AERODYNAMICS AND FLYING QUALITIES REQUIREMENTS FOR A LONGRANGE

TRANSPORTATION SYSTEM

Rodrigo Haya Ramos (1)

, Jordi Freixa(1)

, Tobias Schwanekamp(2)

, Martin Sippel(2)

, Rafael Molina(3)

(1) DEIMOS Space S.L.U., Ronda de Poniente 19, 28760, Tres Cantos, Spain, Email: rodrigo.haya@deimos-space.com

(2) DLR, Robert-Hooke-Straße 7, 28359 Bremen, Germany, Email: martin.sippel@dlr.de

(2) European Space Agency, ESTEC, Keplerlaan 1, 2201 AZ Noordwijk, Netherlands, Email: rafael.molina@esa.int

ABSTRACT

The objective of a long range hypersonic transportation

system is to take advantage of high altitude and high

velocity to perform intercontinental flight in the order of

one hour. The SpaceLiner concept has been proposed by

the Space Launcher Systems Analysis group (SART) of

the German Aerospace Center (DLR). In the frame of

the European Commission project FAST20XX, this

concept has been further investigated in several areas.

This paper presents the results of the aerodynamics and

Flying Qualities activities conducted to support the

design and evolution of the system. Engineering

methods have been combined with rapid CFD analysis

in order to mature the dataset and to increase the

reliability of the Flying Quality analyses. Areas for

further design iteration of the SpaceLiner concept have

been identified.

1. INTRODUCTION

The European Commission (EC) co-funded project

FAST20XX (Future High-Altitude High-Speed

Transport 20XX) aims at exploring the frontier between

aviation and space by investigating suborbital vehicle

concepts (1).

The main focus is the identification and mastering of

critical technologies for such vehicles rather than the

vehicle development itself. The technology

development is based on two advanced suborbital high-

speed transportation concepts one for the medium term

(5-10 years) and another for the longer term (>30 years).

The long term concept is related to suborbital point-to-

point long distance transport in very short time. Within

this type of transportation, the SpaceLiner concept

proposed by the DLR has been investigated [2]. It is a

vertically launched two-stage rocket space vehicle

system concept used to identify technologies required

for suborbital ultra-fast long-range transport of the long-

term future.

One of the areas of investigation for such future

transportation system is Flying Qualities, whose

objective is the characterisation of the performance of

the aerodynamic shape combined with the Mass,

Centring and Inertia (MCI) properties in a given flight

envelope. The Flying Qualities also provide inputs for

the specification of the Guidance, Navigation and

Control (GNC) system and feedback about the

aerodynamic control surfaces sizing.

The Flying Qualities provide an extended

characterisation of the aerodynamic performance of the

shape. Therefore, the reliability of the Flying Qualities

conclusions and recommendations is linked to the

maturity of the aerodynamic database. The aerodynamic

database available in early phases of the design is

normally derived with rapid engineering methods

lacking detailed characterisation of critical effects like

non-linearity and cross-couplings. As a result, initial

conclusions tend to be optimistic about the Flying

Quality performance and hence about the feasibility of a

given aerodynamic shape.

In the frame of FAST20XX it has been mitigated by

combining engineering methods with CFD analyses at

specific flight conditions. The objective is not only the

verification and correction of the dataset but also the

improvement in the estimation of the stability and

control derivatives and the identification of the range of

validity of the engineering methods. Thus, a combined

aerodynamic database covering the expected flight

envelope, (hypersonics to subsonics), angle of attack

and control surfaces deflections as input to the

trajectory, flying qualities and GNC design activities is

created with a higher level of realism. The selected CFD

needs to be rapid enough to explore the complete flight

envelope. Thus an Euler solver with an unstructured

mesh and adaptive Mesh Refinement has been selected.

The objective of the Flying Qualities analyses was to

support the system configuration definition (Centre of

gravity namely), to validate the control surfaces sizing

and to characterise the performance in terms of stability,

controllability and expected static and dynamic

response. For the candidate reference trajectory, the

Flying Qualities have been evaluated in detail both in

nominal condition and with uncertainties for several

SpaceLiner concepts in order to validate the evolution

of the shape and layout.

Main results and recommendations from the early

SpaceLiner concepts to the latest configuration are

presented. The aerodynamic activity was the result of a

joint effort between DEIMOS (CFD for SL4 and SL7),

DLR (engineering AEDB) and ESA (CFD for SL7.1).

