Launch Systems - Lecture Notes, Dispense di Ingegneria Aerospaziale

Scarica Launch Systems - Lecture Notes e più Dispense in PDF di Ingegneria Aerospaziale solo su Docsity!

Launch SYSTEMS

Prof. Filippo Maggi

A.Y. 2020/

Lecture Notes by Andrea Pizzetti

Launch Systems Pagina 1

We can classify rockets in three main categories: Low-Lift Devices. They can be launchers or sounding rockets. The main factor here is the thrust. The launch is vertical, the Mach number is low. Low lift, high thrust, the levelling (null flight path angle) occurs in space environment.

High-Lift Devices. They can be atmospheric missiles or space planes (e.g. from Sydney to NY in 1 hour). In these devices there is lift and drag, more than thrust. The speed is much higher and the insertion takes much more time. There is a much higher Mach number, often the device is airbreathing. In this way, there's no need to carry the propellant. However, there is a limitation on the loading for manned applications, specifically on the maximum dynamic pressure sustainable. For this reason the envelope must be within a specific flight corridor:

Moreover, there is the need of a rocket propulsion, when atmosphere is no more present. In atmospheric missiles the aerodynamic is important to extend the maximum range. Firearms/Fireworks. For instance, the project HARP (High Altitude Research Project) used a cannon to supply speed and send to LEO, in order to study the re-entry of vehicles. It was abandoned because combustion products had high MM (about 40 g/mol), so they allowed only a bad expansion: the projectile couldn't go faster than the gases expanding in the barrel.

Propulsion Types

The type of propulsion for launch systems can be based on three different technologies: ○ SRM. T/W ratio can arrive to 75:1, but they have low specific impulse. Turbojet. T/W ratio only 5:1, but impulse of about 1500 s. For instance, Tomahawk missile is based on turbojet:

Classification lunedì 14 settembre 2020 19:

1. Introduction Pagina 2

sounding rockets, even if the aim is different, so, as said before, it's sometimes difficult to distinguish them. Launch Mode. Usually launchers are Surface-to-Surface (SS) or Surface-to-Air (SA). A surface-to- Sea example is the SpaceX booster. Airborne launches are released as payloads from a flight carrier: in this case we have Air-to-Air missiles (AA).

Type of target. This classification divides the target in Fixed or Movable/Unknown. In case of space applications, we have a fixed target and the guidance is on inertial basis. If the target is movable or unknown, we need to rely on passive (emission), active (research through autonomous target recognition) or semiactive (target illuminated by external sources) guidance.

Type of guidance. The guidance can be of different types. The Wire/Command consists of a radio signal connecting launch platform and the missile. It's used only for short ranges. The Terrain Comparison is typical of cruise missiles, the guidance is provided with a comparison of a map saved in the internal memory of the missile with the view. The missile follows the track recognizing some key elements. There are also the possibilities to use Terrestrial Guidance through star-tracking, Inertial Guidance through a gyroscope and Laser Guidance through a laser beam.

Range/Ground Track. The range is the length of the projected trajectory between launch and final points. The ground track is practically the same, it's the path followed by a Space system projected on the Earth surface. Military missiles can be Short range, few km, Medium-to-Long range, up to some hundreds of km, Ballistic range, hundreds to thousands of km.

Examples

For instance, the Ariane V ground track: Boosters are shut down in about 130 seconds, so the points are almost coincident in the map. Then the cryogenic motors ignite and do the majority of the impulse, because the fairing is detached since the air is very rarefied and there is no more need to resist to the aerodynamic forces. After that, from H2 to H it's the turn of the upper stage of storable propellant. From H3 to the final orbit insertion the spacecraft is on its own. For cruise missiles the path is inside the atmosphere. We can have a subsonic regime or a supersonic regime and they can be high-lift (such as the Tomahawk missile, which has wings and guidance capabilities) or low lift devices. If the target is steady, the mission is not time-critical and we can use a turbofan system. Otherwise, if the target must be hit before a certain time, we should use ramjet or

