putting you in a non-optimal position around the Sun, but still not a bad position, nor as disadvantageous as a further few weeks or months delay (and scrapping all prior calculations).
Sources/Proof of the above answer.
On NASA's Blog an article titled "Parker Solar Probe Launch Window Extended to August 23" (Aug 2 2018) they wrote:
"NASA and its mission partners have analyzed and approved an extended launch window for Parker Solar Probe until Aug. 23, 2018 (previously Aug. 19). The spacecraft is scheduled to launch no earlier than Aug. 11, 2018, at 3:48 a.m. with a window of 45 minutes.".
So day one the window is 45 minutes. If they can't launch within that window it's on to day two.
But on August 7 2018, in the article: "Launch Week Begins for Parker Solar Probe" they wrote:
"... scheduled for Saturday, Aug. 11, at 3:33 a.m. EDT, the opening of a 65-minute window.".
So now it's earlier, and longer. Their newest Blog entry: "Parker Solar Probe Proceeds Toward Launch Aug. 11" confirms the same date and time with no mention of the day's window.
On NASA's webpage: "Chapter 9 - From Earth to Venus" they write about the Magellan mission:
"Fortunately, a 64-minute launch window had been designed for May 4. After 59 anxiety-filled minutes, the winds dissipated and the clouds parted just enough for launch at 2:46:59 p.m., eastern daylight time (see Figure 9-2), only 5 minutes before the end of the launch window for that day. The shuttle slowly rose out of the billows of steam and accelerated toward the low clouds. It went briefly out of sight and then reappeared for a few seconds, framed in a blue window amid the clouds. It was truly picture perfect.
The Space Shuttle Atlantis compensated for the delay in launch by yaw steering into the correct orbit plane. After five revolutions around the Earth at an altitude of 296 kilometers (160 nautical miles), Magellan was slowly deployed from the shuttle (see Figure 9-3). Sixty minutes later, with the solar panels extended as shown in Figure 9-4, the IUS ignited its two SRMs in rapid succession and propelled the spacecraft on very nearly the precise trajectory to Venus. After firing its attitude-control thrusters for a small course correction, the IUS separated from Magellan and used its remaining fuel to move away from the spacecraft.
Magellan's Path to Venus
The original May 1988 launch period would have allowed Magellan to reach Venus 4 months later via a Type-I trajectory, meaning that from launch to destination, the spacecraft would have traveled less than 180 degrees around the Sun. There was a similar opportunity in the October 1989 launch period initially set aside for Magellan but sub-sequently assigned to the Galileo mission to avoid further delays in its launch.
However, the positions of Earth and Venus during the late-April to late-May 1989 launch period required a Type-IV trajectory (see Figure 9-5). This meant that the spacecraft would travel between 1-1/2 to 2 times around the Sun (slightly more than 540 degrees) and that it would arrive at Venus on August 10, 1990. While it dictated a longer cruise duration (15 months), the Type IV actually had the advantages of reductions in launch energy and Venus approach speed.
Back to the Drawing Board
Magellan's Type-IV trajectory and the resultant Venus arrival date brought about some changes in the basic mapping plan developed for the 1988 mission.
Superior conjunction (where the Sun is positioned between Venus and the Earth) will now occur during the primary mapping mission, instead of at the end. The result is that up to 18 days of mapping data will be lost around November 2, 1990, because radio interference from the Sun will make it impossible to communicate with the spacecraft. Fortunately, the missing data can be recovered in early July 1991, if the mission is extended for additional 243-day mapping cycles.
The trajectory also dictates an approach over the north pole; this will result in a mapping swath from north to south, the reverse of that planned for the 1988 mission.
So you can see how utilizing a different portion of the window can alter (but not ruin) the execution of the mission.
The Venus DRM mission, scheduled for April 30, 2021 on an Atlas V 551 L/V, uses modern calculations based on what we have learned since the Magellan mission; it would be more representative of what is to be done during the week starting Saturday, Aug. 11 2018, at 3:33 a.m. EDT.
We spin (at the equator) at close to 1000 miles per hour (1600 km/hr) and move through space around the Sun at a speed of 66,000 miles per hour (107,000 km/hr).
Relative to the local standard of rest, our Sun and the Earth are moving at about 43,000 miles per hour (70,000 km/hr) roughly in the direction of the bright star Vega in the constellation of Lyra. [Source: AstroSociety.org - "How Fast Are You Moving When You Are Sitting Still?"].
The Earth rotates 360° in 23 hours, 56 minutes, and 4 seconds (1,436.06667 minutes), so the 65 minute window represents 360 / (1 436.06667 / 65) = 16.2945081 degrees - quite a wide range of trajectories.
Venus travels around the Sun at an average speed of 78,341 miles per hour or 126,077 kilometers per hour in its orbit around the Sun.
See Wikipedia's webpage: "Hohmann Transfer Orbit - Application to interplanetary travel", "Bi-Elliptic Transfer", and "Interplanetary Transport Network" for information about "Gravity Assist":
"With any Hohmann transfer, the alignment of the two planets in their orbits is crucial – the destination planet and the spacecraft must arrive at the same point in their respective orbits around the Sun at the same time. This requirement for alignment gives rise to the concept of launch windows.".
A demonstration of the orbits of Earth and Venus around the Sun is offered in the YouTube video "Earth Venus Tango Round the Sun" and is explained at the Wikipedia page "Venus - The Pentagram of Venus":
"The pentagram of Venus is the path that Venus makes as observed from Earth. Successive inferior conjunctions of Venus repeat very near a 13:8 orbital resonance (Earth orbits 8 times for every 13 orbits of Venus), shifting 144° upon sequential inferior conjunctions. The resonance 13:8 ratio is approximate. 8/13 is approximately 0.615385 while Venus orbits the Sun in 0.615187 years.".
Because of the resonance period, due to the orbital speed versus the distance from the Sun, the planets Earth and Venus remain relatively near each other for a longer period than if the ratio were 13:1 - still, precise calculations are especially important over extremely long periods of time.