Thursday, November 10, 2011

The Scout Launch Vehicle

The Scout was an all solid rocket motor launch vehicle. It was one of the smallest and earliest launchers developed by NASA, used for payloads less than about 550 lbs into low Earth orbit (LEO). Its development began in 1957 and its first test launch was in 1960, just 2 years after the US launched the Explorer satellite. The first Scout successfully placed a payload into orbit in 1961. The Scout family of rockets had a long lifespan of over 30 years with 118 launches and the last one being launched in 1994.



There's a lot of information on the Scout launch vehicle available on the web, and there were many different variants over the years with payload capabilities from a few pounds up to 550 pounds. I will only concentrate on aspects relevant to the discussion of developing small launchers.

GENERAL
The Scout was a four stage all-solid propellant vehicle (except that liquids were used for orientation control of the upper stages). The first stage was an Algol motor which used aerodynamic fins and jet vanes in the rocket motor exhaust stream for control. The second stage was a Castor motor which used hydrogen peroxide reaction control rockets for guidance and orientation. The third stage was an Antares motor which also used hydrogen peroxide reaction control rockets for orientation. The fourth stage used an Altair motor which was unquided but spun on its long axis for orientation stability.



GUIDANCE AND CONTROL
It is worth focusing on the guidance and control approach used on the Scout vehicles since it has lessons for those designing small launch vehicles. First, the guidance and control system of the Scouts was a relatively simple pitch rate system controlled by a timer. The roll and yaw axes used a gyro system to ensure that the vehicle maintained a specific roll and yaw attitude throughout its flight. This reduced the guidance problem from three rotational orientation dimensions down to a one-dimensional pitch over problem. Clockwork mechanisms used pre-programmed pitch rates to ensure that the vehicle had the proper pitch throughout the flight to orbit. At the end of the third stage flight, spin rocket motors spun up the fourth stage to provide it some amount of orientation stability. Then the fourth stage motor was ignited to add the last amount of delta V required to get the payload to orbit. Like other small launchers, the final stage was unguided and the lower stages were responsible for orienting it before firing.

MASS CONSIDERATIONS
We can also learn something by considering the side effects necessary to take a raw motor and turn it into a useful stage. For the following analysis, I will consider the Scout D configuration.

The first stage of the Scout D used a Algol III motor. In its basic configuration, the motor weighs 31305.10 pounds and has a total propellant weight of 28,059.77 pounds. Subtracting the propellant from the total weight leaves an inert weight of about 3245.33 pounds. This results in an inert:propellant ratio of 0.116. However, if we look at the stage weights of the first stage, we see that its inert weight is 4211.37 pounds. Therefore, the control system (which includes fins, jet vanes and other hardware) adds about 966.04 pounds resulting in a stage inert:propellant ratio of 0.1489. So, the inert weight increased from 3245.33 lbs to 4211.37 lbs for a weight increase of 130 % (30% more weight for the control system).

The second stage of the Scout D used a Castor IIA motor. In its basic configuration, the motor weighs 9774.26 pounds and has a total propellant weight of 8206.15 pounds. Subtracting the propellant from the total weight leaves an inert weight of about 1568.11 pounds. This results in an inert:propellant ratio of 0.1911. Looking at the full second stage weight, however, we see that the stage inert weight is 2399.06 pounds. Therefore, the control system and staging mechanisms of this stage weighs about 831.49 lbs resulting in a stage inert:propellant ratio of 0.2891. So, the inert weight increased from 1568.11 to
2399.06 lbs for a weight increase of 153 % (50% more weight for control and staging systems).

The third stage of the Scout D used an Antares II motor. In its basic configuration, the motor weighs 2796.80 pounds and has a total propellant weight of 2559.43 pounds. Subtracting the propellant from the total weight leaves an inert weight of about 237.37 pounds. This results in an inert:propellant ratio of 0.0927. Looking at the stage weight, though, we see that the stage inert weight is 771.95 pounds. Therefore, the control system and staging mechanisms added about 534.58 pounds with a resulting stage inert ratio of 0.2982. The staging and control system added 534.48 lbs to the motor's 237.37 lbs and thus added 225 percent (or 125 percent increase) to this stage.

The fourth and final stage of the Scout D used an Altair III motor which weighs 666.63 pounds full and has 607.15 pounds of propellant; the inert weight is thus 59.48 pounds for an inert:propellant ratio of 0.0980. The third stage inert weight is 104.62 lbs, 45.14 lbs worth of staging mechanism (since there is no control system). This is an increase of 176% (or 76% increase) just for staging mechanisms.

Although this section might seem like just a bunch of useless weight breakdowns and percentages, it is sometimes good to know these percentages to provide a grounded estimate of control system weight increase. One trend that might be noticed is that as we go up the stages, the mass of the control and staging mechanisms become a higher percentage of the stage weight. This is because it's not always easy to scale these down equally with the stages. This issue is especially significant for smaller launch vehicles.

AERODYNAMIC AND GRAVITY LOSSES
Just to be complete and provide some basis for comparison, I will give my estimate of the aerodynamic and gravity losses.

Because the Scout had a high fineness ratio (length to diameter ratio) of about 20, it had relatively low aerodynamic losses. Of course, the trajectory and other factors affect the overall aerodynamic losses and gravity losses, but using a simulation close to that of the Scout 185C, my simulations show that the aerodynamic losses were about 327.76 fps and gravity losses were about 2640.40 fps for the first stage.

LESSONS THAT ONE MIGHT LEARN
There are three major lessons that I think are worth learning from the Scout family of rocket launchers. First, it is possible to have an orbital rocket launcher which uses only simple one-degree pitch control guidance (once the other two axes are stabilized). Second, it is possible to have no guidance in the final stage. Third, it is possible to have orbital rockets with length to diameter ratios of 20 or more at lift-off. These facts provide some credibility for smaller orbital rocket designs that might also utilize these design techniques.


REFERENCES

Scout: The Unsung Hero of Space
YouTube http://www.youtube.com/watch?v=btRk6AhoOmI

Scout Launch Vehicle Program, Part 2, Phase 6 Final Report
NASA, 19820073029_1982073029.pdf

Project Development Plan: Scout Program
NASA, 19810069924_19810069924.pdf

No comments:

Post a Comment

Note: Only a member of this blog may post a comment.