Rocket Engines

Technology Roadmap Sections and Deliverables

Roadmap Overview

The primary function of rocket engines is to convert chemical energy to kinetic energy. This means that a rocket engine would be placed squarely on the cell “Transforming Energy” in our 5x5 technology matrix. There are several different styles of rocket engines that are used today (all of which meet the primary functional requirements) but the perform those functions in slightly different ways with slightly different architectures. These architectures are all determined by their fuel type and their fuel consumption method. The rocket engines we will examine here include: solid rockets, open cycle liquid fuel, closed cycle full flow liquid fuel, and nuclear. Below is a generalized OPD describing the operation of a liquid fuel rocket engine.

Design Structure Matrix (DSM) Allocation

Below is a schematic DSM for the simplified and generalized liquid fuel rocket engine described in the first section. Cells colored black indicate a physical connection between formal elements. Red indicates a mass flow between formal elements. Green illustrates an energy flow between formal elements. There can be multiple colors for a single cell.

Roadmap Model Using OPM

Solid Rocket Boosters

Solid rocket boosters are a comparatively simpler system. However, once ignition starts, there are no ways to stop the process. Ammonium perchlorate acts as the oxidizer, and aluminum powder acts as the fuel source.

Examples of solid rocket boosters getting used in large quantities are the space shuttle and the lower stages of the Ariane-6 rocket.

Gas Generator Open-Cycle Engine

A gas generator open cycle engine is one of the simplest forms of liquid fueled rocket engines. In order to supply more fuel and oxidizer to the combustion chamber – which in turn increases pressure of combustion, and generally higher thrust – turbopumps are installed in the lines of the engine. The turbopumps are rotated by a small rocket engine called a preburner which is itself fueled by the same liquid fuel and oxygen. The spent preburner exhaust is directed outside of the main throat and bell as exhaust.

Examples of this style of rocket engine are the F-1 and the Merlin Engine.

Oxygen-rich, Closed-Cycle, Staged-Combustion Engine

An oxygen-rich, closed-cycle, staged-combustion engine is one of the more complex than open cycle gas turbine engines, but they are more efficient because they use more fuel and oxidizer in the main combustion chamber. In these engines, all oxidizer is pumped into the preburner, with some fuel. The remainder of the fuel is pumped directly to the main combustion chamber. The preburner exhaust is oxygen rich, and is pumped into the main combustion chamber where it supplies the required oxidizer for combustion.

Examples of this style of rocket engine are the RD-180 and the BE-4.

Fuel-rich, Closed-Cycle, Staged-Combustion Engine

A fuel rich dual staged combustion closed cycle is similar in concept, but much more difficult in execution to the already complex oxygen rich staged combustion closed cycle engine. In the fuel rich closed cycle engine design, all of the fuel is sent to the two preburners, and the oxidizer is sent to the preburners as well as the main combustion chamber. Super-heated, fuel-rich exhaust is then directed from the preburner to the main combustion chamber. The advantage to this setup is that it is very efficient and generally has a high thrust to weight ratio as it is quite compact in its dimensions.

An example of this engine is the RS-25.

Full-flow, Closed-Cycle, Staged-Combustion Engine

A full flow staged combustion closed cycle engine takes advantages of oxidizer rich and fuel rich closed cycle designs. This design is technically the most difficult to successfully built, and only one engine has ever been flight tested with this configuration. The key differentiator is that flowing fuel and oxidizer are never directly connected to the main combustion chamber. Instead, they are all directed through turbopump assemblies. One assembly is responsible for producing an oxygen rich exhaust – which drives the oxidizer turbopump – and the other assembly produces a fuel rich exhaust – which drives the fuel turbopump.

There are only three engines which have ever attempted this design, and only one has ever flown. The soviet RD-270, the Rocketdyne integrated powerhead demonstrator (never tested), and the Raptor engine, which was first flown in August 2019 for a 1 minute 150m flight demonstration.

Nuclear Engine

Nuclear propulsion is slightly different then chemical rockets, in that chemicals are not getting combusted, but rather chemical reactions are taking place. A nuclear rocket uses the expansion of liquid hydrogen to gaseous hydrogen in confined spaces to generate thrust. The heat required to generate this expansion is achieved by forcing hydrogen through a nuclear core. Nuclear rockets have significant advantages to chemical rockets in their Isp values, but the major downside is their very low thrust to weight ratios.

The NERVA rocket is one of several examples that have ever been built.

