Review on Technical Characteristics and Development Laws of Solid Launch Vehicles

BU Xiangwei ,YANG Haoliang ,SHI Xiaoning ,WEI Kai ,YANG Yiqiang

1 Institute of Mechanics,CAS,Beijing 100190

2 Beijing CAS Aerospace Exploration Technology Co.Ltd.,Beijing 100176

Abstract:Solid-propellant and liquid-propellant launch vehicles have their own characteristics,both playing an important role in space transportation systems of space powers.This paper reviews and summarizes the development history of solid-propellant launch vehicles,and analyzes their technical characteristics including multiple stages,large payloads,complex separation,diverse operation modes,fast response,and mission adaptability as well as unique advantages in launch activities.This paper analyzes and proposes four development laws for solid-propellant launch vehicles,including improving comprehensive performance,infusing heritage with innovation,unitization and seriation,and optimizing power systems.Finally,this paper proposes the opportunities and challenges faced by solid-propellant launch vehicles based on market demands.

Key words:solid-propellant launch vehicles,technical characteristics,development laws,unitization design

1 INTRODUCTION

Solid-propellant launch vehicles have always been an indispensable part of the transportation system of major space powers.Solid-fuel rockets do not require propellant fueling before firing and can be stored for a long time.With simple,fast,and efficient operation,solid launch vehicles are widely used in the field of small satellite launches which have flourished in recent years,because they can perfectly match the demand for small satellite launches.This paper summarizes the development of solid-propellant launch vehicles in the world,analyzes the technical characteristics and development laws,and provides a reference for their future development.

2 DEVELOPMENT OVERVIEW

Solid-propellant launch vehicles have a development history of 60 years.Solid-propellant and liquid-propellant launch vehicles have their own characteristics in terms of carrying capacity,operation mode,response time,and mission adaptability.Solid rockets are an important part of the space transportation system for major space powers.The United States,Russia,Europe,Japan,China,India,Israel,and Brazil have all developed their own solid-propellant launch vehicles,as shown in Table 1[1].

The United States has developed the largest number of solid-propellant launch vehicles in the whole world,forming a series of solid launch vehicles with different payload capabilities,different launch preparation periods,and different uses.The Minotaur series of rockets and Pegasus rockets currently in service enable the U.S to have the rapid launch capability of solid rockets from land and air,and the United States is currently the only country in the world with an airborne launch capability.

Table 1 Statistics of launched solid-propellant launch vehicles

Vega developed by Arianespace is a European launch vehicle specifically designed for launching small satellites.Together with Ariane 5 and Soyuz,Europe now has a complete launch vehicle spectrum capability[2].

Russia’s solid-propellant launch vehicle named as Start is basically in retirement.India has developed two types of solid-propellant launch vehicles,but due to low success rate,they have not been widely used.The Shavit solid-propellant rocket is the only launch vehicle currently in service in Israel .

Public reports show that the United States,Europe,China,and Israel currently have solid-propellant launch vehicles under development,as shown in Table 2.European and Chinese solid-propellant launch vehicles are pursuing higher performance,while American ones are mainly engaged in new technology verification.Israel’s research on solid launch vehicles is mainly used for technology accumulation and verification.

3 TECHNICAL CHARACTERISTICS

3.1 Multiple Stages

It can be seen from Table 1 that solid-propellant launch vehicles with a launch-to-orbit capability have at least a three-stage configuration.Minotaur series of rockets have also developed five-stage and six-stage rockets,which are mainly determined by the characteristics of solid rocket motors,including specific impulse,mass ratio,working hours,and exhaustion shutdowns.The specific impulse of a solid rocket motor is generally between 250 s and 290 s,lower than that of the liquid-propellant rocket engine.The working time is generally below 140 s.Once the motor is ignited,it can only be shut down after all the propellant is depleted.In order to obtain a higher efficient launch capability,the solid-propellant launch vehicle generally adopts at least a four-stage configuration.

Table 3 shows the comparison of the stages,takeoff weight,and carrying capacity of the Enhanced Epsilon[3]and Minotaur IV[4]rockets.It can be seen that the cost-effectiveness of the 500 km SSO orbit launched by the Enhanced Epsilon in a threestage configuration is slightly lower.After adding a two-component four-level for a 300 kg charge,it only has a carrying capacity of 590 kg/500 km SSO.Compared with the Minotaur IV which directly adopts the four-stage configuration,Epsilon’s performance is much lower than that of the Minotaur IV.At a takeoff weight of 8%,the LEO launch capacity of the Enhanced Epsilon is similar to that of the Minotaur IV,but the 500 km SSO launch capacity of the Enhanced Epsilon is 44% lower than that of the Minotaur IV.Except for the impact of low mass ratio of the Enhanced Epsilon’s rocket motors,the main gap is still determined by the number of stages.

