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Striepe, Scott A.

Publications and source records attributed to Striepe, Scott A..

31 records · Page 2
All sourcesNASA NTRSDOE OSTI
NASA NTRS1994

Radiation exposure predictions for long-duration-stay Mars missions

In this study, the ionizing radiation environment is estimated, using the Mission Radiation Calculation (MIRACAL) program, for several long-duration-stay Mars missions proposed for early in the 21(sup st) century. Both minimum energy and fast transfer missions are evaluated and their 30-day maximum, annual maximum, and total slab skin and blood-forming organ (BFO) doses are compared. When large flares were included while the astronauts were on the surface, the delivered dose did not significantly contribute to the total dose (less than 4 cSv BFO dose, or 8 percent of the guideline annual limit, for the most energetic event simulated) due to the substantial protection provided by the Martian atmosphere. However, dose delivered by large flares during transit is dependent on vehicle shielding and distance from the Sun. All of the fast transfer missions studied had lower total and annual maximum doses than the corresponding minimum energy transfer missions (on average, 30% less for missions having no large flares and the shielding thicknesses evaluated in this study). For all the missions studied, having the astronauts spend one-third of their day during transit in a 10 g/sq cm storm shelter resulted in an approximate 10% reduction in the total mission dose.

TOTAL SLAB SKIN↗
NASA NTRS1993

Influence of interplanetary trajectory selection on Earth atmospheric entry velocity of Mars missions

Many current manned Mars mission studies are using low lift-to-drag ratio vehicles to aerobrake at both Mars and Earth. This paper will demonstrate that if entry velocity constraints are incorporated into the interplanetary analysis of aerobraking Mars missions, more opportunities can be achieved for only a small increase in initial mass in low-Earth orbit (IMLEO). These additional opportunities result from varying the initial launch date and the encounter dates and possibly using a powered Venus swingby on either the inbound or outbound transfer. This paper not only presents unconstrained entry velocity missions but also includes results for entry velocities below 12.5 and 14 km/s on Earth return and between 6.0-8.5 km/s at Mars arrival. The results indicate that, regardless of the Mars entry velocity range selected, an Earth entry velocity below 14 km/s is easily attainable for a minimal IMLEO increase. Although there are fewer 12.5 km/s Earth entry velocity missions possible, both Mars entry velocity constraint cases have over 50 percent of their missions requiring a negligible IMLEO increase.

Striepe, Scott A.↗
NASA NTRS1993

Influence of interplanetary trajectory selection on Mars atmospheric entry velocity

Many current manned Mars mission studies are using low lift-to-drag ratio (L/D) vehicles to aerobrake at both Mars and Earth. The use of these low L/D vehicles could limit the allowable velocity at the atmospheric interface. This paper will demonstrate that if entry velocity constraints are incorporated into the interplanetary analysis of aerobraking Mars missions, many opportunities can be achieved for a small increase in initial mass in low-Earth orbit (IMLEO). These opportunities result from varying the initial launch date and the encounter dates and possibly using a powered Venus swingby on either the inbound or outbound transfer. This paper demonstrates this technique by using three atmospheric entry velocity ranges at Mars arrival (6.0-8.5, 6.4-8.1, and 7.2-7.3 km/s), unconstrained Mars entry velocities, and an Earth return entry velocity below 14 km/s. The results indicate that, by carefully selecting the interplanetary trajectory, an optimum IMLEO mission can be found for even highly restrictive entry velocity missions in practically all of the 15 yr studied.

Striepe, Scott A.↗
NASA NTRS1993

Piloted Mars missions using cryogenic and storable propellants

The paper presents an analyses of piloted long-duration stay Mars missions using a chemical or nuclear thermal propulsion Earth-departure stage, storable propellant Mars arrival and departure stages (to avoid propellant boiloff), and an Earth entry vehicle, conducted for three-year segments from 2000 to 2020. The study initially determines the minimum initial mass in low-Earth orbit (IMLEO) missions and then investigates their radiation exposure levels and Mars parking orbit selection. The impact of reducing the outbound (Earth-Mars) transfer time on IMLEO and predicted radiation exposure levels is included. The results indicate that a vehicle using an advanced chemical propulsion system (Isp = 480 s) for Earth departure is feasible throughout 2000-2020 with an IMLEO less than 640 metric tons, while an Earth departure stage using a nuclear thermal propulsion (NTP) system with Isp of 925 s is even more appealing with IMLEO's less than 405 tons. Vehicles with 875 and 825 s specific impulse Earth departure stages have only slightly higher IMLEO's than the advanced NTP system (10 and 20 tons higher, respectively). Reducing the time spent in space may not always result in a significantly reduced level of radiation exposure, especially if the mission encounter dates are changed to a less favorable period of the solar cycle.

Striepe, Scott A.↗
NASA NTRS1993

Minispacecraft for a multiple-asteroid rendezvous mission

The development of the Multiple Asteroid Rendezvous Tracker and Explorer spacecraft, which is designed for the analysis of the composition and the motion of several main-belt asteroids, is discussed. The mission includes a laboratory lander craft (the main lab) and several minispacecraft. The main lab will be equipped with a complete sample analysis laboratory and multiple external sensors. Each of the minispacecraft, which are much smaller than the main lab and contain only a few instruments, is targeted to rendezvous with an asteroid, on which it will land and perform a preliminary surface analysis. This will identify the most scientifically interesting asteroid. The main lab will then be sent to this asteroid for performing a detailed analysis including soil sampling.

