SOUNDING ROCKETS  – NEAR EXTINCTION OR APPROACHING REVIVAL?

Thirty years ago, the sounding rocket was seen as obsolete as a tool for science and technology. Its purported nemesis then was Spacelab, the manned laboratory in the Shuttle cargo bay. Well, the 80’s saw an intensive series of sounding rocket launches for microgravity and space science. The micro-gravity experiments were seen as preparatory to Spacelab and later ISS experiments and the space science rockets became increasingly more complex.

The past twenty years has seen constant or slowly waning activity in the use of sounding rockets here in Sweden. The reasons are manifold – more satellite experiments are available to scientists and the extensive Swedish program in the 70’s and 80’s touched many of the relevant scientific issue that can be explored with sounding rockets. But, instrument technology, information technology and branches of space technology like control engineering have advanced rapidly in the last 20 years.

Is it time to revisit some of the scientific branches and topics that used sounding rockets (and also balloons) in the past? Have these new technologies possibly changed how we see the role of sounding rockets and balloons? Or, are there new fields that can use the sounding rocket and the balloon as tools? Isn’t it time for scientists and space industry to take a systematic common look at these questions?

A short look at history
The graph below shows the payload mass launched by sounding rockets in the Swedish national sounding rocket program up till now. Of course accumulated payload mass is not a perfect measure of activity, but it is easy to measure and in some way indicates the level of activity. After Sweden took over Esrange and the Swedish Space Corporation was formed there was a surge of activities that lasted about ten years. Almost 1/3 of the total payload mass was launched in the five-year period 1976-1980.




It has long been conventional wisdom that while sounding rocket launches have been fewer and fewer the payload mass per rocket has increased. Again, let us see what history tells us:


Already at the end of the 70’s the average payload mass reached the 150 kg level. The peak in the late 90’s is due to two single launches and therefore not so significant. The drop to below 100 kg in recent years is due to the switch to single-stage Orion rockets instead of two-stage vehicles. Thus, average payload mass has been around 150 kg for the period 1976-1995, i.e. during a period of twenty years. Admittedly, microelectronics advanced rapidly to provide more functionality per kg during this period. But anyhow, conventional wisdom is not completely true.

However, these overall graphs hide some interesting details which appear if one distinguishes between payload mass for atmospheric studies and that for magnetospheric and purely ionospheric investiga-tions. Admittedly, sometimes the distinction between D-layer research and atmospheric physics is blurred, but I have tried to separate projects into categories to the best of my ability. The graph below shows that only atmospheric research shows a relatively constant level of activity. 

 

Magnetospheric scientists entered the sounding rocket game late since they were busy with early satel-lite experiments in the 60’s and “dropped out” of sounding rockets in 1986 after a truly magnificent activity peak at the end of the 70’s. The last magnetospheric research rockets in the Swedish program launched from Esrange were those that were coordinated with the Viking satellite at the end of 1986. The magnetospheric scientists in Sweden went back to satellites as carriers of their instruments, while the atmospheric research group (i.e. the Institute of Meteorology at the University of Stockholm) kept up its interests in in-situ measurements despite their deep engagement in the Odin satellite project.

If one sums the payload masses for the two categories one finds that about 5000 kg of payload mass have been launched for atmospheric research and 3000 kg for magnetospheric research.

Advances in technology since the “halcyon days” of sounding rockets
So, sounding rockets in the Swedish programme and launched from Esrange have not been used by magnetospheric scientists since twenty years. But some rather sophisticated experiments were tried during the “halcyon days” like “mother-daughter” payloads – both with an instrumented “daughter” and with a chemical release “daughter”. When one looks back at these experiments one can note that they were both advanced and unsophisticated.

Formation flying – an example
For example, the S-17 payloads were launched by Nike-Tomahawks in the winter of 1976. This was a very sophisticated sounding rocket project, even according to today’s standards. Both “mother” and “daughter” were instrumented with booms, electron density probes and particle detectors. The daugh-ter was manoeuvred sideways at 5 m/s by a small solid rocket motor. A Doppler radar on the “mother” provided range-rate from which relative range was calculated. Magnetometers on both payloads showed the spin orientation vis-à-vis the magnetic field lines. A star camera also provided data that could be reconstituted as attitude after the flight. The idea was to distinguish between temporal and spatial variations in key magnetospheric parameters.
Let us just imagine what we could do with modern technology in an experiment like this;

Monopropellant thrusters coupled with a miniature inertial platform (e.g. the IMT30 from IMEGO) could be used to manoeuvre the “daughter” or multiple “daughters” to exactly the desired location relative to the “mother”. The inertial platform could also provide accurate attitude data. Back-up relative position data could be provided by onboard GPS receivers and attitude updates to the IMU could be provided by miniature star trackers with direct attitude read-out now available “off-the-shelf”.

