Interpolation-Enhanced Powered Descent Guidance for Onboard Nominal, Off-Nominal, and Multi-X Scenarios
Interpolation-Enhanced Powered Descent Guidance for Onboard Nominal, Off-Nominal, and Multi-X Scenarios
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DOI：
10.2514/6.2015-0850
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发表时间：
2015-01
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通讯作者：
D. Scharf;S. Ploen;Behçet Açikmese
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文献类型：
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作者：
D. Scharf;S. Ploen;Behçet Açikmese
用于行星际探索的下一代软着陆任务可能需要在动力下降期间进行大的转向，以实现精确/精确着陆或避免大的危险。然而，阿波罗时代的多项式动力下降制导可能会导致禁止系统级的大转移质量处罚。最近在凸化的完全约束，燃料最优，功率下降问题的进展可以消除这些处罚，并使大分流。要做到这一点，必须使用抗辐射处理器在着陆器上近乎实时地解决凸化问题。凸化动力下降问题需要线性搜索来找到最佳飞行时间，这可能需要求解10个或更多的转向轨迹。在这里，它是经验表明，在工程范围内感兴趣的登陆火星，最佳飞行时间和最佳推进剂质量的动力下降可以准确地预测通过插值从一个合理大小的网格的初始条件。这种工程级的可插值性非常重要。首先，飞行时间线搜索可以用一个小表的插值来代替，立即将运行时间减少了10倍。第二，为了在着陆器沿降落伞或离轨轨道下降时触发动力下降，可以在有限范围内查找到达目标的最佳推进剂质量。如果现在开始转向到达目标的推进剂是局部最小值和/或可行的，则可以启动动力下降。第三，对于Multi-X，着陆器从一系列安全目标中进行选择，只需查找每个目标的推进剂质量并选择最低的，就可以从数百种可能性中快速选择最佳目标。第四，如果没有足够的推进剂到达目标，可以通过使用内插推进剂质量的基于网格的搜索来近似可以到达的最接近目标的点。第五，也是最后一点，最远可能的转向也可以通过在目标网格上找到最大的推进剂可行距离来近似。
Next-generation soft-landing missions for interplanetary exploration can require large diverts during powered descent for precision/pinpoint landing or avoiding large hazards. However, Apollo-era polynomial powered descent guidance can incur prohibitive system-level mass penalties for large diverts. Recent advances in convexification of the fully constrained, fuel-optimal, powered-descent problem can remove these penalties and enable large diverts. To do so, the convexified problem must be solved in near-realtime onboard a lander using a radiation-hardened processor. The convexified powered descent problem requires a line-search to find the optimal time-of-flight, which can require solving for 10 or more divert trajectories. Here it is shown empirically that, over the engineering range of interest for landing on Mars, both the optimal time-of-flight and optimal propellant mass for powered descent can be accurately predicted via interpolation from a reasonably-sized grid of initial conditions. This engineering-level interpolability is significant. First, the time-of-flight line-search can be replaced with interpolation from a small table, immediately reducing the run-time by a factor of 10. Second, to trigger powered descent while a lander is descending on chute or a deorbit trajectory, the optimal propellant mass to reach the target can be looked-up over a finite horizon. If the propellant to reach the target starting a divert now is a local minimum and/or viable, then powered descent can be initiated. Third, for Multi-X, in which a lander selects from a list of safe targets, the optimal target can be selected quickly from hundreds of possibilities by simply looking up the propellant mass for each target and selecting the lowest. Fourth, if there is insufficient propellant to reach a target, the point closest to the target that can be reached can be approximated through a grid-based search using interpolated propellant mass. Fifth and finally, the farthest possible divert can also be approximated by finding the largest propellant-feasible distance over a grid of targets.