2. THE SPACELINER CONCEPT

Since 2005, the Space Launcher Systems Analysis

group (SART) of the German Aerospace Center DLR

has been working on a novel vehicle concept for long

range hypersonic passenger transport. The “SpaceLiner”

is a large, rocket propelled vehicle (length ~60 m) that is

launched vertically with launch and ascent being

assisted by a reusable booster. Both, the hypersonic

passenger stage and the booster utilize liquid

propellants.

In contrast to other hypersonic vehicle concepts the

SpaceLiner does not incorporate radically new or

unproven technologies. Instead, rather conventional

rocket propulsion systems and vertical ascent

trajectories are used. The SpaceLiner concept contains

several technical and logistical challenges, such as

active cooling technologies, passenger accommodation

and safety together with the provision for suitable

launch and landing sites.

The final objective of the SpaceLiner development is to

dramatically reduce intercontinental travel times

compared to today’s subsonic passenger aircraft flights

by travelling at hypersonic velocities. For example, a

trip from Europe to Australia, which is the current

reference mission, will last only 90 minutes carrying 50

passengers. Also other intercontinental routes such as

New York to Australia have been considered.

It is a gain to lose concept, in which the vehicle gains

energy thanks to the rocket stage and hence glides back

towards the destination losing energy by friction. The

profile is suborbital reaching altitudes around 80 km and

speeds around Mach 20.

Several SpaceLiner concepts have been produced. This

paper covers from SpaceLiner 4 concept (SL4) up to

configuration 7.1, named SL7.1.

Figure 1. Artist’s impression of the SpaceLiner7 with

the booster

3. AERODYNAMICS

The aerodynamic dataset (AEDB) is the driver for

Mission and GNC performances. Two main activities

have been carried out. First, an aerodynamic inspection

and verification from a Flight Mechanics perspective to

validate its suitability for design activities. Then, CFD

calculations have been performed to improve the

characterization of the stability derivatives. The

requirements for the CFD matrix have been derived

from the mission, Flying Qualities and GNC needs.

These CFD results have been used to refine the

Aerodynamic Dataset (AEDB) used for engineering

activities. Computations have been performed between

DEIMOS and ESA to support SpaceLiner concepts SL4

to SL7.1.

Figure 1. CAD model of the Spaceline SL4, SL7 and

SL7.1 concepts.

The software used to run the CFD simulations is an in-

house code based on: Euler flow modelling with

tetrahedral spatial discretization; adaptive Mesh

Refinement, feature-based (Mach number has been

implemented for this work) involving anisotropic

remeshing and mesh movement/adaptation; node

Centred, blended anisotropic second- and fourth-order

dissipation; local time stepping. This configuration

ensures good coverage of the flight envelope with

moderate computational resources.

The adaptation feature allows characterising the bow

shock properly. The streamlines and mesh for a 2D

ramp of 20º at Mach 2 are shown in Fig. 2. The

theoretical pressure coefficient is 0.6582, while the CFD

provides 0.6587 at the centreline and 0.6589 at the

wedge. The theoretical shock angle is 53.41º, while the

CFD captures a shock at 53.2º. This benchmark case

validates the use of this adaptive unstructured mesh

solver in supersonic and hypersonic regime.

Figure 2. Streamline and mesh (10584 elements) for a

20º 2D ramp at Mach 2.

An initial aerodynamics of the SpaceLiner 4 concept

was available for the conceptual design of the vehicle.

As long as it is a key input that strongly drives the

Flight Mechanics and GNC performance of the vehicle,

a verification and extension of the dataset through

detailed CFD analyses has been conducted.

A total of 38 CFD cases have been conducted covering

transonic to hypersonic regime with and without

elevator deflection (see Tab. 1). Not all Mach, Angle of

Attack (AoA) and elevator combinations have been

inspected, but those of interest according to the flight

envelope and initial trim assessments. The CFD

representation of the vehicle includes a highly detailed

surface mesh in which all geometrical aspects of the

SpaceLiner CAD geometry have been modelled. Body

flaps and wing elevator have been considered. CFD

results were used to detect artificial steps and gaps and

hence to refine the CAD modelling. The Mach

distribution for Mach 19.8 is shown in Fig. 3.

