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The baseline configuration focuses on a design where the entirety of the propulsive force is produced by the propulsor array at design point. Whilst some thrust may be produced by the turbomachinery at off-design (Table 3.3), the majority of the aircraft’s thrust will be produced by the propulsor array for the duration of a flight. However, performance benefits may be possible by splitting the source of thrust between the propulsor array and the main engines. This intro-duces the thrust split parameter defined in Equation 3.49. Although the electrical transmission efficiency of a superconducting system is high in comparison to conventional electric machines, there is still power lost in the transmission system. Energy is also lost in the power turbine, as the turbomachinery is not 100% efficient. Benefits may therefore be gained by instead directly converting some energy into thrust in the turbomachinery, rather than losing energy in the gen-eration and transmission of power. This also leads to a smaller propulsor array, as its thrust requirement is lower. Depending on how the array is scaled, this can avoid high speed flow at the extreme edges of the array and/or reduce the array height, with the associated performance benefits.

The net propulsive force produced by the propulsor array will depend on the selected thrust split. The remaining propulsive force requirement for the aircraft must be provided through the turbomachinery. For relatively high thrust splits, a turbojet may still provide reasonable efficiency. However, as a turbojet is not the most efficient configuration for producing thrust, turbofan configurations will be necessary as more thrust is required from the main engines.

A turbofan configuration adds an additional optimisation parameter, as any given bypass ratio and engine configuration will have a fan pressure ratio that minimises the engine’s specific fuel consumption. This minimum specific fuel consumption for the engines is not the same as the minimum effective specific fuel consumption for the propulsion system as a whole. Although there may be benefits to thrust split, reducing the thrust split will reduce the propulsor array size, and hence will reduce the high effective bypass ratio that results from a distributed propulsor array. A very low thrust split is therefore unlikely to be beneficial without compensating by including a high bypass ratio turbofan.

Depending on design-related factors such as intake total pressure loss, a fan pressure ratio that provides a minimum power configuration can be identified (Figure 3.19a). However, for the thrust split analysis, a constant fan pressure ratio of 1.3 was assumed and array length was kept equal to the length of baseline array. In each case, the net propulsive force required from the propulsion system as a whole is constant. The primary design variable is therefore thrust split and the main engine size. Any reduction in thrust from the propulsor array is obtained by reducing the height of the propulsors. In addition to a turbojet thrust split configuration, three

94.1%

Figure 5.1: Specific fuel consumption as a function of thrust split and turbomachinery bypass ratio

85.0%

Figure 5.2: Bypass ratio influence over optimum thrust split and effective specific fuel consumption

turbofan sizes were considered with bypass ratios of 1.0, 2.0, and 4.0. In each thrust split case and for each turbofan configuration, an increase in thrust from the turbomachinery is obtained by increasing the mass flow and hence size of the engine. Fan pressure ratio for the turbofans in each case is selected to minimise the engine’s specific fuel consumption. All other design variables are kept the same as for the baseline configuration of the N3-X turbojet (Table 3.1).

As with the baseline turbogenerators, power and thrust required from the main engines are split equally between each of the two turbomachines.

By combining the use of thrust split with the optimum array fan pressure ratio, efficiency may be improved in comparison to the baseline configuration. For the turbojet configuration, specific fuel consumption is minimised at a thrust split of 94.1% (Figure 5.1). At this thrust split, the effective specific specific fuel consumption is approximately 1% lower than the effective specific fuel consumption of the baseline N3-X propulsion system. The thrust split for minimum eSFC decreases linearly with an increase in the bypass ratio (Figure 5.2a). A reasonable im-provement in eSFC is provided in the jump from a turbojet to a turbofan, however, the eSFC levels off for further increases in turbofan bypass ratio. For turbofan configurations, the effective specific fuel consumption decreases linearly with an increase in bypass ratio (Figure 5.2b). For the purposes this study, the maximum bypass ratio is limited to values that may be compatible with an embedded engine configuration, as estimated from a model of the aircraft [145]. How-ever, further efficiency improvements may be gained by further increases in bypass ratio for the main engines.

5. Alternative Propulsion System Configurations

Figure 5.3: Effective specific fuel consumption as a function of thrust split and electrical transmission efficiency for a turbojet configuration

An optimum can be found with respect to thrust split due to the balance between the effi-ciency boost offered by the propulsor array, the effieffi-ciency of producing and transmitting power to the array, and the efficiency of producing thrust directly from the main engines. Increasing elec-trical transmission losses or reducing the power turbine efficiency the will therefore reduce the minimum eSFC thrust split (Figure 5.3). Conversely, a more efficient electrical transmission sys-tem and power turbine will increase the minimum eSFC thrust split, as less power is ‘wasted’.

The main goal of the thrust split variable is to find a balance between the high efficiency offered by a distributed BLI propulsor array and an electrical power production/transmission system. If the production and transmission of power was 100% efficient, there would be little benefit to producing thrust from the main engines as opposed to the array (assuming minimal distortion or location-specific performance loss).