Table 2 Statistics of solid-propellant launch vehicles in research

Table 3 Comparison between Enhanced Epsilon and Minotaur IV

3.2 High Force Load,Thermal Load and Flight Overload

A solid-propellant launch vehicle has greater acceleration when flying in the atmosphere.The take-off thrust of Minotaur IV is about 2200 kN,and the maximum acceleration of the firststage flight segment is about 4.5g.The takeoff thrust of Minotaur IV is about 2261 kN,and the maximum acceleration during the first flight segment is about 4.1g,which creates a large force load and thermal load during the flight of the solid-propellant launch vehicle.Structural strength,stiffness,and thermal protection are the key elements in the development process.

In addition,the maximum axial overload of a solid-propellant launch vehicle during flight is generally larger than that of a liquid-propellant launch vehicle,which basically occurs in the third stage flight segment.The average thrust of Vega’s thirdstage motor is about 225 kN.According to the maximum capacity,its maximum overload does not exceed 5.5g[5].The average thrust of Minotaur IV’s third-stage motor is about 329 kN.When launching a 620 kg payload,the overload reaches 11g.

Figure 1 Relationship between the maximum overload curve and the payload weight in the flight process of Minotaur series

The first-and second-stage separation altitude of a solid-propellant launch vehicle is relatively low,and the aerodynamic interference is large.After the first-and second-stage separation,the second-stage body needs to be quickly controlled.Otherwise,the rocket body will diverge quickly due to aerodynamic interference.In addition,the residual after-effect thrust of the first-level solid motor is large,and a large separation force is required to separate the two bodies.For example,the first-and second-stage separation height of Minotaur IV is about 25 km.At this time,the rocket’s flight speed is about 1.3 km/s,and the first-stage residual thrust is expected to be 100 kN to 150 kN.The design of the separation process is very difficult.Most solid launch vehicles use a thermal separation process for the first and second stages,that is,the upper-stage motor ignites first,and then performs the inter-stage separation.The pressure generated by the upper-stage motor provides the separation force.At the same time,the upper-stage motor can quickly obtain the thrust required for ensuring control,but the thermal environment between the stages is extremely harsh.

The first-and second-stage separation altitude of the Vega rocket is about 40 km.At this time,there is almost no atmospheric interference,so the cold separation method is used.The separation energy is generated by 6 retro rocket motors installed in the interstage section.

The fairing of a solid launch vehicle generally adopts the separation mode of two-lobed separation structures.During the separation process,the joint between the two flaps needs considerable energy to be separated.The fairing of the launch vehicle is severely affected by external pressure loads and aerodynamic heating during flight,which requires high connection strength and stiffness between the two flaps,but naturally makes the separation process more difficult.In addition,the modal shape and frequency of the two flaps need to be adapted to the separation process to avoid the separation mechanism occupying too much space.

3.3 Complicated Separation Process

The multi-stage design creates more separation links,meanwhile the large force and thermal load bring difficulties to the design of the separation devices.

Table 4 analyzes the reasons for 6 failures of solid launch vehicles since 2000.There have been 4 occasions of flight failure due to problems during the separation process,namely 2 separations of the first stage and the second stage and 2 fairing separations.In addition,the cause of the 15th flight anomaly of Vega is still under investigation.Whether the anomaly was caused during separation still needs to be confirmed.

Table 4 Statistics of flight failures since 2000

3.4 Diverse Operation Modes

A solid-propellant launch vehicle’s charge has been cured internally before leaving the factory,and it has a certain strength and stiffness,which can adapt to complex mechanical environments and load conditions,which greatly enriches the use and launch modes for solid launch vehicles.Solid launch vehicles have land-based,sea-based and air-based launch modes.Land-based launches include road mobile,temporary field scaffolding,rail-launch,simple towers,and service towers.

Figure 2 The mobile transporter erector launcher of KZ-1A

Figure 3 Sea launch of LM-11

Figure 4 Rail-launch of Super Strypi

Figure 5 Temporary field scaffolding of Taurus

Figure 6 Air-based launch of Pegasus

Figure 7 Service tower of Vega

3.5 Fast Response

Pegasus,Minotaur,and Super Strypi all serve the ORS office in the United States,and their rapid response cycles include weekly,daily and hourly levels.With the support of ORS,Super Strypi was developed by the Sandia National Laboratory in the United States to verify a simple,fast,efficient,and inexpensive micro satellite launch system.