The results show that the initial aerodynamics was

appropriate in the hypersonic regime, but refinement

was needed in the supersonic and transonic part, where

larger differences were noticed (Fig. 4). The CFD

results have been used to populate the aerodynamic

dataset. Fig. 5 and Fig. 6 show a comparison of the

longitudinal aerodynamics (lift, drag, pitch moment and

L/D) between the conceptual aerodynamic dataset,

which is based on rapid engineering methods, and the

CFD results at Mach 3 and Mach 10. CFD results

confirm trends of dataset with offsets as expected due to

the simplified pressure coefficient (Cp) characterisation

used in the Newtonian flow model implemented within

the engineering methods.

The CFD results were integrated within the

Aerodynamic Database (AEDB) to create a new release

and solve the issues detected in the initial release:

unrealistic supersonic data and non-linearities and

inconsistent elevator efficiency in some conditions. This

improvement had a significant impact on mission and

Flight Mechanics predictions, which stresses how

relevant is to improve the aerodynamic characterization

since first step to reduce design iterations. The updated

AEDB incorporates preliminary viscous effects.

Figure 3. Mach distribution at Mach 19.8

Table 1. CFD matrix of cases for SL4

Mach AoA Elevator

1.1 to 19.8 0 to 10º -5º to 5º

Figure 4 Raw comparison between the initial database

based on engineering methods and the CFD results

0 5 10-0.1

0

0.1

0.2

0.3

0.4

0.5

0.6

CL

AoA [deg]

0 5 100

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

CD

AoA [deg]

0 5 10-0.4

-0.35

-0.3

-0.25

-0.2

-0.15

-0.1

-0.05

0

0.05

CM

AoA [deg]

0 5 10 15-3

-2

-1

0

1

2

3

4

5

6

7

AoA [deg]

LoD

Figure 5. Comparison between the conceptual

aerodynamics and CFD at Mach 3 (CD, Cm and L/D)

0 5 10-0.05

0

0.05

0.1

0.15

0.2

0.25

0.3

CL

AoA [deg]

0 5 100

0.01

0.02

0.03

0.04

0.05

0.06

AoA [deg]

0 5 10-0.14

-0.12

-0.1

-0.08

-0.06

-0.04

-0.02

0

0.02

CM

AoA [deg]

0 5 10 15-3

-2

-1

0

1

2

3

4

5

6

AoA [deg]

LoD

Figure 6. Comparison between the conceptual

aerodynamics and CFD at Mach 10 (CD, Cm and L/D)

Figure 7. integration of the CFD results within the

AEDB for engineering activities (lift coefficient)

Fig. 7 shows for instance the lift coefficient from the

updated AEDB and the matching with the available

CFD data as a function of Mach number for several

angles of attack and elevator conditions.

The vehicle evolved during the project towards SL7,

which implemented a new design for the wing. A CFD

campaign was performed to assess the impact into the

major aerodynamic performances, namely in terms of

drag and lift-to-drag ratio. A comparison with the SL4

CFD campaign was conducted (Tab. 2). The hypersonic

efficiency (both cases without viscous correction)

improves in hypersonics and it is similar in supersonics.

Now, at null AoA it is possible to generate lift. There is

a reduction of drag that will be beneficial during the

ascent but it will increase heat fluxes during the entry

phase. The SL7 shape rapidly evolved in terms of wing,

fuselage and empennage towards the SL7.1 concept.

For the SL7.1, a characterisation of the vehicle

aerodynamics was directly conducted through CFD. A

matrix of 36 cases was designed (Tab. 3) to cover the

complete unpowered flight in clean configuration (no

control surfaces deflection). The SL7.1 shape introduces

a higher level of complexity in terms of CFD modelling

compared to SL4. Thus, the following steps were

followed: from the CAD of the vehicle provided at

system level, an initial surface mesh was created and

subsequent surface mesh decimation and smoothing was

performed with YAMS. Volume meshing was

performed for two domains: 6-chord and 20-chord

domain for supersonic-hypersonic and transonic-

subsonic computations respectively. Systematic mesh

adaptation by enrichment and node movement was used.

Table 2. Comparison between CFD for SL4 and SL7.