Figures of Merit

Tsiolkovsky Rocket Equation (m/s)

       Δv = ve*ln(m0/mf)

• ve = effective exhaust velocity
• m0 = initial total mass including fuel and oxidizer
• mf = final mass without fuel and oxidizer

Specific Impulse (s)

       Isp = ve/g0 

• ve = average exhaust speed along the axis of the engine (either at sea level or vacuum)
• g0 = standard gravity in m/s2

Net Thrust (N)

       Fn = M*Ve

• M = exhaust gas mass flow
• Ve = effective exhaust velocity

Thrust-to-weight ratio (N/kg)

       TTW = Net Thrust (N)/m (kg)

• m = dry mass of the engine

Comparison of FOMs Across Platforms

Engine Name	Engine Type	Use Case	Year of First Development	Isp vac (s)	Thrust vac (kN)	Dry Mass (kg)	Thrust/Weight Ratio	Cost ($Million)	Isp/Weight (s/kg)	Thrust/Cost (kN/$)	Isp/Cost (kN/$)
Raptor	Full flow staged combustion closed cycle	Starship	2019	330	3,297	1,700	198	1	170	3,297	330
BE-4	Oxygen rich staged combustion closed cycle	New Glenn	2020	340	2,400	Unknown	Unknown	8	Unknown	300	43
Merlin	Gas generator open cycle	Falcon 9	2007	305	981	470	213	0.75	146	1,308	407
Vinci	Gas generator open cycle	Ariane 6	2017	465	180	93	197	Unknown	240	Unknown	Unknown
Vulcain 2	Gas generator open cycle	Ariane 6	1995	431	1,140	1,800	65	Unknown	682	Unknown	Unknown
P120	Solid rocket booster	Ariane 6	2018	279	4,500	11,000	42	Unknown	682	Unknown	Unknown
Space Shuttle SRB	Solid rocket booster	Space Shuttle	1975	242	12,000	91,000	13	8	1,835	1,500	30
RS-25	Fuel rich staged combustion closed cycle	Space Shuttle	1980	452	2,297	3,177	73	50	630	46	9
RD-180	Oxygen rich staged combustion closed cycle	Atlas V	1989	338	4,150	5,480	77	25	446	166	14
F-1	Gas expansion open cycle	Saturn V	1964	304	7,770	8,391	94	30	328	259	10
NERVA	Nuclear solid core	Test Only	1964	850	334	18,144	2	Unknown	46,175	Unknown	Unknown

Alignment with Company Strategic Drivers

The table below shows some potential strategic drivers and alignment of the Raptor engine technology roadmap.

Number	Strategic Driver	Alignment and Targets
1	To develop a liquid fueled, reusable, full-flow, staged combustion, closed-cycle rocket engine that can enable an affordable launch marketplace in which we have a distinct first-mover advantage.	The liquid fuel rocket engine roadmap will target the development of a methane fueled rocket engine that will provide at least 3,000 kN of thrust in vacuum. This would represent a disruptive technology in the rocket engine marketplace. It is aligned with the liquid fuel rocket engine technology roadmap.
2	Capture marketplace advantage through low cost manufacturing capacity ensuring dominant design architecture from a cost-normalized standpoint	This liquid fuel rocket engine roadmap will target the development of a rocket engine that can be rapidly produced in an assembly line style to minimize cost. This rocket engine will be produced at a rate of 500 engines per year, or 1 engine every 12 hours at dedicated fabrication facilities – enabling costs of <$1 million per engine. This would represent a disruptive technology in the rocket engine marketplace. It is aligned with the liquid fuel rocket engine technology roadmap.
3	To ensure we are the rocket engine of choice for multiple use contexts, the engine shall provide a sufficiently high Isp such that clients requiring less thrust (with lighter payloads) will still find the product ideal for their projects.	The liquid fueled rocket engine roadmap will target the development of a rocket engine that will deliver an Isp in vacuum of at least 330s, and a thrust to weight ratio of at least 190. This would represent a competitive position in the existing marketplace. It is aligned with the liquid fuel rocket engine technology roadmap.

The company’s technology roadmap is aligned with all three of its strategic drivers. There is sufficient organizational capability, funding, and time to meet our ambitious target launch date using these new rocket engines by the second half of 2020. All of the targets are within the parameters set forth by the governing equations, and thus they are technically feasible. They would indeed represent a disruptive technology in the aerospace field, and so there may be hurdles that are not captured by the roadmap (e.g., certification, new regulations at launch sites, or unscheduled rapid unplanned disassembly events).

Positioning of Company vs. Competition

Summary

At the highest level, there are two semi-competitive solutions – governmental funded agencies and private companies. Publicly funded agencies are nation specific, and account for between 50% and 75% of all orbital class launches annually. For orbital class launch vehicles, China, India, Iran, and Japan utilize exclusively publicly funded organizations. Russia uses almost exclusively publicly funded organizations with rare exceptions. Chinese and Russian launches together account for ~90% of all publicly orbital class launches in the world on an annual basis. The remaining 50% to 25% of all orbital launches annually are publicly and privately funded but launched by private companies.

