The genesis for this systematic literature review was to search for industry case studies that could inform a decision of whether or not to support the change process, investment, training, and tools needed to implement an MBSE approach across the engineering enterprise. The question asked was, how the change from a document-based systems engineering approach (DBSE) to a modelbased systems engineering approach (MBSE) is justified? The methodology employed for this systematic literature review was to conduct a document search of electronically published case studies by authors from the defense, space, and complex systems product engineering industries. The 67 case studies without metrics mainly attributed success to completeness, consistency, and communication of requirements. The 21 case studies with metrics on cost and schedule primarily attributed success to the ability of an MBSE approach to improve defect prevention strategies. The primary conclusion is that there is a significant advantage to project performance by applying an MBSE approach. An MBSE approach made the engineering processes on a complex system development effort more efficient by improving requirements completeness, consistency, and communication. These were seen in engineering processes involved in requirements management, concept exploration, design reuse, test and qualification, Verification and Validation, and margins analyses. An MBSE approach was most effective at improving defect prevention strategies. The approach was found to enhance the capability to find defects early in the system development life cycle (SDLC), when they could be fixed with less impact and prevented rework in later phases, thus mitigating risks to cost, schedule, and mission. However, if a program only employed an MBSE approach for requirements management, advantages from finding defects early could not be leveraged in later phases, where the savings in cost and schedule from rework prevention is realized. Significant performance success was achieved when the systems engineer (SE) held a leadership role over engineering processes. A number of the case studies addressed a general lack of skilled MBSE engineers as a major hindrance to implementing an MBSE approach successfully.

paring the SCF total energies obtained by using the 4-31G basis set for propane,1,4 **propene,7 the n-propyl radical, and the hydrogen atom.8 In this manner, the computed energies for eq 1 and 2 are 37 and 82 kcal/mol, respectively.In contrast to the n-propyl radical is the energy required for C-H bond rupture in cubane and the cubyl radical, i.e., for reactions 3 and 4. Since experimental measurementsare not available for this system, the results of the ab initio calculations in this report will be used. When one uses the total energies for cubane, cubene, and the cubyl radical, the AH for rupture of a C-H bond in cubane, as indicated in eq 3, is found to be 91 kcal/mol. The AH for the ß C-H bond scission reaction in the cubyl radical, as shown in eq 4, is 106 kcal/mol. Because of the neglect of correlation energy in the SCF total energies, only two significant statements may be made about the energetics of reactions 3 and 4. The first is that the C-H bond energies in cubane and the cubyl radical are about equal. The second is that the energy required for ß C-H scission is clearly much (7) Total energy for propene was taken from

The infrared emission intensities from the HC1 and HF products have been used to assign relative rate constants for the reaction of H atoms with Cl2, F2, GIF, and IC1. The experiments were done in a fast flow apparatus utilizing ~1 torr of Ar carrier gas. The vibrational distributions obtained from analysis of the chemiluminescence recorded with a Fourier transform spectrometer at the first window of the flow reactor were very close to the initial distributions produced by the chemical reaction. Except for a small residue of population in the high J levels of HC1 from the H + IC1 reaction, the rotational populations had relaxed to a room temperature Boltzmann distribution. The relative rate constants for HC1 formation from the F2, Cl2, GIF, and IC1 series are 0.053:1.00:1.99:2.42. Since the absolute rate constant is well known for Cl2, these data, plus the vibrational-rotational distributions of product states, give absolute rate constants for formation of individual product quantum states. Summation of the HC1 and HF relative intensities from + GIF gave a macroscopic branching ratio of 5.2 favoring the HC1 channel. For the H + IC1 reaction, the HI/HC1 ratio is <0.5. Dynamical models based upon the data of this and the preceding paper are discussed.

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Rate constants and initial HBr and HF product distributions for the title reactions were measured in a fast-flow apparatus using infrared chemiluminescence techniques. The spectra were interpreted using a new set of Einstein coefficients for HBr, which are listed in the Appendix. The rate constants for HBr(v⩾1) and HF(v⩾1) formation, relative to the H+Cl2 reaction, are 3.3, 0.39, 0.50, 3.4, and 0.003, for Br2, SF5Br, PBr3, SF5, and SF4, respectively. This directly measured Br2 rate constant supports the smaller values that have been estimated in the literature. The initial HBr vibrational distribution (v1:v2:v3:v4:v5=0.03:0.20:0.40:0.31:0.06) from H+Br2 corresponds to 〈fV〉=0.49. The observed HBr vibrational distributions (v1:v2:v3:v4) are 0.28:0.43:0.23:0.06 and 0.63:0.24:0.13 for SF5Br and PBr3, respectively. The SF5Br results are close to the initial distribution and give 〈fV〉=0.36. The low vapor pressure of PBr3 limited the [PBr3] and high [H] was required to observe HBr emission; correcting the observed distribution for vibrational relaxation gives 〈fV≅0.47. These 〈fV〉 values include estimates for HBr(v=0). Based upon the highest HBr level observed from SF5Br and PBr3, D0(Br–SF5)&lt;55 and D0(Br–PBr2)&lt;62 kcal mole−1. The HF vibrational distributions from SF5 and SF4 decline with increasing v, which suggests that these reactions proceed via a long-lived complex. For these cases the formation of HF(v=0) is important, and significant corrections must be made to the HF(v⩾1) formation constants to obtain the total HF formation rate constants. The rate constants and energy disposal data are used to discuss models and to compare the H+Br2 reaction to H+Cl2 and F2.