Coefficient M = 2

AoA = 10 deg M = 2.31

AoA = 0 deg M = 19.6

AoA = 6 deg

CFD SL7 CFD SL4 CFD SL7

CFD SL4 (M=2)

CFD SL7 CFD SL4

CD 0.1071 0.1205 0.0204 0.0209 0.0105 0.0133

LoD 4.6592 4.8880 1.1814 ~0 6.0286 4.2556

Table 3. CFD matrix of cases for SL7.1

The same CFD code has been used to cover from

hypersonics to subsonics with different grids. Fig. 3

shows the comparison of the Mach field at Mach 0.9 for

two angles of attack. Transonic behaviour is observed in

the leeward side with the formation of an aft shock

wave that becomes stronger and moves forward as the

angle of attack increases from 6º to 10º.

Fig. 8 to 10 show the Mach contour and the surface and

fluid grid in subsonics and hypersonics. The efficiency

of the mesh adaption procedure is verified in

hypersonics, where more elements are needed.

Figure 8. Mach 0.9 alpha 6 & 10, Mach contours

Figure 9. Mach 0.4 alpha 0, Mach contours (left) and

Figure 10. Mach 0.7 alpha 7, Mach contours (left) and

Figure 11. Mach 10 alpha 6, Mach contours (left) and

grid (right). NNODES~0.5e5 NELEM~2.7e5

grid (right). NNODES~0.5e5 NELEM~2.7e5

grid (right). NNODES~5.5e5 NELEM~3.5e6

An aerodynamic dataset (AEDB) has been built around

this CFD campaign covering from subsonics to

hypersonics. Elevator efficiency as been added using

engineering methods and hence it will require future

refinement using dedicated CFD analyses.

The drag, lift and pitching moment coefficient with

respect to the Moment Reference Centre (MRC) is

shown in Fig. 12 as a function of Angle of Attack

(AoA), Mach and elevator deflection. Trimmability for

an AoA of 10º is achieved in nominal conditions down

to subsonic. Trends are deemed adequate for the Flying

Qualities analyses.

Starting from the raw AEDB tables, an application rule

has been created to calculate longitudinal aerodynamic

coefficients C as well as the stability and control

derivatives needed for Flying Qualities and GNC:

C=(C0(,M)+CDE(,M,e)+CDBF(,M,bf))(1+UC(M)) (1)

Where C0 is the coefficient in clean configuration (no

control surfaces deflection) and CDE and CDBF are the

contribution of the wing elevator and body flap.

Uncertainty is modelled as a scale factor dependent on

Mach as a first approach.

0 5 10 150

0.2

0.4

AoA [deg]

CD

dE = -10 deg

0 5 10 15

0

0.5

1

1.5

AoA [deg]

CL

dE = -10 deg

-30 -20 -10 0 10 20 30-0.2

0

0.2

dE [deg]

CM

AoA = 10 deg

Figure 12. SL7.1 aerodynamic database

4. FLYING QUALITIES

Flying Qualities (FQ) constitute the exhaustive

performance metrics for the aerodynamics combined

with the Mass, Centering and Inertia properties of the

vehicle. The classical definition of Flying Qualities for

airplanes is applicable for re-entry vehicles: Flying

Qualities are defined as the stability and control

characteristics that have an important bearing on the

safety of flight and on the […] ease of flying an airplane

in steady flight and in manoeuvres. However, aircraft

Flying Qualities are not directly applicable and a

specific approach and methodology is required.

According to the Space Vehicles classification for

Flying Qualities (FQ) proposed in [3], the SpaceLiner

vehicle is of Class III, space plane, which comprises

winged vehicles that generate aerodynamic lift through

its body and wings and whose manoeuvrability exceeds

that of lifting bodies (Class II) and capsules (Class I).

The SpaceLiner flight covers hypersonic entry flight,

descent and approach and landing into runway.

Therefore, categories A, B and C apply.

In a hypersonic vehicle the angle of attack profile is

strongly linked with the mission feasibility and hence its

selection and assessment cannot be uncoupled from the

trajectory design. The objective is the evaluation of the

Centre of Gravity (CoG) box provided by system team

and the identification of the associated entry corridor.

This process provides the range for the design of the

nominal angle of attack profile during the entry and

identifies the available corridor for trajectory design.