1. Introduction Pagina 4

turbofan system. Otherwise, if the target must be hit before a certain time, we should use ramjet or scramjet system, such as in the case of the supersonic Brahmos missile: In most cases these missiles are multi-stage: a booster one and then a constant high speed one. The Surface-to-Air missiles have limited manoeuvre capability, their mission is to intercept targets (such as the anti-missiles Patriot) and they have a small ground track and a steep mission profile. They are characterized by a dead zone in which they cannot engage the target. Finally, regarding ballistic missiles, several possible ranges exist:

1. Introduction Pagina 5

Launch Systems Market

The Launch services are a small share of the overall space market but they are consistent, all the industries need them. The biggest part is taken by the light/medium class vehicles. Regarding satellite sizes, the trends for the nano and micro satellites are growing very fast. The possibility to use CubeSats helped a lot. However, the trend changed suddenly from 2019: Market Overview lunedì 14 settembre 2020 19:

1. Introduction Pagina 7

The game-changing was the introduction of Satellite Constellations, such as the Starlink system. However, this was possible also thanks to the launch systems capabilities. The deployment of an high number of satellites at the same time, for instance, enabled a low-cost for payload insertion. Launch Costs If the launcher has high payload capabilities the cost is smaller, so the biggest launchers are the most affordable for high payload masses: The trend is still the same also for GEO orbits, apart from the up shift due to the higher requirement.

1. Introduction Pagina 8

to avoid the propellant costs: To be convenient, the number of uses must be over 10 in case of booster fly back systems, while it can be just over 2 for this innovative system. Since the descent in case of parachute is depending on the environmental factors, the launching and landing sites must be in both good weather conditions. Space Exploration The space exploration sector is still a state company sector because privates do not want to spend money on this, without a gain. The major state space agencies are from USA, USSR/RUSSIA and PRC. The first one dropped their launches due to the retirement of Space Shuttle. The Chinese space company is improving very fast. The payload launch capability is increasing more and more with the years, not to answer a commercial need but to explore farther and have a large cargo size. The escape performance is dictated by the launch energy C3: C3 can be negative, if we are not escaping Earth's gravity such in case of a Moon trip. In case of long travels such as to Pluto, we need gravity assist otherwise it would require more than 10 years with the current launchers.

1. Introduction Pagina 10

The payload mass influences a lot the C3. The lower the payload mass, the higher the value of C3 that can be obtained: With the Proton M Russian launcher, for instance, we cannot launch a 6500 Kg payload because we couldn't escape the Earth's gravity. In the following tables the values of payload for the different launchers are reported:

1. Introduction Pagina 11

That's why we need heavy launchers to prepare an interplanetary mission, there is the need to create an adequate environment.

1. Introduction Pagina 13

Planning

This is defined as all the requirements to plan and execute the project from beginning to end, at all levels, from customer to suppliers.

1. Purpose and Objectives of the project, TBD with the customer. TRL Advancement plans. We need to assess the TRL for each component of the project and their advancement.

3. Reusability of existing equipment, if it's possible and feasible. HR and Skills, Technical Facilities. We need to know if we have people able to do the task and allocate the consequent resources in number and quality of the employees.

5. Risk Assessment allows to predict the risk of failures, prevent or repair them. Development Approach is strictly connected to the points 2,3,4 and 5. It can be based on timing, performances, etc…

Project Deliverables are documents or products derived from the workflow. For instance, a specific subsystem technical specs, or physical items/software. A Work Package is composed of a subject, a detailed description, a timing, the outputs (deliverables and milestones and the inputs which usually are the deliverables of a previous WP). Deliverables are time-consuming, they cannot be too much because time is spent to write them. However, they cannot be few otherwise the customer will not be satisfied.