Two of China’s KZ-1A were launched at an interval of 6 hours on December 7,2019,creating a record of the shortest interval,launching the same type of launch vehicle at the same launch site,fully demonstrating the fast response launch capability of the solid-propellant launch vehicle.

3.6 Strong Task Adaptability

Launch vehicles with four stages or higher configurations also have the advantage of high adaptability for launch missions.The trend of the launch capacity of a two-stage liquid-propellant launch vehicle and Minotaur IV as a function of altitude is shown in Figure 8,and the ordinates refer to the percentage of the launch capacity of each orbit in the 200 km SSO launch capacity.It can be seen from Figure 8 that as the orbital height increases,the launch capacity of the solid launch vehicle declines slowly,the characteristics demonstrate the adaptability of solid-propellant launch vehicles for launch missions of various types of small satellites below 1000 km.Liquid-propellant launch vehicles are generally designed for a certain type of orbit.In recent years,the ability to switch on and off the engines has been enhanced,which has improved the ability of liquid launch vehicles to launch satellites into various types of orbits,but the difficulties and cost are relatively high.

Figure 8 Comparison of the launch capacity of Minotaur IV and a two-stage liquid-propellant rocket with altitude changes

The high design load of solid-propellant launch vehicles is also one of the foundations for their ability to adapt to various types of orbital launches.They have a strong adaptability with regard to the large heat flow and large dynamic pressure after reducing the trajectory,and can be used for various types of near space flights and hypersonic flight tests,for example,the light-weight version of Minotaur IV,which was used to launch the second-generation Falcon Hypersonic Technology Vehicle 2 (HTV-2).

4 DEVELOPMENT LAWS

4.1 Focus on Overall Design and Improvement of Overall Performance

In recent years,solid-propellant launch vehicles have obtained stable orders from the commercial small satellite launch market due to their outstanding advantages of “flexibility and fastness,convenient operation,and strong task adaptability”,as well as appropriate carrying capacity and cost.As for solid-propellant rocket design,researchers had to focus on the overall design and improvement of the comprehensive performance at the beginning.In addition to optimizing the important parameters such as cost,launch capacity and orbital accuracy,it is important to focus on matching the product to the market needs and niches,the use of processes and guarantee performance of the entire process.In addition,scalability of multi-platform and multi-launch site launches,high-density launch technology and rapid test launch will be included in the scope of overall optimized design,thereby the unique comprehensive advantage of solid-propellant launch vehicles will be established.

4.2 Infusing Heritage with Innovation

In order to shorten the development cycle,reduce development cost,and control development risks,solid-propellant launch vehicles use mature technologies and mature products,for example,Minotaur IV uses the motors,servo mechanisms,launch mode,manufacturing and test facilities of Peacekeeper.However,due to special requirements for application scenarios and tasks,the military equipment have very high requirements in terms of process quality control,environmental adaptability,reliability of individual units,product performance,customization,and miniaturization,and the cost is very high.

The design,production,and use of launch vehicles are very cost-sensitive.In order to win the market,solid-propellant launch vehicles should balance cost and performance,and incorporate innovative research and development in systems and product design,supply chain system,and process control.In terms of systems and product design,system-level redundant design can reduce the requirements for component level and reliability of individual units,thereby achieving the aerospace application of industrial-grade components and equipment; by adaptively designing flight profiles and increasing environmental control means,it should provide a good environment for launcher and ground equipment,and reduce development difficulties; introduce new tools,new materials,new processes,such as additive manufacturing,while reducing production costs,and shorten the product supporting cycle.In terms of the supply chain system,a socialized,open supply chain and a quality control model based on market-oriented facilities should be established; in terms of process control,information and digitalization should be fully used to build a batch final assembly test and integrated production line to increase production efficiency,and reduce the cost of manufacturing.

In addition,the development period of a solid-propellant launch vehicle is relatively short.Combined with its technical characteristics,it has become a carrier for new technology verification and a pioneer in a new field exploration.For example,Conestoga solid-propellant launch vehicle is the earliest commercial launch vehicle,and Pegasus is the only airlaunched launch vehicle at present.Long March 11 is the only one which can be launched from mobile platform and a ship at sea.Epsilon adopts considerable innovations in intelligent testing and launch control.Pegasus and Minotaur are widely used in hypersonic flight technology verification.

