Words have meaning: Delta-V

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.


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