Public Competitive Landscape

China and Russia are the primary consumers of state-funded and state-organized (and state-researched, and state-produced) products. For these examples, the pricing scheme is extraordinarily difficult to find detailed reports. However, one can safely assume that massive amounts of funds are dedicated to the research of new technologies. In examining the development of the “Long March” rocket (the current iteration has roots back to 1993) there have been over 100 launches of various incarnations. Most of which have been almost 100% expendable. There are currently three new Long March rockets in development with differing fuselage diameters to fulfill different mission profiles – including a 100m tall, heavy lift behemoth called the Long March 9. This development of this program must have cost a minimum of tens of billions of dollars.

The Russian space program is another important example to examine. Most of the lift vehicles that the KSRPSC operate are old ICBMs that have been decommissioned. Specifically, there is a major use of R-7 rockets that have provided lifting capability for payloads ranging from sputnik to the Soyuz (after modifications). In fact, the Soyuz-U, a member of the R-7 rocket family, is the single most launched carrier rocket in the world with 786 launches and 22 failures (success rate = 97%). A single launch of a Soyuz-U is claimed to cost ~$50 million bringing the total cost of launching Soyuz-U rockets to ~$40 billion. This excludes development costs, doesn’t account for any other Soyuz launches, and excludes monies paid to the Russian government to use their services. Regardless, it is a reasonable claim to estimate the cost of the R-7 space program to be on the same order of magnitude (tens of billions of dollars, possibly hundreds of billions of dollars) as the Chinese program.

From a competition perspective, the Russian government has been much more willing to work with other governments (including the United States government, the Israeli government, the Ukrainian government, and more). In this sense then, the Russian publicly funded space program is in direct competition with the private sector while simultaneously achieving some levels of internal funding guaranteed and specific research requirements dictated by national defense. The Chinese space program, on the other hand, is not in competition with anyone and in fact is likely not going to be a user of any other space program pending political resolutions.

Private Competitive Landscape

Within the private sector there was a duopoly that existed prior to 2016 between Arianespace and ULA. Arianespace is partly owned by 17 separate private entities. Arianespace was initially founded with European governmental agencies but is currently mostly privately held. The largest shareholders of Arianespace are Airbus and Safran SA. Arianespace was founded with the specific intent of enabling commercial launches to space. From 2007 to 2015, Arianespace accounted for between 20% and 30% of the private space market for orbital launch vehicles. ULA is a joint venture that has existed between Boeing and Lockheed Martin since 2006. The venture was chiefly formed to help the US government launch their own military and civilian satellites, but ULA also allows for commercial launches.

Prior to 2016, Northrop Grumman, Arianespace, and ULA all occupied slightly different markets as launch providers. Northrop Grumman was largely a small payload and cheap launch provider. If you needed an orbital class rocket, and a small-lift vehicle didn’t have the diameter to hold your satellite, Northrup dominated because of their cheap launches. If you had a light, but large payload, then Ariane dominated with a massive 5.2m hull and a cost that was comparable to much smaller diameter rockets. If your payload mass exceeded 16,000kg, then ULA was your only option with their Atlas and their massive Delta rocket offerings. Given that each provider was well positioned in this landscape, they were all in the position of defenders with the emergence of SpaceX’s Falcon 9 rocket.

Everything changed in 2016. SpaceX now dominates the private sector launch market – and in fact, in 2018 China launched the most orbital class rockets (38) and the second most rockets were launched by SpaceX (21) with Russia launching 20, and ULA launching 8. In 2018, SpaceX alone accounted for 57% of all the launches in the private sector. In a span of one year, SpaceX upset the duopoly held by Arianespace and ULA and became the largest single force in the private launch provider sector. See the graphs and tables below to summarize the private launch capabilities that currently exist.

By 2016, the Falcon 9 had flows 20 times with one failure. With a cost of $50 million the Falcon 9 and SpaceX were unquestionably in the position of attacker and pioneer. The Falcon 9 could outperform every other vehicle in almost every realistic launch scenario. There were a few circumstances where a payload was too heavy or too wide to work with the Falcon 9, and in those situations either Ariane or Delta was used. It is no surprise however, that clients flocked to the affordable solution provided by SpaceX. In almost every scenario, the Falcon 9 is a dominating strategy.

SpaceX continues this strategy of attack and pioneering by developing the Starship platform. No firm claim for launch costs have been settled on for Starship, but it will have payload capacity of ~100,000 kg and a diameter of 9m dwarfing every other private provider option. Estimates for cost range from gobsmackingly low ($10 million – Elon Musk at the IAC 2017) to shockingly low ($100 million – Business Insider estimate). If the real cost per launch is in that window, then Starship paired with Falcon 9 will become dominant designs.