The technology for such “formation flying” could possibly be derived from the Swedish Prisma formation-flying demonstrator satellite project. Nowadays the design of complex guidance, navigation and control systems is much simplified by modern tools for integrated design, simulation and test so such a project is more affordable.

Modern data handling
The data rate from the S-17 “mother” was 205 kbps and from the “daughter” 128 kbps. Of course this pales in comparison with for example the MASER service module which can provide a total data rate of almost 20 Mbps (3×5 Mbps for digital video and 1.5 Mbps for housekeeping and experiment telemetry). Modern systems could easily provide 100 times the data rate of 1976.

Sounding rockets have now adapted to the international CCSDS telemetry and telecommand standard where packet telemetry is now “conventional technology”. For sounding rockets where data rates need to be extremely high during short flights this is probably still the most efficient data transfer mode. But for balloon payloads, where rapid changes in data are uncommon, computer-to-computer links with extensive “handshaking” for file transfer could be a good new standard to develop. SSC has made some initial strides in this direction with its E-LINK system.

Science sensor technology
Science sensors also have been reduced in size while increasing functionality. An example is the mass analyzer (PRIMA) from the Institute of Space Physics in Kiruna using micromechanical shutters to be tested on the Swedish PRISMA satellite (see picture on the lower right).

Recovery aids
An important factor behind the survival of the sounding rocket as a research tool has been the use of recovery systems even for payloads that do not return physical samples. One driving force behind the use of parachute recovery has been the need for post-flight calibration of sensors, but mainly it has been a way of saving money through the re-use of both service systems and science instruments.

If one takes a look at today’s recovery systems the only innovation is that we use GPS for locating the payload after landing. However, recovery is still a risky business. The payload may land in difficult terrain and be damaged or fall into water. But technology now provides the ability to steer parachutes to selected landing site by GPS navigation. Why couldn’t there be, for example, sites in the Esrange impact area that have certified as reasonably smooth for landing that the GPS-supported parachute navigation computer can choose between as it descends? This could potentially save money and in-crease the efficiency of recovery. Perhaps this is worth examining as a means of rejuvenating sounding rocket technology? Or could air-bags such as used for landing on Mars be used to protect the payload from damage upon landing?

Hopeful/useful signs
So, is the sounding rocket still a useful tool for space science and technology? I think so. Let me men-tion three factors that give a glimmer of hope:

• The examples above hopefully show that there is plenty of powerful new technology available that perhaps could be attractive to users of sounding rockets both for refining old measure-ments and for solving new scientific problems…
  
• Also, upper-atmosphere research has a “second wind” due to the climate debate. Upper-atmosphere conditions seem to be precursor indicators of larger changes in the climate at lower altitudes. Providing in-situ validation for remote sensors on satellites is another obvious application in this context. The sounding rocket (and indeed balloons) may be precisely what is required a this point in history.
  
• Despite much talk of small satellites and even student satellites as training tools for young sci-entists and engineers sounding rocket projects still are attractive because they can be carried out in a couple of years from first idea to real data.
  
  Recruiting good people to our field needs a bolder approach. Sounding rockets are still a cost-effective training ground not only for graduate students at research institutes but also for engi-neers and managers in the space industry.
  
  In my personal experience it is hard to overestimate the benefits that SSC’s Space Systems Division has had from its sounding rocket activity, both for training the technical and manage-rial staff and for providing a technology test-bed.

Recommendations
So, what should we do if we want to keep the sounding rocket as a useful tool in the future?

Revisit scientific themes in the light of new technology
The most obvious action is to gather space agencies, scientists and space industry in a study where scientific themes are revisited in the light of new available technology. The study should examine:

• Which themes can benefit from a rejuvenated sounding rocket technology.
• Which new technology is worth incorporating into new sounding rocket vehicles and payloads.

Provide regular flight opportunities
An equally important action – once the future direction of sounding rocket technology has been sketched – is to provide regular flight opportunities. I am not talking about a scheduled “sounding rocket bus” departure schedule, but a stable “line item” in the space budgets of space agencies. If this is provided science groups can spend serious efforts on studying and proposing ambitious sounding rocket projects and recruit the young scientists to use them. Also, space industry can build at least a modest product line – otherwise the price of an occasional sounding rocket project will be ex-orbitant. A certain business volume is required to attain at least a semblance of industrial efficiency.

Hopelessly romantic about sounding rockets –

Best Wishes

Sven Grahn