Fowler, Wallace T.↗
NASA NTRS1992

Radiation exposure predictions for short-duration stay Mars missions

The human radiation environment for several short-duration stay manned Mars missions is predicted using the Mission Radiation Calculation program, which was developed at NASA Langley Research Center. This program provides dose estimates for Galactic cosmic rays (GCR) and large and ordinary solar proton flare events for various amounts of effective spacecraft shielding and a given time history of the spacecraft's heliocentric position. The results of this study show that most of the missions can survive the most recent large flares if a 25 g/sq cm storm shelter is assumed. The dose predictions show that missions during solar minima are not necessarily the minimum dose cases, due to increased GCR contribution during this time period. The direct transfer mission studied has slightly lower doses than the outbound Venus swingby mission, with the greatest dose differences for the assumed worst case scenario. The GCR dose for a mission can be reduced by having the crew spend some fraction of its day nominally in the storm shelter.

Striepe, Scott A.↗
NASA NTRS1992

Radiation exposure predictions for long-duration-stay Mars missions

In this study, the ionizing radiation environment is estimated, using the Mission Radiation Calculation (MIRACAL) program, for several long-duration-stay Mars missions proposed for early in the 21st century. Both long-fast and minimum energy transfer missions are evaluated, and their 30-day maximum, annual maximum, and total slab skin and blood-forming organ (BFO) doses are compared. When large flares were included while the astronauts were on the surface, the delivered dose did not significantly contribute to the total dose (less than 4 cSv BFO dose, or 8 percent of the guideline annual limit, for the most energetic event simulated) due to the substantial protection provided by the Martian atmosphere. However, dose delivered by large flares during transit is dependent on vehicle shielding and distance from the sun. All of the long-fast transfer missions studied had lower total and annual maximum doses than the corresponding minimum energy transfer missions (on average, 30 percent less for missions having no large flares and the shielding thicknesses evaluated in this study). For all the missions studied, having the astronauts spend one-third of their day during transit in a 10 g/sq cm storm shelter resulted in significantly lower total doses.

Striepe, Scott A.↗
NASA NTRS1992

MIRACAL: A mission radiation calculation program for analysis of lunar and interplanetary missions

A computational procedure and data base are developed for manned space exploration missions for which estimates are made for the energetic particle fluences encountered and the resulting dose equivalent incurred. The data base includes the following options: statistical or continuum model for ordinary solar proton events, selection of up to six large proton flare spectra, and galactic cosmic ray fluxes for elemental nuclei of charge numbers 1 through 92. The program requires an input trajectory definition information and specifications of optional parameters, which include desired spectral data and nominal shield thickness. The procedure may be implemented as an independent program or as a subroutine in trajectory codes. This code should be most useful in mission optimization and selection studies for which radiation exposure is of special importance.

Nealy, John E.↗
NASA NTRS1991

Interplanetary trajectory optimization of Mars aerobraking missions with constrained atmospheric entry velocities

Many current manned Mars mission studies are using low lift-to-drag ratio (L/D) vehicles to aerobrake at both Mars and earth. The use of these low L/D vehicles imposes constraints on the allowable velocity at the atmospheric interface. This paper will demonstrate that if these entry velocity constraints are incorporated into the interplanetary analysis, more opportunities can be achieved for a small increase in initial LEO mass. These additional opportunities result from varying the initial launch date, the encounter dates, and possibly using a powered Venus swingby on either the inbound or outbound transfer. This paper presents results for three atmospheric entry velocity ranges at Mars arrival and one velocity limitation upon Earth return. The results indicate that by carefully selecting the interplanetary trajectory, an optimum initial LEO mass mission can be found for even highly restrictive entry velocity missions in practically all of the 15 years studied.

Striepe, Scott A.↗
NASA NTRS1989

Effects of a Venus swingby periapsis burn during an earth-Mars trajectory

Interplanetary missions were analyzed that utilize the gravity-assist orbital transfer technique to increase mission capabilities. This swingby maneuver provides a nonpropulsive change in the spacecraft's heliocentric energy which can reduce the amount of propellant needed to complete an interplanetary mission. For certain planetary orientations, incorporating a propulsive manuever at the periapsis of the swingby trajectory decreases the mission's propellant requirement. This decrease results in a reduced initial vehicle mass in low-earth orbit. This study analyzed the feasibility of using a three-dimensional powered swingby to reduce the initial vehicle size for an excursion class mission during the 2010-2025 time frame.

Striepe, Scott A.↗
NASA NTRS1988

Aeroassist flight experiment guidance Quiet Time

The science experiments for the Aeroassist Flight Experiment (AFE) will be greatly enhanced by taking measurements with no Reaction Control System (RCS) contamination just before perigee. Methods of modifying the AFE guidance to accomplish this are discussed. Several methods that could give up to 30 seconds of quiet time were investigated and the results of these guidance modifications shown. A 20 second quiet time is definitely possible and a 30 second quiet time may be possible if the guidance can be inactive past perigee. Some of the most significant being the criterion for determining if the mission is threatened. A limited follow-on test program is outlined.

Striepe, Scott A.↗