The Flying Qualities analyses are performed with the

Flying Qualities Analysis (FQA) Tool. This FQA Tool

will enable a Flight Mechanics engineer to follow the

steps that build the FQA Framework to successfully

perform the required flight qualities analyses within a

given program. The FQA Tool software package has the

flexibility to connect to different vehicles models (e.g.

capsule, space plane and lifting body) and data. The

FQA Tool performs computations based on the user’s

inputs (mainly: vehicle models, flying quality

objectives) in order to derive the criteria allowing the

characterization of the flying quality of the vehicle and

the definition of guidelines for the design of the GNC

for the atmospheric re-entry.

The system context makes reference to operational

context in which the system or the simulator will

operate, giving indication of its interaction with the

environment in terms of inputs and outputs. Fig. 13

presents the context use of the FQA Tool. The user of

the FQA Tool will be an experienced flight mechanics

analyst. It will use the FQA Tool to derive the criteria

allowing the characterization of the flying quality of the

vehicle and the definition of guidelines for the design of

the GNC for atmospheric re-entry. The FQA Tool can

also interface with an external Worst Case Analysis

Tool for the identification of worst-case combinations

of dispersions for the flying quality analysis.

Flying Qualities

Analysis

Framework

Tool

Flying Quality of Vehicle

Flight Mechanics

Analyst

Vehicle AEDB

Flight data

Trajectory GNC Guidelines

Worst-Case

Analysis Tool

Environment

Vehicle FCS

Figure 13. FQA Tool Context

Uncertainties must be incorporated in the Flying

Qualities analyses since the first step as they might

change the feasibility and hence conclusions at system

level. For instance, Fig. 14 and Fig. 15 show the angle

of Attack corridor for a given CoG position for the SL4

concept with and without uncertainties. This corridor is

limited by trim authority, minimum L/D performance

and static stability. The supersonic and hypersonic

regime shows a feasible corridor with wide margins for

AoA design. However, if uncertainties are considered,

the corridor shrinks and it is practically closed in the

Mach 7 to Mach 10 region, which is critical due to the

transition from hypersonic AoA to supersonic AoA.

For what concerns the SL7.1, the Angle of Attack

corridor has identified several areas for improvement

either on the aerodynamic side or in the system layout.

It is shown in Fig. 16. A minimum angle of attack in

hypersonics is needed to guarantee longitudinal stability

in the region with high heat fluxes. There is a stability

barrier that prevents the vehicle from being stable for

Mach lower than 11. The vehicle is trimmable is the

whole domain except in low supersonics, where

saturation occurs. It indicates the need of using the body

flap to improve the trim authority and/or to revisit the

control surface sizing.

For the angle of Attack profile coming from an

optimised trajectory (black wide line in Fig. 16), a

Monte Carlo campaign has been performed to assess the

trim, stability and controllability characteristics of the

SL7.1 concept against uncertainties and perturbations.

Perturbations on the Mass, Centre of Gravity and Inertia

properties, aerodynamics, trajectory and angle of attack

have been considered (Tab. 4)

-0.05

-0.05

-0.05

-0.05

-0.0

5

-0.0

5

-0.0

5

-0.0

25

-0.0

25

-0.025

-0.0

25

-0.025

-0.0

25

-0.0

25

-0.02

5

-0.0

25

-0.0

1

-0.0

1-0

.01

-0.0

1

-0.0

1

-0.0

1

-0.01

-0.0

1

-0.01

1 11 1

2

2

2 2

3

3

3

3

3

3

3

34

4

Mach

Ao

A (

de

g)

AoA entry corridor, AEDB1.1 with k222 extrapolation, 99%ile with 90% CI, COG [-34.566; 0; -1.056] m

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 220

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20d

E = -35 deg

SM = 0.0

SM = 0.0

LoD

Figure 14. SL7.1 AoA corridor

Table 4. Uncertainties considered in the FQ analyses.