The customer comes with his needs, but there is a Requirements Pull due to this. Let's take an example to understand. ArianeGroup can carry to LEO 20 tons with Ariane 5, 1.5 ton with Vega and 5 tons with Soyuz (there's an agreement for European use). So we have heavy, light and medium lift products. Let's say we want to develop a new launcher dedicated to small satellites, according to the market needs. For instance, a Vega Light with a LEO payload capability of 500 kg. However, this new product cannot be a stand-alone, but it must be integrated with all the existing ones. We have several integration specifications regarding for instance propellants or launch sites. This way of thinking is called System-to- Systems Consideration. The customer should be aware of this but usually it's not. This, together with Programmatic Requests (political reasons and regulations) and Performance Needs of the customer, lead to the Performance Requirements which are eventually different from the needs. At this point we Project Management sabato 19 settembre 2020 10:

2. Requirements And Specifications Pagina 14

but it needs to be demonstrated at big scale. Also for AD^2 level 5 is ok because there is similarity with existing experiences but a dual development approach can grant a success with high confidence. When developing a project, we can follow different Development Approaches :

• Lowest Cost. It minimizes the cost but it must respect a risk and performance minimum threshold. Best Performance. In this case we have cost and risk thresholds but we have the maximum performance.

Lowest Risk. The risk is minimized and we have thresholds for cost and performance. For instance, military projects need low risk and development time, maybe in order to overtake a competitor, while usually they do not have budget problems.

• Best Value. In this case there is a specific objective to match. It's a mix of the above three.

Risks

A lot of aspects enter the risks management, from human errors to external events and institutional failures. This part of risks is not restricted to the mission, but to the general project development. In most of the cases these falls into delays of the project chain. How to approach this problem? According to European approach, we need to have this type of workflow:

1. Define the Policy Management of the risk
2. Identify and assess the risk Probability and its Impact Rating
3. Find the Acceptability of the risk and take decisions on its Reduction
4. Monitor communicating and acceptance of the risk. To define the policy management we need to answer to two questions: how to rate a risk and its impact on the project? What is the acceptance level of risk?

2. Requirements And Specifications Pagina 16

To find the acceptability scale, we multiply the probability of an event with its impact and then we look on the last table the consequent risk magnitude. If this value is too much, it must be reduced before the beginning of the project. To do that, we can exploit different sources:

• Experience
• Expert Judgement
• Analysis
• Analogy with other projects The risks to consider come from different fields. There can be technology, design or integration risks. Programatic and politics risks comes for instance, the cut of funds to a space agency or the introduction of new regulations. Moreover, also risks on safety and logistics occurs, as well as risks on the project management itself.

Organization

The project can be organized by defining:

• Organizational Structure to clarify the role of each partner and the disciplines between them. Communication and Reporting to define the collaboration interfaces and when/how to deliver the reports. Every company often tries to write as less as possible in the reports in order to not exchange too much information in a collaboration with other companies. Everyone should agree to share a certain amount of data.

• Audits to identify independent examiners that enter the development project and monitor it, to
  2. Requirements And Specifications Pagina 17

Only after that, we can write the WPs.

Phasing

Typically, we have 7 phases for the life cycle of a space project: NASA ed ESA both employs this scale. A preliminary risk management activity is done since phase 0. A preliminary technical requirements specification is done by identifying the mission needs and goals. The final technical requirements are obtained at the end of phase A, after the risks and feasibility assessments. The first technical designs are done. In phase B also the system requirements is done, while at the end after trade-offs and shortlisting of solutions we have a general fixed design, or maximum two alternatives. If we begin phase C, there is no more time to change the design: we need a defined configuration to work on. In phase C we improve the design and add all the details (screws, connections, interfaces), in order to carry out the Critical Design Review. This activity is the most important, because it is the threshold between the design and the manufacturing processes.

2. Requirements And Specifications Pagina 19

It should be noted that a system will never be designed and built on performance alone. Economy and other programatic aspects play a fundamental role. For instance, for a space launch device we have: The final design must balance both performance and programatic requirements and it's a sub-optimal solution. Indeed, programatic requirements reduce the performance.

Performance Drivers

The Performance Drivers enhance and improve the performances of the vehicle. The two most important ones are the Mass Efficiency and the Propulsion Efficiency. The first one is dictated by the Mass Ratio because it influences directly the velocity impulse, according to the Tsiolkovsky Law. To have a good value for MR, we should use the less inert mass possible. The technology of filament winding for the production of rocket cases in composite can be of help to improve the mass efficiency and therefore the overall performance:

2. Requirements And Specifications Pagina 20