Being a big fan of both Apollo 13 and From the Earth to the Moon (FTETTM ), I have heard this expression a lot and I assume that it was often used by Apollo astronauts after an engine had been fired.
I initially thought that the “no residuals” part meant something chemical as in “the burn was complete and no bad gases were formed” which never quite made sense to me. After watching a recent FTETTM episode I thought that the true meaning of the phrase may be closer to “everything went good and no warning lights went off, there were no bad after effects”
If the second option is correct then my question would be, why not just say that it was a “good burn” and leave it at that? “there were no residuals” is redundant.
There must be more to this.
Any insights?
I’m not sure, and I don’t have my copy of the book with me to check, but I remember reading something about how after they did a burn, they would sometimes have to make a “correction burn” if the first one was mis-timed even by a little.
I believe this was particularly an issue when the Apollo 13 crew made the manual (unassisted by computer) burn to (I think) get out of the slingshot trajectory and into a safe re-entry trajectory. IIRC, they didn’t have to make a correction burn, which was surprising since it was performed manually.
Anyhow, I’m not sure, but it could have to do with this, maybe.
When you use a rocket for orbital maneuvers, your goal is not a position as much as it is a velocity (which is comprised of a direction and a speed, but let’s focus on the speed part).
You may make a burn now that is expected to put you on a course to reach your desired position in a few minutes, or a few hours, or even a few days. By using very very weak rockets with very very fine timing, you can control your velocity to within centimeters (millimeters?) per second of velocity. This is important, because a velocity error causes drift: if your velocity is off by one meter per second, then at the end of a minute you’ll be off-target by sixty meters. At the end of an hour you’ll be off by 3.6 kilometers. Imagine directions for driving to Dayton Ohio from Baltimore MD that said: “get on I-70, aim west, achieve a speed of 60 mph, and hold that speed for exactly 8.000 hours.” If your speedometer was off by a few mph, you could end up in Columbus or perhaps even Indianapolis.
If you’re aware of the error, and it’s a shortfall, you can “trim” it off by adding a little bit more speed. If it’s an excess of speed (e.g. if a valve leaked a little and your burn lasted .01 seconds too long) then you correct it by pointing in the other direction and firing your rockets again but this uses up fuel. That’s the “correction burn” mentioned in the replies above. All of the fuel that went into the extra speed is wasted twice: once to speed up and again to slow down. So a “clean” burn means you use exactly as much fuel as you budgeted for, and traded it for exactly as much velocity as you wanted.
But how would they know this immediately after a burn? The movies make it seem that they know right away whether the burn was good or not. Do they not have to check reading or measure something?
Two methods: one is inertial tracking, i.e. sensitive instruments on-board (gyroscopes, accelerometers) measure the axial thrust, yaw, and pitch. You integrate this all up over the burn time (accounting also for lost mass of propellant) and see if it matches the planned course within acceptable error bounds (usually 1.5-sigma allowable for satellite launches, I think); if not, you have to calculate correction burns. The second is from remote measurement (ground-based radar, GPS/GLONASS, et cetera) which measures position and velocity instantaneously over a short course and compares to the predicted course. The latter method is what was typically used during the early manned space program as on-board instruments and computing power were insufficient to really get a good handle on position and velocity–those Mercury capsules were really little more than reinforced boilerplate and some rudimentary avionics and life support–but more modern systems can perform calculations from inertial measurements that are as precise if not more than ground tracking systems.
A third way of tracking position and velocity is to use stellar parallax; that is, you take sightings on a set of stars, and then take them again a little while later, et cetera, similar to ocean-going stellar navigation, and in fact a sextant was part of the equipment on the Apollo missions as a backup to inertial guidance, as radio-based parallax from Earth was not sufficiently accurate. Ballistic missile systems like Minuteman III have stellar guidance systems as a backup to the inertial systems, although it is generally acknowledged that the on-board inertial systems are more than sufficient to achieve a high degree of accuracy for warhead deployment; the stated circular error accuracy (CEP) of the Peacekeeper system was ~100m, but the AIRS gimbal-less navigation system is capable of placement on the order of ~1m, the remaining inaccuracy coming from variations in re-entry profile from RV ablation and dynamic instability.
To address the question of the o.p., I’m not sure of the terminology use with manned spacecraft, but with satellite launches “residuals” are usually gyroscopic errors, i.e. unintended pitch, yaw, or roll. Sometimes a deliberate roll is induced to help spin-stabilize the spacecraft or assist in deployment of antennae or solar panels, but residual pitch and yaw (rotations perpendicular to the spacecraft’s main axis) are almost always extremely undesirable. This is usually corrected by the spacecraft’s on-board propulsion and RCS systems rather than by the delivery system, although the Agena Upper Stage/Agena Target Vehicle was intended to remain attached to spacecraft like the Corona surveillance satelites for propulsion assist and stabilization.
With regard to the film Apollo 13, there are numerous (albeit mostly niggling) technical errors, but one glaring one is that the Aquarius LM engine was actually fired three times: once to place it into the correct lunar swing-by trajectory, one to achieve lunar escape velocity, and a final correction burn. This is alluded to in the dialogue–Haise say something like, “Gee, I hope we don’t have to do that again!”, and the Grumman representative objects to the concept of throttling and restarting the LM descent engine–but only one burn is actually indicated. In reality, the LM descent engine was in fact designed to be restartable for eventual use as a Lunar Exploration System, making hops from site to site using previously deployed refueling stations, and to that end used a more simple and robust (if lower performance) pressure-fed system with hypergolic propellants. Also, far from having one obtuse, bureaucratic representative from Grumman, the Grumman corporation in fact is reported to have provided extensive support, with hundreds of engineers and technicians reporting to the plant as soon as they heard of the Apollo 13 problem. Although using the LM as a lifeboat and alternate main propulsion system was never specifically addressed, Grumman had done a number of studies involving adapting the LM to serve as an orbiting lab and observatory module, space tug, orbital ambulance, et cetera.
Stranger
Right, but then shouldn’t it be the ground people saying “good burn, no residuals” no the astronauts?
Well, gyroscopic errors (residual pitch, yaw, and roll) are only going to be discernable to the crew and on-board systems. As previously mentioned, modern inertial guidance systems (even those used on Apollo) are going to be at least as accurate and provide information on velocity faster than ground tracking. Of course, in reality all of this information is being relayed in the telemetry stream, so all the astronaut is doing is confirming what the mission controllers already know; that, and it gives the astronauts something to do, because for the most part the crew is actually pretty useless in terms of flying the craft, which is mostly done by a combination of computer control (analog, in the case of Gemini, or a crude ballistic autopilot for Mercury) and Newton’s laws. The only time having people up there really comes in handy is when things get seriously screwed up, like Armstrong’s famous save on Gemini 8, but of course it also increases perceived risk.
Stranger