Variable SL4 SL7.1 Type Unit

Aerodynamics +/- 1 Uniform -

AOA trim AOA +/- 2 Uniform deg

CoG location – X xG +/ 1 Uniform m

CoG location – Y yG +/- 0 yG +/- 0.1 Uniform m

CoG location – Z zG +/- 0.1 Uniform m

Mass +/- 5% Uniform kg

Roll, Pitch, Yaw inertia [A, B, C] +/- 10% Uniform kg.m2

Figure 15. SL4 AoA corridor without uncertainties

Figure 16. SL4 AoA corridor with uncertainties

5 10 15 20-35

-30

-25

-20

-15

-10

-5

0

5

10

15

Mach

d E [

deg]

5 10 15 20-0.05

0

0.05

0.1

0.15

0.2

Sta

tic M

argi

n [L

ref]

5 10 15 200

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

Mach

LoD

[-]

5 10 15 200

0.5

1

1.5

2

2.5

3

3.5

4

4.5

Mach

Pul

satio

n of

Sho

rt P

erio

d [r

ad/s

]

5 10 15 20-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

Mach

Dam

ping

of

Sho

rt P

erio

d [-

]

5 10 15 2010

-1

100

101

102

103

104

105

Mach

Tim

e to

Dou

ble

[s]

Figure 17. Longitudinal flying Qualities of SL7.1 for a

trajectory and CoG: 99% range with 90% Confidence

Results are presented in Fig. 17: elevator deflection,

Static Margin (SM), lift-to-drag ratio (LoD), pulsation,

damping and time to double/time to half for the short

period response. The red and blue curves represent the

bounds of the 99% range of variability with 90%

confidence level derived from a 2000 shots Monte Carlo

campaign. The green line corresponds to the mean

value. As anticipated in the entry corridor analysis, only

using the wing flaps it is not possible to trim the vehicle

in supersonics. Stability issues are present below Mach

10. However, the time to double (time required by the

response to a step input to become double) is greater

than 10 s above Mach 4 and hence it does not represent

an issue from a control standpoint. For Mach lower than

4 the instability becomes severe.

This instability has an impact into the GNC physical

architecture (ex: redundancies) and hence in order to

mitigate the impact several actions to be tackled in

future design loops have been identified. For instance,

to improve the system layout by moving the CoG or

revisiting the vehicle wing position and controls

surfaces layout accordingly.

5. CONCLUSIONS

The aerodynamics of a long range transportation

concept like the SpaceLiner has been extensively

characterized though CFD in order to support the

vehicle evolutions.

The resulting aerodynamic database available for the

design is much more mature than the usual databases

available at this stage. This higher level of reliability

also anticipates to early design stages the identification

of issues. As a result, modifications can be rapidly

injected into the design without waiting to later phases

where changes are more difficult to accommodate.

Areas for further maturation and characterisation of the

shape performance have been identified, in particular in

terms of lateral-directional aerodynamics, efficiency and

sizing of the control surfaces and uncertainty policy.

Flying Qualities issues have been identified as a result

of entering into details. It is a valuable input for driving

the future evolutions and consolidation of the shape.

A strong interaction between System and Flying

Qualities/GNC team is required to speed up that shape

maturation.

Uncertainties must be considered since the beginning as

they might completely change the conclusions and

recommendations. It comprises not only aerodynamics

but also the vehicle, for instance in terms of mass

properties design envelope.

CFD Euler based computations combined with

engineering methods has been proven as a effective

approach in terms of availability in early design stages

of a mature aerodynamic dataset for system and

subsystem activities. Flying Qualities are used to

provide a complete and exhaustive picture of the

aerodynamic performances for a given configuration.

6. ACKNOWLEDGMENTS

This work was performed within the ‘Future High-

Altitude High-Speed Transport 20XX’ project

investigating high-speed transport. FAST20XX,

coordinated by ESA-ESTEC, is supported by the EU

within the 7th Framework Programme Theme7

Transport, Contract no.: ACP8-GA-2009-233816.

Further information on FAST20XX can be found on

http://www.esa.int/Our_Activities/Space_Engineering_

Technology/FAST20XX_Future_High-Altitude_High-

Speed_Transport_20XX.

7. REFERENCES

1. Mack, A. et al (2011). FAST20XX: Achievements on

European Suborbital Space Flight. Proceedings of

the 7th European Symposium on

Aerothermodynamic,. Noordwijk, Netherlands:

European Space Agency, 2011, id.35

2. Sippel, M., van Foreest, A., Bauer C., Cremaschi F.

(2011). System Investigations of the SpaceLiner

Concept in FAST20XX. Proc. of 17th AIAA

International Space Planes and Hypersonic

Systems and Technologies Conference, San

Francisco, CA, USA.

3. Haya Ramos, R. at al (2011). Flying Qualities

Analysis for Re-entry Vehicles: Methodology and

Application. Proc. of AIAA Guidance, Navigation,

and Control Conference, Portland, USA