NTR助推NASA火星任务设计：降低7次发射的方案
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本研究论文"AIAA-2009-5308"探讨了美国国家航空航天局（NASA）在最新的火星设计参考架构（DRA）5.0中，对人类火星任务的未来规划，特别是关注于减少依赖阿瑞斯五号重型运载火箭的多次发射。核热火箭（NTR）因其优越性能再次被选为首选的太空运输系统。NTR之所以被青睐，主要因为它具备以下关键优势：
1. 高特异性比值（Isp）：NTR的高Isp意味着其每单位燃料能产生的推力更大，从而在保持同等推进效率的情况下，可以携带更多的有效载荷，减少了总体发射重量需求。
2. 负荷适应性：NTR系统能够承受更大的载荷增长，这对于长期火星任务中的设备升级和技术迭代至关重要，同时降低了任务风险。
3. 低初始轨道质量（IMLEO）：NTR在低地球轨道（LEO）的初始质量较低，这有助于减少火箭发射时所需的能源和成本，进而降低发射次数，整体上降低了任务的经济负担和复杂性。
DRA 5.0的核心理念是采用一个地面停留的“分段任务”策略，即通过将货物和人员分批运送至火星表面，而不是一次性完成所有任务。这种分步执行的方法有助于优化资源分配，提高任务的灵活性和生存能力。
通过这种基于NTR的太空运输系统，NASA的目标是在降低发射成本、风险和时间消耗的同时，推动人类向火星目标的稳健进展。此研究结果对于理解NASA在后续火星探索计划中的技术选择和战略考虑具有重要意义，预示着未来的太空探索可能会更加高效且可持续。
AIAA-2009-5308
3
American Institute of Aeronautics and Astronautics
45
th
Joint Propulsion Conference & Exhibit, August 2-5, 2009
Phase II analysis results showed that NTR provides an IMLEO savings over chemical propulsion of ~400 t, the equivalent
of 3–4 Ares-V launches. However, despite an ~50% greater lift capability for the Ares-V, DRA 5.0 required 9 Ares-V
launches to deliver the cargo and crewed MTV components needed for the mission compared to 6–80 t SDHLVs plus a
Shuttle launch (for TransHab and crew delivery) used in DRM 4.0
4,5
. To better understand the causes for the mission mass
growth between DRM 4.0 and DRA 5.0, ESMD and JSC tasked GRC to identify the “cause and effect” of specific changes in
the transportation system and mission payloads between the two studies. Furthermore, because large numbers of heavy lift
launches has been identified as a major mission risk area, ESMD tasked GRC to identify solutions for getting the number of
Ares-V launches down.
Figure 1 traces the growth in total IMLEO associated with key transportation, mission and payload changes from DRM
4.0. DRA 5.0 baselined Al/Li over composite propellant tanks, lower Isp “composite” NERVA fuel, higher dry weight
contingency, and slightly higher Mars mission ΔVs over the timeframe of interest. To accommodate the increased LH
2
propellant loads needed to push heavier mission payloads, larger diameter tanks were required. Besides an ~37.5% increase
in the TransHab mass, the mass of the AC/EDL aeroshell used on the cargo flights increased by ~400% (from ~10 t to 40 t)
increasing the mass of each cargo vehicle by over 100 t. To accommodate this significant mass growth, the NTR engine
thrust level was increased to 25 klb
f
. The addition of the contingency consumables and doubling of the CEV mass led to
further increases in tank diameter. The net result is an IMLEO increase of ~392 t (from ~451 t to 844 t) with each cargo MTV
accounting for ~118 t and the crewed MTV for ~156 t. With an assumed lift capability of ~110 t, 8 Ares-V launches were
required with NTR. To reduce the launch count further and also accommodate design deficiencies in the crewed mission
payload mass identified at the end of the Phase II analysis cycle, GRC developed the “7-Launch” NTR Mars mission strategy
which is the subject of this paper.
The paper addresses the following key areas. First, the operational principles and characteristics of the 25 klb
f
NTR engine
baselined in DRA 5.0 are discussed along with performance projections using recent MCNP transport models of the engine’s
reactor core. Also included is a brief summary of the technical accomplishments of Rover/NERVA nuclear rocket programs.
Mission and transportation system ground rules and assumptions are then presented, followed by a discussion of NTR stage
and mission payload sizing issues and their impact on the Ares-V lift and payload shroud requirements. The overall mission
scenario, including assembly activities, is discussed next followed by a description of the key features and characteristics of
the cargo and crewed MTVs. The paper ends with a summary of our findings and a brief discussion of benefits of NTP for
future NASA exploration missions and its significant growth potential.
II. NTR System Description and Performance Characteristics
The NTR uses a compact fission reactor core containing 93% “enriched” uranium (U)-235 fuel to generate the large
quantities of thermal power (100’s of MW
t
) required to heat the LH
2
propellant to high exhaust temperatures for rocket
thrust. In an “expander cycle” NERVA-type engine (Fig. 1), high pressure LH
2
flowing from twin turbopump assemblies
(TPAs) cools the engine’s nozzle, pressure vessel, neutron reflector, and control drums, and in the process picks up heat to
drive the turbines. The turbine exhaust is then routed through the core support structure, internal radiation shield, and coolant
channels in the reactor core’s fuel elements where it absorbs energy from the fissioning U-235 atoms, is superheated to high
exhaust temperatures (T
ex
~2650-2800 K depending on fuel type and uranium loading), then expanded out a high area ratio
(ε ~300:1-500:1) nozzle for thrust generation. Controlling the NTR during its various operational phases (startup, full thrust
and shutdown) is accomplished by matching the TPA-supplied LH
2
flow to the reactor power level. Multiple control drums,
located in the reflector region surrounding the reactor core, regulate the neutron population and reactor power level over the
NTR’s operational lifetime. The internal neutron and gamma radiation shield, located within the engine’s pressure vessel,
contains its own interior coolant channels. It is placed between the reactor core and key engine components (e.g., TPAs) to
prevent excessive radiation heating and material damage.
The NERVA-derived engine assumed in DRA 5.0 uses a “graphite matrix” material fuel element (FE) containing the U-
235 fuel in the form of a dispersion of uranium and zirconium carbide (UC-ZrC) within the matrix material, referred to as
“composite” fuel. It utilizes an epithermal neutron energy spectrum. A typical NERVA FE has a hexagonal cross section
(~0.75” across the flats), is 52” long and produces ~1 megawatt of thermal power
7
. Each FE has 19 axial coolant channels,
which along with the element’s exterior surfaces, are coated with ZrC using chemical vapor deposition to reduce hydrogen
erosion of the graphite. Composite fuel, with its higher exhaust temperature potential (T
ex
~2550-2800 K for ~4-6 hours
8
), was
the preferred fuel form at the end of Rover/NERVA program. An alternative fast energy spectrum NTR fuel is a ceramic-
metallic or “cermet” fuel consisting of uranium dioxide (UO
2
) in a tungsten (W) metal matrix. It was developed during the
GE-710 and ANL nuclear rocket programs
9,10
as a backup to the carbide-based fuels of the Rover/NERVA programs. While
no integrated reactor/engine tests were conducted, a large number of fuel specimens were produced and exposed to non-
nuclear hot H
2
and irradiation testing with promising results.
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