Delta (rocket family)

Note before you even start to read. I have started with this article in 2018. I have tried to go through all rocket types and families, which were launched or used or planned, but I needed to take a break. So, I had to change several things in the following article. I hope it will still stay consistent.  

Delta rocket family is American expendable launch system since 1960 with 95 per cent of successful launches. Currently Delta IV Heavy rocket remains in use as by November 2020. 

The original Delta rocket was developed and built by Douglas Aircraft Company and it has origin in Thor IRBM (deployed in the UK between the years 1959 and 1963). IRBM is abbreviation which stands for Intermediate Range Ballistic Missile. After Sputnik 1 was launched in October, 4, 1957, Thor was developed into a space program direction and become a basis of Delta rocket. Thor was developed into Delta, so basically Thor is the first rocket in Delta rocket family. The original Delta rockets used Thor as their first stage. 

Delta was supposed to have communicational, meteorological, lunar and scientific goals. At this point launch vehicles were divided into civilian variants launched from the Cape Canaveral carrying Delta name, and military variants launched from Vandenberg Air Force Base carrying Thor name.

Delta deployed many communications satellites during the 1960s and 1970s. It retired in 1981 as NASA prioritized space shuttle satellite deployments over commercial rockets at that time. However in 1986, President Reagan decided to go back to use rockets again and the design of the Delta II began. The first launch occurred in 1989. 

Delta II has flown 154 times and carried missions including the Kepler space telescope, Mars spacecraft Phoenix, Spirit and Opportunity, and many GPS satellites. The first stage of the Delta II was propelled by a Rocketdyne RS-27 main engine burning RP-1 and liquid oxygen. This stage was referred to as the "Extra-Extended Long Tank Thor", as well as all Delta rockets until Delta IV. 

For additional thrust during launch, the Delta II used solid boosters. The vehicle could have different amount of boosters, three, four, or, nine boosters. When three or four boosters were used, all ignited on the ground during the lift off, while models that used nine boosters would ignite six on the ground, then the remaining three in flight after the burnout and jettison of the first six.

The second stage of Delta II was the Delta-K, which was powered by a restartable Aerojet AJ10-118K engine burning hypergolic Aerozine-50 and N2O4. These propellants are very toxic and corrosive, and of it was once loaded the launch had to occur within 37 days, otherwise the stage would have to be replaced. 

For low Earth orbit, Delta II was not equipped with a third stage. For higher orbits such as geostationary transfer orbit or to reach Earth escape velocity for trans-Mars injection or other destinations beyond Earth used a solid propellant third stage. 

Engineers started to design the Delta III rocket in 1995 as a larger, and more powerful launch vehicle than the Delta II. But the Delta III flew only for three years, between 1998 and 2000, before retiring and giving its place to Delta IV. During this time Douglas Aircraft Company was combined with Boeing Company, taking over the rockets. 

Delta III was an expendable launch vehicle made already by Boeing. The first Delta III launch was on August 26, 1998. Of its three flights, the first two were failures, and the third, though declared successful, reached the low end of its targeted orbit range and carried only a dummy payload. The second stage of the Delta III had newly developed Delta Cryogenic Second Stage (DCSS), which burned liquid hydrogen and liquid oxygen. 

The Delta Cryogenic Second Stage (DCSS) is a family of cryogenic rocket stages used on the Delta III and Delta IV rockets, and which is planned to be used on the Space Launch System (known as SLS) Block 1.

Delta IV is a group of five expendable launch systems in the Delta rocket family introduced in the early 2000s. Delta IV first launched at 2002. Delta IV had two main versions: medium, which already retired and Heavy, which should retired by the year 2023. 

Delta IV Heavy is expendable heavy lift launch vehicle, the largest from the Delta IV family and world's second highest capacity rocket in operation behind SpaceX's Falcon Heavy rocket. It consists of a central Common Booster Core, with two additional CBCs as liquid rocket boosters instead of the GEM-60 solid rocket motors which were used for Delta IV Medium version. CBC is 40.8m long, with 5.1m diameter. It is powered by a single engine RS-68 burning liquid hydrogen and liquid oxygen. Liquid rocket booster uses liquid fuel and oxidizer to give a liquid propellant or hybrid rocket extra boost at the take off, or if need to increase payload to be carried out. Solid rocket booster on previous version used to provide thrust from initial launch through the first ascent. For example, Space Shuttles have used them as well. 

At a lift off, all three common booster cores operate at full thrust. 44 second after lift off center core throttles down to 55 per cent to conserve fuel until booster separation. Which is the different from the Solid rocket booster. Solid rocket boosters are not controllable and generally it must be burn until complete exhaustion after ignition. 

The rocket uses three RS-68 engines, one in the central core, and one in each booster.  It is a liquid fuel rocket engine that uses liquid hydrogen and liquid oxygen LOX as propellants in a gas generator power cycle. 

The gas generator power cycle is power cycle of bipropellant rocket engine. The gas generator turbine does not need to deal with the counter pressure of injection the exhaust into the combustion chamber, compare to staged combustion cycle. The engine is lighter. Gas generator cycles tend to have lower specific impulse than staged combustion cycles. 

The upper stage of the Delta IV is the Delta Cryogenic Second Stage, which is based on the Delta III upper stage with increased propellant capacity. Two version were produced. The one version had 4m diameter and has been used for Delta IV Medium. The second version has 5m diameter and is used for Delta IV Heavy. Each Delta Cryogenic Second Stage is powered by one RL10B2 engine, with an extendable carbon-carbon nozzle to improve specific impulse.

Capacity of the Delta IV Heavy:

• Low Earth orbit (LEO), 200 km × 28.7°: 28,790 kg (63,470 lb) 
• Low Earth orbit (ISS), 407 km × 51.6°: 25,980 kg (57,280 lb) 
• Geosynchronous transfer orbit (GTO): 14,220 kg (31,350 lb) 
• Geosynchronous orbit (GEO): 6,750 kg (14,880 lb) 
• Lunar transfer orbit (LTO): 10,000 kg (22,000 lb)
• Mars transfer orbit: 8,000 kg (18,000 lb) 

The Delta IV Heavy's total mass at launch is approximately 733,000 kg (1,616,000 lb) and produce around 952,000 kg (2,099,000 lb) of thrust to power the rocket skyward at liftoff.

And here comes the question. What is the resulting force upwards? It is only approximate number, since the mass and related engines, boosters differ accordingly. 

Mass of the rocket m = 733 000 kg

Calculate the weight force W = m  g = 7 190 730 N

Thrust should be higher than the weight to be able to lift off T  > W

Calculated thrust force from the given value T = 9 339 120 N

Resultant force upwards is the difference between the forces = 2 148 390 N. 

In any case, positive value says the rocket goes upward, otherwise it stays where it is. And that could be the best case.