A different thrust split will lead to a change in the weight of the propulsion system. A domi-nant aspect of this change is the fact that a lower thrust split reduces the size of the propulsor array and the weight of the electrical power system, which accounts for approximately 75% of the propulsion system’s weight (Table 3.5). Given the relationships used, the electrical system weight decreases linearly with thrust split. The propulsor weight similarly decreases linearly with a decrease in thrust split (assuming all other array design variables are kept the same).

It is worth highlighting here that propulsors with a lower fan pressure ratio would be heavier, due to an increase in size. However, their lower power consumption would reduce the electrical system weight. As the array weight is the main component of the propulsion system weight, a reduction in thrust split leads to a reduction in the total weight of the propulsion system. A turbofan is heavier than a turbojet due to the addition of a fan bypass, leading to a higher weight than a turbojet for the same thrust split (Figure 5.4). The optimum configuration from the aircraft perspective must include the influence of thrust split over both the total weight and eSFC of the propulsion system. A configuration may be selected that minimises eSFC, however, this will not correspond to a minimum weight configuration. The influence of thrust split over mission fuel consumption can be found by combining both factors (Figure 5.5). For the turbojet config-uration, fuel consumption can be reduced by 1.6% by using a 93.7% thrust split. This minimum fuel thrust split is slightly lower than the 94.1% thrust split that provides the minimum eSFC, as a slight benefit in fuel consumption is gained by reducing the propulsion weight at the expense of a slight increase in eSFC. Although the turbofan configurations have a higher weight than the turbojet configurations, the relatively larger eSFC improvement leads to an overall decrease in mission fuel consumption. In each case, the minimum fuel thrust split is approximately 1%

9.5 10.0 10.5 11.0 11.5 12.0

80% 85% 90% 95% 100%

Total Weight (tonnes)

Thrust Split Turbojet

BPR 1.0

Figure 5.4: Total propulsion system weight as a function of thrust split

Figure 5.5: Fuel saving for a 7500 nmi mission as a function of thrust split

below the minimum eSFC thrust split. However, as the peak of the trend is relatively flat, the actually net difference in fuel consumption is less than 0.5%. As with the eSFC trend, there is a relatively large jump in fuel savings from a turbojet to turbofan configuration. Subsequently, fuel savings increase linearly with an increase in bypass ratio. The increase in weight from turbojet to turbofan means that the jump in fuel savings is not as large as the reduction in eSFC.

Performance of an example thrust split configuration may be compared to the performance of the baseline over the key operating points identified from previous research (Table 3.3). A configuration with a BPR 4.0 turbofan was selected, with thrust split and turbofan fan pressure ratio selected to minimise the specific fuel consumption (Table 5.1). The array is sized for a lower net propulsive force than the baseline array, and hence produces a lower net propulsive force at the key operating points. The slightly lower thrust of the propulsor array is counter-balanced by the higher thrust capability of the turbofans. The reduction in propulsor array size leads to a reduction in its specific power consumption for each operating point. Although a reduction in the propulsor array’s size will reduce the effective bypass ratio, the introduction of the turbofans returns the effective bypass ratio to a similar level to the baseline propulsor array.

Although the net propulsive force produced by the propulsion system is lower at SLS and RTO, the thrust produced still exceeds the take-off thrust targets established in previous research.

This lower thrust is the result of a smaller propulsor array. Overall, the configuration leads to a lower eSFC than the baseline and a 2.3% improvement in the specific fuel consumption during cruise. In combination with a lower weight due to a reduction in array size (Table 5.2), this will

5. Alternative Propulsion System Configurations

Table 5.1: Performance of a sample thrust split propulsion system (Turbofan BPR = 4.0, Array FPR = 1.3)

ADP Cruise RTO SLS

Altitude (ft) 30000 40000 0 0

Mach Number 0.84 0.84 0.25 0

Engine

TET (K) 1811 1728 1895 1922

Net Thrust (kN) 8.5 5.3 28.4 42.3

Power (MW) 15.2 9.2 27.6 27.6

Mass Flow (kg/s) 124.04 79.52 220.17 206.99

Fuel Flow (kg/s) 0.690 0.417 1.383 1.401

Array

NPF (kN) 101.9 63.9 353.6 530.1

Mass Flow (kg/s) 1411.4 903.7 2728.6 2686.9

Power Consumption (MW) 30.3 18.5 55.3 55.2

Specific Power Consumption (W/N) 297.6 289.1 156.3 104.1

Propulsor RPM 100.0% 100.1% 91.4% 92.8%

Length (m) 20.1

Propulsion System

eSFC (mg/Ns) 11.60 11.19 6.74 4.56

eBPR 32.4 32.4 32.4 32.2

eST (N/kg) 71.7 70.1 129.5 198.3

NPF (kN) 118.9 74.5 410.4 614.8

Thrust Split 85.7% 85.8% 86.2% 86.2%

Table 5.2: Overall weight of a thrust split propulsion system (Turbofan BPR = 4.0, Array FPR = 1.3)

Component Weight % of Total

Distributed Propulsors (total) 4410 kg 40%

Turbofan (×2) 1650 kg 30%

HTS Generators (×2) 609 kg 11%

Motors (total) 1500 kg 14%

Misc. HTS 545 kg 5%

Total Weight 10980 kg

reduce the fuel consumption of the aircraft.