Almost 45 years ago man walked on the Moon. Space exploration seemed to progress at very fast pace. Just 12 years before the first human-made object was inserted into a stable orbit and 8 year before there was the flight of Gagarin.
An manned mission to Mars seemed feasible in a near future, science fiction even thought possible a manned station orbiting around Jupiter in 1999 (obviously I am referring to 2001: A Space Odyssey). It didn’t happen. Human exploration of our Moon was soon stopped and all the following manned mission were confined to the LEO (low Earth orbit).
Don’t get me wrong, space exploration has given amazing images/data so far, let’s only consider the following ones:
- Voyager probes
- Mars probes (landers, rovers and satellites)
- ISS space station
- Space telescopes (Hubble, Chandra, XMM)
- SoHo probe (Sun observation)
- Cassini–Huygens probe (Saturn exploration)
When Neil Armstrong stepped on the Moon we had very little knowledge about the Solar System. For instance, we didn’t know that Saturn has hexagon storm at the North pole, that Mars hosted water or Iapetus has a dramatic equatorial ridge.
Nevertheless, I think we can say that most of the people in 1969 would have been disappointed by knowing in advance the future of space exploration in the following 50 years. Let’s face it:
- Manned orbital missions are still very expensive, dangerous and limited to a little crew.
- There is not any manned mission or station outside the LEO.
- Mars exploration program seems still very far.
What happened? Well, obviously there are many technical difficulties to manned missions like life sustaining, radiation exposure, negative effects of lack of gravity on the human body. Also political and strategic choices made by governments have influenced the timeline of space exploration. However, I think there is a key factor that is enough to explain the limited progress in the last 50 years:
The theoretical limit of chemical propulsion was already reached 60 years ago
The most important number of a propulsion technology is the effective exhaust velocity (strictly related to the specific impulse), i.e. the average speed of the exhaust jet as it leaves the vehicle. For LOX (Liquid Hydrogen+Oxygen) propulsion this speed is 4.46km/s, for solid fuel rockets is in the range 2.6-2.8km/s. Solid fuel propulsion is usually used only in initial stages in rockets. Well, the effective exhaust velocity hasn’t increased from the Sputnik rocket or Saturn V basically because further improvements were not possible. Look at these examples of space rockets:
- Sputnik (rocket), 1957: ~3km/s
- Saturn V, 1967-1973: 2.58km/s (first stage), 4.13km/s (other stages)
- Space shuttle, 1981-2011: 2.64km/s (boosters), 4.46km/s (shuttle propulsion)
- Soyuz-2, 2004-current: 3.0km/s (first stage), 3.5km/s (second stage), 3.2km/s (third stage)
- Delta IV, 2003-current: 2.7km/s (boosters), 4.0km/s (first stage), 4.53km/s (second stage)
- Ariane V, 1996-current: 2.7km/s (boosters), 4.3km/s (other stages)
- Vega, 2012-current: 2.7-3.1km/s
As you see, the most modern rockets (Ariane V, Delta IV, Vega) are just slightly better than Saturn V. We can improve the reliability, the automatic control, the equipment but we can’t improve the propulsion because this is the maximum we can get from chemical propulsion. What does it mean that we can’t improve the effective exhaust velocity?
It means that we will continue to need a 137tons rocket to put a 1.5tons satellite into orbit or 334tons rocket to send a 900kg probe to Mars. This huge ratio between fuel and payload is described by the Tsiolkovsky rocket equation. Space exploration requires maneuvers with high delta-v (velocity variation), usually 15-20 km/s for interplanetary missions. The amount of fuel increases exponentially with delta-v. This leads to all the main issues of space engineering:
- Mass is the most critical factor in a payload. Everything must be optimized to be light, with huge increase in cost and complexity.
- Cost, complexity and intrinsic dangers of rockets can’t decrease much as their size will always be huge. Single-stage-to-orbit are still unfeasible.
- Interplanetary journeys are often very long and complex. About 9 months are necessary to go to Mars, 7 years were needed for Cassini-Huygens to go to Saturn through 4 gravity assists (Venus-Venus-Earth-Jupiter). This makes a manned mission to Mars very critical for its long duration.
The conclusion to this is that we can’t expect huge progresses in space exploration until we develop a propulsion based on higher energy-density fuel than the chemical ones. Basically we are talking about nuclear propulsion. Unfortunately this won’t happen in the next 30 years, so let’s manage our expectations.