4.3 Unitization Design and Seriation Development

A solid-propellant motor has a simple structure and high reliability,and has strong adaptability for various applications and flight profiles.Solid motors with different thrusts and charge can be used as flexible power modules.Through the unitization design,a series of solid launch vehicles with different orbital adaptabilities,gradient carrying capabilities,operation modes,and response speeds can be quickly formed.Through seriation development,the maximum use of each type of solid-propellant motor reduces cost and period of the R & D process of a new launch vehicle.

Taking Minotaur IV,IV+and V as examples,the third-stage motors and structural sections,fairings,separation devices,control methods,and avionics systems of the three versions are all the same.By replacing and combining the fourth-stage power modules,the three launch vehicles have different launch capabilities.Minotaur IV and IV+are mainly used to carry out LEO and SSO missions.The LEO carrying capacity of Minotaur IV’s and Minotaur IV+’s are 1636 kg and 1931 kg respectively.Minotaur V is mainly used for launching Geostationary Transfer Orbit (GTO),Medium Earth Transfer Orbit (MTO),and Trans-Lunar Injection (TLI),with 630 kg launch capacity of GTO,and 340 kg launch capacity of TLI[8].

4.4 Motors Are the Basis for Improving the Performance of Solid Rockets

Improving the performance of a launch vehicle requires a higher mass ratio and higher specific impulse of its power system.When designing a solid-propellant launch vehicle,it is also necessary to focus on the thrust and internal trajectory optimization of the first-stage motor,the selection of the fourthstage motor,and the unitized configuration optimization of the second-stage and third-stage motors.

The thrust of the first-stage motor basically determines the size and capability of the rocket.The takeoff weight of Vega is 137 t,its first-stage motor thrust is 2261 kN,its carrying capacity is 1500 kg/700 km 90° circular orbit.The takeoff weight of the Vega C will be 210 t and its first-stage motor will generate a total of 2880 kN thrust.Its carrying capacity will be 2200 kg/700 km 90° circular orbit.In addition,in order to improve the capabilities of solid-propellant launch vehicles and create better conditions for the first-and second-stage separation,the internal trajectory of the first-stage motor is also an important item in the overall optimization.

In addition to affecting the carrying capacity,the scheme and performance of the fourth stage motor also directly affect the rocket’s guidance and attitude control plan,mission adaptability,and launch support facilities,which need to be determined comprehensively in combination with the technical foundation and market positioning.

Figure 9 Unitization and seriation design of Minotaur IV and V

The configuration of the second-and third-stage motors needs to be determined by unitizing the inter-stage ratio,the fairing envelope requirements,and research and development capabilities.For example,the Minotaur IV series of launchers have the same diameters.The design of the third-stage motor is basically spherical,which is difficult to design and manufacture.The diameters of rockets such as Vega and Taurus have become smaller from the second stages,which has reduced the design difficulty of the third-stage motor.

5 OPPORTUNITIES AND CHALLENGES

Compared with liquid-propellant launch vehicles,solid-propellant launch vehicles have the advantage of easily meeting the launch conditions,convenient for use,with a fast and flexible launch capability,and play an important role in space transportation systems.In addition,solid-propellant launch vehicles have relatively short development period,lower development costs,and fast applications,allowing them to be the first choice for many countries or companies to enter the aerospace field.

In recent years,the requirements for low-Earth orbit small satellite technology verification,deployment and replacement launches have been increasing.The features of these launches include mission variables,high-frequency launches,diverse orbits,and emergency response,which are well matched with the technical characteristics of solid-propellant launch vehicles including rapid response,easy to meet launch conditions,high-density launch capability,and suitable for land,sea,and air launch,thus provide more opportunities for the development of solid propellant launch vehicles.

However,the payload of existing solid-fueled carrier rocket is generally less than 2000 kg/500 km SSO.Compared with large liquid-fueled rockets,the solid-fueled rockets are not advantageous in terms of payload and launch cost per payload kilogram.The launch cost per payload kilogram of a solid-fueled rocket is usually between $20,000 and$40,000,but it is usually between $10,000 and $30,000 for a liquid-fueled rocket.In 2019,the Falcon 9 rocket provided the opportunity to launch small satellites,and the launch cost per payload kilogram was reduced to less than $10,000 by the recycling technology of the first stage.

The general view is that if the launch cost per payload kilogram can be reduced to between $10,000 and $15,000,the solid-fueled rocket would be able to take a place in the field of small satellite launch,by combing with the features of a solid-fueled rocket.However the payload would need to be at least 1000 kg/500 km SSO to obtain an acceptable profit.Thus,in the face of opportunities and challenges,solid-fueled rockets can achieve a broad market and profit through innovative development of a combinational design,serial development,large-scale production,and market-oriented integrated supply chain,while enhancing capabilities,and reducing costs.